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Recent applications of N-sulfonyloxaziridines (Davis oxaziridines) in organic synthesis
Tetrahedron 74 (2018) 3198e3214
Contents lists available at ScienceDirect
Tetrahedron journal homepage: www.elsevier.com/locate/tet
Tetrahedron report 1161
Recent applications of N-sulfonyloxaziridines (Davis oxaziridines) in organic synthesis Franklin A. Davis Department of Chemistry, Temple University, Philadelphia, PA, 19122, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 December 2017 Received in revised form 12 February 2018 Accepted 13 February 2018 Available online 19 February 2018
N-Sulfonyloxaziridines are the most commonly used oxaziridines in organic synthesis. Most applications of these stable, commercially available reagents involve the stereo- and regioselective oxidation of nucleophiles which have found many applications in the synthesis of architecturally complex molecules. In addition, these oxaziridines have been used in cycloaddition reactions (oxyamination), epoxidation of alkenes, silyl enol ethers and enamines, as well as C-H oxidation and amination reactions. The object of this review is to highlight recent applications of N-sulfonyloxaziridines in organic synthesis. © 2018 Published by Elsevier Ltd.
Keywords: N-sulfonyloxaziridines Davis oxaziridines Oxidation Asymmetric a-Hydroxy carbonyl compounds
Contents 1. 2.
3.
4.
5. 6. 7.
8. 9. 10. 11.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3199 Asymmetric synthesis of N-sulfonyloxaziridines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3199 2.1. Asymmetric catalytic oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3199 2.2. Kinetic resolution of racemic N-sulfonyloxaziridines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3201 a-Hydroxylation of enolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202 a-Hydroxylation of enolates in racemic syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3203 3.1. a-Hydroxylation in asymmetric syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3204 3.2. 3.3. Asymmetric a-hydroxylation of enolates using chiral catalysts and racemic N-sulfonyloxaziridines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3206 g-Hydroxylation of enolates and allylic anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3207 g-Hydroxylation of enolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3207 4.1. gHydroxylation of allylic anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3208 4.2. a-Hydroxylation of N-sulfinyl metallodienamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3210 Oxyamination reactions of N-sulfonyloxaziridines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3210 Epoxidation of alkenes, silyl enol ethers, and enamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212 7.1. Epoxidation of alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212 7.2. Epoxidation of silyl enol ethers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212 7.3. Epoxidation of enamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 CH oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 Amination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213
E-mail address: [email protected]. https://doi.org/10.1016/j.tet.2018.02.029 0040-4020/© 2018 Published by Elsevier Ltd.
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1. Introduction Oxaziridines are heterocyclic compounds containing oxygen, nitrogen and carbon atoms in a three-membered ring. Because of the weak NeO bond and the strained three-membered ring, these compounds function as both oxidizing and aminating reagents with nucleophiles. Regioselectivity is dependent on the group size of the nitrogen substituent where large and small groups direct attack of nucleophiles at oxygen and nitrogen, respectively. The high configurational stability of the oxaziridine nitrogen means that stable, optically active oxaziridines where the asymmetry is due only to nitrogen is possible. The chemistry of oxaziridines has been extensively explored and the subject of numerous reviews.1,2 The most widely used oxaziridines in organic synthesis are the N-sulfonyloxaziridines (Davis reagents or Davis oxaziridines), introduced by Davis and co-workers in 1977.3 As illustrated in Fig. 1, these stable, aprotic, neutral oxidizing reagents oxidize a wide variety of nucleophiles in a highly regioselective and stereoselective manner.1,2 Oxidations by this class of oxaziridines include amines to amine oxides and nitrones, sulfides to sulfoxides, disulfides to thiosulfinates, thiols to sulfenic and sulfinic acids, selenides to selenoxides, lithium and Grignard reagents to alcohols, alkenes to epoxides and enolates to a-hydroxy carbonyl compounds.1,2 With chiral N-sulfonyloxaziridines, the absolute stereochemistry of the product is predictably controlled by the configuration of the oxaziridine three-membered ring based on steric arguments.1 As shown in Fig. 2, many types of N-sulfonyloxaziridines have been used in synthesis. While they are structurally similar, they all have somewhat different reactivity and properties. For example, oxaziridine of types I and II, for steric reasons, are more reactive than the enantiomerically pure (camphorsulfonyl)oxaziridines III and V. Electron withdrawing groups on the oxaziridine 2- and 3-position can substantially increase their oxygen transfer abilities. The benzothiazine type oxaziridines IV, introduced by Du Bois, are similar to dioxiranes and capable of CH oxidations.4 Several of the oxaziridines listed in Fig. 2 are commercially available. The objective of this short review is to summarize the chemistry of N-sulfonyloxaziridines since the last, excellent review by Yoon and co-workers in 2014.2 Some overlap with earlier reviews is
Fig. 2. Types of N-sulfonyloxaziridines.
unavoidable and necessary for continuity. 2. Asymmetric synthesis of N-sulfonyloxaziridines Baeyer-Villiger type oxidation of N-sulfonylimines with buffered potassium peroxymonosulfate (Oxone) is the method of choice for the preparation of N-sulfonyloxaziridines (equation (1)).5 Yields are excellent and reactions can be carried out on multi- kilogram scale.
(1)
2.1. Asymmetric catalytic oxidation Recent efforts have been focused on the asymmetric synthesis of enantiomerically enriched N-sulfonyloxaziridines using chiral catalysts.2 These methods include the oxidation of N-sulfonylimines
Fig. 1. Oxygen transfer reactions of N-sulfonyloxaziridines.
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Scheme 1.
with m-chloroperbenzoic acid (m-CPBA) using cinchona alkaloid based organocatalysts and metal-catalyzed oxidations (equation (2)). Kinetic resolution of racemic N-sulfonyloxaziridines has also been described. Good yields and excellent levels of stereoinduction for the trans-2-arylsulfonyl-3-aryloxaziridines are realized.6
Scheme 2.
(2)
Earlier studies by Jin and co-workers explored the use of cinchona alkaloid sulfide 2 (R ¼ TBS) catalyst for oxaziridination of N-tosyl arylaldimines 1 with ee's up to 95% for meta- and para-aryl substitution (Scheme 1).7 In continuing, studies this group examined a series of cinchona alkaloid ester derivatives 2 for asymmetric oxaziridination of 1 with m-CPBA to give (R,R)-()-3.8 The highest ee's were found for the 4-nitrophenyl ester 2, whereas lower ee's were noted when R was acyl. For oxaziridine 3, best ee's >90% were generally found for meta aryl substitution and for Ph and 4-t-BuPh (Scheme 1). In an attempt to make (S,S)-3, the use of pseudoenantiomer of 2 resulted in racemic material. A series of thiourea organocatalysts were evaluated in the catalytic oxaziridination of N-tosyl arylaldimines 1 with m-CPBA at 40 C in toluene (Scheme 2).9 The highest ee's were observed for thiourea organocatalyst 4 affording the (S,S)-oxaziridines 5 in good yield and good to excellent ee's. Oxidation of the p-nitrophenylsulfonyl (Ns) and methanesulfonyl (Ms) benzaldimines resulted in much lower ee's for oxaziridines 6
Scheme 3.
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Scheme 4.
and 7 (Scheme 2). Using basic P-spiro chiral triaminoiminophosphorane organocatalyst 8 (Ar ¼ 3,5-Cl2C6H3-), Ooi and co-workers reported that the asymmetric oxidation of N-sulfonylimines 9 (R2 ¼ H) gave (R,R)oxaziridines 10 (R2 ¼ H, Scheme 3).10 The oxidizing reagent utilizes hydrogen peroxide and trichloroacetonitrile in a Payne-type oxidation. The enantioselectivity and yields are uniformly excellent for a wide range of substrates including aromatic, heteroaromatic, and aliphatic N-sulfonylimines 9 (R2 ¼ H). Importantly, epoxidation of the alkene in oxaziridine 11 was not observed under these conditions.10b N-Sulfonyl a-imino ester derived oxaziridines, 10 (R2 ¼ CO2Me), were prepared from N-sulfonylimino ester 9 (R1 ¼ 4-MePh, R2 ¼ CO2Me, Ar ¼ Ph).11 Interestingly higher ee's for 10 (R2 ¼ CO2Me) were noted in the absence of trichloroacetonitrile, 97 vs 69 %ee, respectively. Bicyclic N-sulfonylketimine 12 on oxidation with m-CPBA and C3-symmetric chiral tris(imidazoline) organocatalyst 13 afforded
the (R,R)-bicyclic oxaziridines 14 with ee's up to 87% (Scheme 4).12 Importantly, a single recrystallization of 14a and 14b from hexane/ CH2Cl2 gave enantiomerically pure materials. 2.2. Kinetic resolution of racemic N-sulfonyloxaziridines Making use of the fact that oxaziridines readily isomerize to amides in the presence of transition metal catalysts, Yoon and co-workers devised an iron (II)-catalyzed kinetic resolution of racemic N-sulfonyloxaziridines.13 With bis(oxazoline) ligand 16 and 5% FeCl2, oxaziridine (±)-15 gave N-sulfonyloxaziridines 17 in high ee and amide 18 (Scheme 5). The reaction is remarkably rapid and complete within a few minutes i.e. ()-19 and 20. This procedure works well for meta- and para-substitution, but lower ee's for C-alkyl substituents. Overall the reaction is scalable affording gram quantities of 17 (R ¼ Ar ¼ Ph). Mixing two equivalents of oxaziridine (±)-21 with one
Scheme 5.
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Scheme 6.
equivalent of indanone b-ketoester 22 in the presence of 5 mol% of chiral bifunctional guanidine catalyst 23 afforded (S,S)-()-N-sulfonyloxaziridines 24 and a-hydroxy b-ketoester (R)-()-25 (Scheme 6).14 The enantioselectivities of ()-24 and ()-25 are excellent, 92e99% ee, with good product yields. The kinetic resolution worked best with catalyst 23 and indanone 22 both having the bulky adamantyl (Ad) group. While racemic oxaziridines 21 having a variety of substitution patterns in the C-aryl work well, much poorer ee's were noted for C-naphthyl, 2-MePh and cyclohexyl groups. In a study aimed at developing catalysts for the asymmetric a-hydroxylation of b-ketoesters, motifs found in natural products and pharmaceutical lead structures, Waser and co-workers explored the use of various catalysts and oxidizing reagents.15 Of the oxidants explored (H2O2, m-CPBA, t-BuOOH), racemic N-sulfonyloxaziridine 26 gave highest enantioselectivities of (R,R)-28 when used with chiral bifunctional urea ammonium salt (S,S)-25 (Scheme 7). At 40 C with 2 equiv. of (±)-26 and 1 equiv. of 27 oxaziridine (R,R)-28 and a-hydroxy b-ketoester (R)-29 were obtained in 88 and 97% ee after 3 h. When 1.5 equiv. of (±)-26 was used the ee of N-sulfonyloxaziridine (R,R)-28 improved to 97%. A variety of enantiopure a-hydroxy b-ketoesters and N-sulfonyloxaziridines were prepared in this manner.
3. a-Hydroxylation of enolates The a-hydroxy carbonyl moiety is a common feature found in many biologically active materials such as pheromones, sugars, antibiotics, terpenes and alkaloids. Chiral a-hydroxy carbonyl compounds are also versatile synthons for the asymmetric construction of natural products and are useful stereodirecting groups for the synthesis of enantiomeric pure materials. For these reasons, the most widely used application of N-sulfonyloxaziridines is the ahydroxylation, of metal enolates because of their aprotic nature and the simplicity for installing this key functional group (eq (3)).1,16 A transition state controlled by steric factors is generally recognized as determining the stereochemistry.1c,f
Scheme 7.
(3)
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3203
Scheme 8.
3.1. a-Hydroxylation of enolates in racemic syntheses Trichodermatide A is a member of a family of natural products isolated from marine-fungi having interesting bioactivities. In a racemic synthesis of trichodermatide A, the bis-lithium enolate of hexahydroxanthene-dione 30 was treated with (camphorsulfonyl) oxaziridine (þ)-31 to give the di-hydroxylated product 32 as the
major diastereoisomer (Scheme 8).17 It was noted that the bulky side chain controls the stereochemistry of the enolate hydroxylations, countering the reagent control of the enantiopure oxaziridine. This phenomenon is often observed in enolate hydroxylations. Steric considerations also play a role in the regioselectivity of enolate hydroxylations by N-sulfonyloxaziridines. Attempts to
Scheme 9.
Scheme 10.
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by treatment of b-ketoester 41 with NaHMDS followed by hydroxylation with 2-(4-toluenesulfonyl)-3-phenyloxaziridine (43) (Scheme 11).21 3.2. a-Hydroxylation in asymmetric syntheses
Scheme 11.
a-hydroxylate the C-1 position in ketone 33 with (camphorsulfonyl) oxaziridine (þ)-31 failed.18 The sterically less demanding 2phenylsulfonyl-3-phenyloxazidine (34) gave only low yields of 35 (Scheme 9). However with LiHMDS/HMPA, 33 gave the C-3 hydroxylation product 36 in good yield. The authors attribute the regioselectivity of 35 and 36 to thermodynamic versus kinetic enolate formation where shielding of the former enolate by the two bulky ortho groups inhibits C-1 hydroxylation. The dearomative ortho-hydroxylation of phenolic substrates is a useful method for the synthesis of highly functionalized polycyclic natural products.19 Grandclaudon and Toulle explored the phasetransfer (benzylcinchonidinium chloride) catalyzed oxaziridinemediated ortho-hyroxylation of substituted phenols and naphthols.20 In the dearomatization of phenol 37, they found that oxaziridines 38a and 38b gave the best yields of dimer 40, resulting in hydroxy cyclohexanone intermediate 39 (Scheme 10) (see Scheme 11). Rocaglamide is a member of a class of compounds that exhibits potent cytostatic and anti-inflammatory activity. C-3 Alcohol 42, an intermediate in the synthesis of racemic rocaglamide, was prepared
In this section, recent applications of N-sulfonyloxaziridines for the a-hydroxylation of chiral nonracemic enolates for the synthesis of natural products and other applications are summarized. Hong and co-workers described the a-hydroxylation of amides 44 derived from Oppolzer's (1S)-()-2,10-camphorsultam auxiliary using oxaziridine 34 (Scheme 12).22 The dr was >20:1, affording the a-hydroxy amide (S)-45 in good to excellent yields. The auxiliary was removed without epimerization with in situ generated MeOMgBr and the sultam recycled. Diastereoselective oxidation of the Evans oxazolidinone of 5-hexenoic acid (R)-46 with 34 gave a-hydroxy amide (R)-47.23 This amide was an intermediate in the synthesis of ()-promysalin, a natural product that exhibits antivirulence phenotypes against pathogenic bacteria (Scheme 12). The Baran group a-acetoxylated ketone 48 with oxaziridine 34 to give 49 as 2:1 mixture of diastereoisomers on a 30 g scale (Scheme 12).24 This ketone was a key intermediate in the total synthesis of the diterpene ()-maoecrystal V. Bicyclic lactenone 52 is a key intermediate in the synthesis of the alkaloid secu'amamine E.25 It was prepared by a-hydroxylation of enone 50 with N-sulfonyloxaziridine 34 followed by treatment with the Bestmann ylide 51 (Scheme 13).25 Many diastereoselective a-hydroxylations of enolates in the synthesis of natural products are performed with racemic 2-phenylsulfonyl-3-phenyloxaziridine (34) because it is more reactive than the enantiomerically pure camphor derived N-sulfonyloxaziridines. Recent examples are given in Table 1.
Scheme 12.
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3205
Scheme 13.
In the eight step gram scale synthesis of ()-jiadifenolide (54), a neurotrophic trace metabolite, the lithium trianion of 53 was treated with CBr4 to give an intermediate a-bromolactone.33 The bromolactone (not shown) on reaction with NaHMDS and
oxaziridne 34 afforded ()-jiadifenolide (54) in 50% yield for the two steps (Scheme 14). The formal synthesis of azadirachtin, an insect antifeedant, involved reaction of 55 with TBSOTf and Et3N to give an intermediate silyl ketal acetal followed by a-hydroxylation
Table 1 Hydroxylation of metal enolates to a-hydroxy carbonyl compounds using racemic 2-phenylsulfonyl-3-phenyloxaziridine (34). Ketone/ester
Base
% Yield
(Ref.)
KHMDS
Product
20%
(26)a
NaHMDS
54%
(27)b
KHMDS
69%
(28)c
KHMDS
87%
(29)d
NaH
66%
(30)e
KHMDS
57%
(31)f
KHMDS
42-50%
(32)g
Synthesis of (a) Viridicatumtoxin B. (b) ()-Jiadifenin. (c) Rubriflordilactone B. (d) Anisodorin 5. (e) ()-Enterocin. (f) mimic of Pramanicin. (g) (þ)-Ryanodol.
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Scheme 14.
Scheme 15.
with N-sulfonyloxaziridine 34 to give 56.34 In the total synthesis of crotophorbolone, hydroxylation of the C-4 position of tetracyclic ketone 57 was challenging because of its steric congestion and its proximity to oxidizable olefins.35 Oxidation of the sodium enolate of 57 with (camphorsulfonyl)oxaziridine (þ)-31 resulted in formation of the undesired isomer 58 (Scheme 15). Hydroxylation of a less congested fragment, 59 gave the desired C-4 hydroxylation product 60 but was highly dependent on the oxaziridine structure as summarized in Table 2.35b
Enantiomeric (camphorsulfonyl)oxaziridine ent-()-31 gave no yield and 2-phenylsulfonyl-3-phenyloxaziridine (34) gave lower yields (Table 2). 3.3. Asymmetric a-hydroxylation of enolates using chiral catalysts and racemic N-sulfonyloxaziridines Using a Cu(II) complex of chiral phenanthroline ligand (S)-63, Naganawa and co-workers explored the asymmetric a-hydroxylation
Table 2
Oxaziridine
% Yield of 60
(þ)-31 ent-()-31 (±)-34
43 0 27
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Scheme 16.
of various oxindole derivatives 61 with racemic N-sulfonyloxaziridine 43 (Scheme 16).36,37 Yields and enantioselectivities of the (R)-3hydroxy-2-oxindole derivatives 62 were excellent. In a related study, this group studied the a-hydroxylation of various b-ketoesters with Cu(II) and ligand (S)-63. Best enantioselectivities for (S)-64 were noted using racemic bicyclic oxaziridine 65 (Scheme 16).38 Attempted kinetic resolution of the 43 with 2 equivalents of 61 gave racemic material.37 It was suggested that the stereoinduction of 62 results from the chiral environment of the prochiral Cu(II) enolate of 61. This result is in contrast to the kinetic resolution of (±)-43 in the
a-hydroxylation of indanone b-ketoesters with chiral bifunctional guanidine catalyst 23 where the ee's for the oxaziridines (S,S)-()-43 and (R)-()-25 were excellent (see Scheme 6).13 4. g-Hydroxylation of enolates and allylic anions 4.1. g-Hydroxylation of enolates On treatment with oxaziridine 34, the potassium dianion of (R)tetrahydroxanthone 66 resulted in a 1:1 mixture of cis/trans
Scheme 17.
Scheme 18.
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Scheme 19.
diacetates 67 and 68 resulting from g-hydroxylation of the extended enolate of 66 (Scheme 17).39 In a related study, g-hydroxylation of the lithium enolate of deoxyaureothin 69 with (þ)-(camphorsulfonyl)oxaziridine 31 afforded alcohol 70 in 35% ee and 76% yield (Scheme 17).40 Alcohol 70 was a precursor to the polyketide metabolite (þ)-aureothin, a potent antiproliferative agent.41 A g-hydroxylation strategy was used in the enantioselective synthesis of the ABC-tricyclic core of phomactin A by Lee and co-workers.42 Enone 71 on treatment with t-BuLi followed by oxidation with 2-phenylsulfonyl-3-phenyloxaziridine (34), gave the ABC-tricyclic furanochroman core 72 of phomactin A in 63% yield (Scheme 18). 5-Hydroxy-3-acyltetramic acid 74 is a structural unit found in natural products such as delaminomycin A. Tetramic acid 73 on treatment with excess LDA, followed by hydroxylation with
oxaziridines (þ)-31 or 34 gave 74 in 27e28% yield (Scheme 19).43 Oxaziridine 34 also gave product 75 in 15% yield. Addition of the enolate to the N-sulfonylimine by-product (PhSO2N¼CHPh) is avoided using bulky (camphorsulfonyl)oxaziridines such as (þ)-32.44 4.2. gHydroxylation of allylic anions A step in the first asymmetric synthesis of ()-marcfortine C, a complex indole alkaloid, was the hydroxylation of the allylic cyanide anion derived from cycloadduct ()-76. The allylic anion of ()-76 was prepared by reaction with LiOt-BuO/n-BuLi and quenched with oxaziridine 34 to give ()-77 in 64% yield (Scheme 20).45 Bromide 78 gave 79 in 29% yield on treatment with 4 equiv. of (camphorsulfonyl)oxaziridine (þ)-31 and 2 equiv. of indium at
Scheme 20.
Scheme 21.
F.A. Davis / Tetrahedron 74 (2018) 3198e3214
Scheme 22.
Scheme 23.
Scheme 24.
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Scheme 25.
Scheme 26.
70 C in a sealed tube for two days (Scheme 20).46
6. Oxyamination reactions of N-sulfonyloxaziridines
5. a-Hydroxylation of N-sulfinyl metallodienamines The first example of the a-hydroxylation of an N-sulfinyl metallodienamine was described by Chatare and Andrade in their total synthesis of the macrolactone antibiotic ()-albocycline.47 Reaction of 80 with LiHMDS, followed by regioselective hydroxylation with oxaziridine 34, gave N-sulfinyl imine 81 as an inseparable mixture of isomers (4:1 dr) in 73% yield, which was elaborated in four steps to ()-albocycline (Scheme 21).
The cycloaddition reactions of N-sulfonyloxaziridines with alkenes involving NO (oxyaminations) and CeO bond cleavages (1,3-dipolar cycloadditions) has been extensively explored and recently reviewed.2,48 An oxamination step was used to prepare a key intermediate in the total synthesis of ()-communesin F by the Movassaghi group.49 Indole ()-82 with stoichiometric copper (II) and 3,3dimethyl-2-(4-nitrophenylsulfonyl)-1,2-oxaziridine (83) afforded oxazoline ()-84 in 68% yield (dr 89:11) (Scheme 22). Formal [3þ2] cycloadditions involving NeO bond cleavage of N-
Scheme 27.
F.A. Davis / Tetrahedron 74 (2018) 3198e3214
3211
Scheme 28.
sulfonyloxaziridines have been used to prepare stereodefined oxazolidin-4-ones.2 Smith and co-workers reported that the reaction of a series of homoanhydrides 85 with racemic oxaziridine 43 and 10 mol% of Lewis-base (2R,3R)-HyperBMT 86 afforded mixtures of anti- and syn-oxazolidin-4-ones 87 in good yield (Scheme 23).50 The ee's of the individual oxazolidin-4-ones were excellent (78e99% ee). When (R,R)-43 (94% ee) was used anti-87 (R ¼ Ph) was obtained in 81% yield and >99% ee (Scheme 23). The mismatched pair (2R,3S)-HyperBMT-86 and (R,R)-43 gave syn-87 (80:20 d.r. syn/ anti) with 98% ee. The authors suggest that these results are more consistent with a-oxidation of an intermediate ammonium enolate
than formation of a transient epoxide by the oxaziridine. With (R,R)-43 (95% ee) and indole 88 adduct 89 was isolated in 76% yield with a dr of 1.2:1 (95%ee/94% ee).10a Much better control of the diastereo- and enantioselectivity in the formal [2þ3] cycloaddition of oxaziridines with a-aroyloxyaldehydes in the synthesis of oxazolindin-4-ones was found using a N-heterocyclic carbene (NHC) catalyst.51 With (R,R)-43 (98% ee) aaryloxyaldehydes 90 with 10 mol% of catalyst (þ)-91 gave antioxazolindin-4-ones 92 in excellent ee and diastereoselectivity (Scheme 24).
Scheme 29.
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F.A. Davis / Tetrahedron 74 (2018) 3198e3214
Scheme 30.
Scheme 31.
7. Epoxidation of alkenes, silyl enol ethers, and enamines
7.1. Epoxidation of alkenes
On heating, N-sulfonyloxaziridines epoxidize alkenes in a synstereospecific manner similar to peracids.1,2,52 With electron rich silyl enol ethers and enamines, reaction rates are much faster and the elusive a-silyl and a-amino epoxides can often be isolated.53,54
Heating oxaziridine (þ)-92 (96% ee) at 60 C for 77 h resulted in intramolecular oxygen transfer to give imine epoxide 93 (Scheme 25).10a Lewis acid mediated reduction resulted in cyclization affording the pyrrolidine methanol derivative ()-94 in 74% yield for the two steps. The more reactive N-sulfonyl a-imino ester oxaziridines are capable of oxidizing non-activated alkenes with high ee's under milder conditions.11 For example stirring (þ)-95 (96% ee) with alkene 96 for 3e7 days at 0 C afforded the corresponding epoxides (þ)-97 in excellent ee and good yield (Scheme 26). 7.2. Epoxidation of silyl enol ethers The oxidation of silyl enol ethers, the Rubottom reaction, is a widely-used method for the synthesis of a-hydroxy carbonyl compounds (eq (4)).55 The intermediate siloxy epoxides are seldom isolated or even detected because trace acid results in facile rearrangement to the a-hydroxy carbonyl compounds. Silyloxy epoxides have been isolated using aprotic oxidizing reagents such as N-sulfonyloxaziridines.53 With chiral N-sulfonyloxaziridines, the ee's are poor (7e11% ee).
(4) Scheme 32.
F.A. Davis / Tetrahedron 74 (2018) 3198e3214
Silyl enol ether 98 was transformed into a-hydroxy carbonyl compound (þ)-99 on reaction with oxaziridine (þ)-95 (Scheme 27).11 Yields and ee's were excellent. A catalytic one-pot method employing the N-sulfonyl a-imino ester corresponding to (þ)-95, (4-MePhSO2N¼C(4-ClPh)COt2Bu) and chiral iminophosphorane organocatalyst 8 (see Scheme 3) was also use to prepare (þ)-99. 7.3. Epoxidation of enamines Disubstituted and trisubstituted enamines are oxidized by N-sulfonyloxaziridines to a-hydroxy and a-amino ketones, respectively.56 Oxidations of indoles by oxaziridines have found numerous applications in the synthesis of architecturally complex alkaloids.2 Recent examples include the asymmetric synthesis of ()-citrinadin A and the ()-trigonoliimines by Martin54 and Movassaghi,57 respectively. In the synthesis of ()-citrinadin A, indole 100 was first treated with pyridinium p-toluene sulfonate (PPTS) to prevent oxidation of the amino groups and followed by reaction with bicyclic oxaziridine 101 (Scheme 28).54 A semipinacol rearrangement gave spirooxindole 103 on treatment of the moderately stable epoxide 102 with AcOH. As part of a model study in the synthesis of the trigonoliimines, homodimeric 2,20 -bistryptamine 104 was oxidized with oxaziridine 101 to give hydroxyindolenine 105 in quantitative yield (Scheme 29).57 Bistryptamine 106 with (þ)-(8,8-dichlorocamphorsulfonyl)oxaziridine (107) afforded hydroxyindolenines (þ)-108 and (þ)-109 (108:109 ¼ 2.2:1) in 95% yield and 96% ee (Scheme 29). 8. C¡H oxidation Du Bois and co-workers, in a series of papers, demonstrated that N-sulfonyloxaziridine 111 derived from benzoxathiazine heterocycle 110 is capable of hydroxylating unactivated 3 CeH bonds similar to dioxiranes.2,58 In addition to stoichiometric oxidations, organocatalytic C-H hydroxylations using H2O2 to generate 111 were reported.59 However this system was limited to a few substrates and required long reaction times (>96 h). Recently this group reported that Oxone and an aqueous hexafluoroisopropanol (HFIP) gave much improved results for 3 CsH oxidation (Scheme 30).60 9. Amination Oxaziridines with large groups on the nitrogen atom, such as N-sulfonyloxaziridine, react almost exclusively at oxygen with nucleophiles.1,2 However, the Banerjee group recently reported that N-sulfonyloxaziridines react at nitrogen with donor acceptor cyclopropanes to give azetidine analogs in the presence of Lewis acids such as MgI2 (Scheme 31).61 Thus, treatment of oxaziridine 43 with cyclopropane 112 and 40 mol% MgI2 afforded azetidines 113 in good yield (Scheme 31). In the presence of the Lewis acid, the mechanism is envisioned as nucleophilic attack of the ring-opened cyclopropane at the N-atom of the MgI2-activated oxaziridine. 10. Reduction Racemic bicyclic oxaziridines 114a-b on hydrogenolysis (H2, 600 psi), in the presence of Pd(OCOCF3)2/(S,S0 ,R,R0 )-115 and ()-camphorsulfonic acid (L-CSA) provide novel sultams 116a-b with ee's up to 97% (Scheme 32).62 Mechanistic studies suggest a mechanism involving NeO bond cleavage and dehydration followed by asymmetric hydrogenation.
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11. Summary N-Sulfonyloxaziridines (commonly known as Davis oxaziridines or Davis reagents), are the most widely used oxaziridines in organic synthesis. Most applications of these stable, commercially available reagents involved the stereo- and regioselective oxidation of nucleophiles. They have found numerous uses in the synthesis of architecturally complex molecules. Most recent applications of these oxaziridines include cycloadditions (oxyamination), epoxidation of alkenes, silyl enol ethers, enolates, N-sulfinyl dienamines, enamines, CH oxidations, aminations and stereoselective reductions. It is anticipated that these applications, as well as new uses of these novel reagents, will find increasing utility in organic synthesis. Acknowledgements I am indebted to Dr. Rodrigo B. Andrade, Temple University, for stimulating conversations and for critiquing this manuscript. The comments and suggestions of the reviewers are greatly appreciated. References 1. (a) Davis FA, Sheppard AC. Tetrahedron. 1989;45:5703; (b) Andreae S, Schmitz E. Synthesis. 1991:327; (c) Davis FA, Chen B-C. Chem Rev. 1992;92:919; (d) Petrov VA, Resnati G. Chem Rev. 1996;96:1809; (e) Davis FA, Reddy RT. In: Padwa A, ed. Comprehensive Heterocyclic Chemistry II. vol. 1. Oxford: Permagon Press; 1996:365; (f) Davis FA. J Org Chem. 2006;71:8993; (g) Davis FA, Chen B-C, Zhou P. In: Katritzky AR, Ramsden CA, Scriven EFV, Taylor RJK, eds. Comprehensive Heterocyclic Chemistry III. vol. 1. Oxford: Elsevier; 2008:559; (h) Kumar KM. Synlett. 2012:2572. 2. Williamson KA, Michaels DJ, Yoon TP. Chem Rev. 2014;114:8016. 3. Davis FA, Nadir UK, Kluger EWJ. Chem Soc Chem Commun. 1977:25. 4. (a) Brodsky BH, Du Bois J. J Am Chem Soc. 2005;127:15391; (b) Litvinas ND, Brodsky BH, Du Bois J. Angew Chem Int Ed. 2009;48:4513. 5. Davis FA, Chattopadhyay S, Towson JC, Lal S, Reddy T. J Org Chem. 1988;53: 2087. 6. For a recent review on the asymmetric catalytic synthesis of N-sulfonyloxaziridines see Sala DG, Lattanzi A. ACS Catal. 2014;4:1234. 7. Zhang T, He W, Zhao X, Jin Y. Tetrahedron. 2013;69:7416. 8. Jin Y, Zhang T, Zhang W, Chang S, Feng B. Chirality. 2014;26:150. 9. Ji N, Yuan J, Xue S, Zhang J, Wei H. Tetrahedron. 2016;72:512. 10. (a) Uraguchi D, Tsutsumi R, Ooi T. Tetrahedron. 2014;70:1691; (b) Tsutsumi R, Kim S, Uraguchi D, Ooi T. Synthesis. 2014:871. 11. Tanaka N, Tsutsumi R, Uraguchi D, Ooi T. Chem Commun. 2017;55:6999. 12. Takizawa S, Kishi K, Abozeid MA, Murai K, Fujioka H, Sasai H. Org Biomol Chem. 2016;14:761. 13. Williamson KS, Swicki JW, Yoon TP. Chem Sci. 2014;5:3524. 14. Lin X, Ruan S, Yao Q, et al. Org Lett. 2016;18:3602. 15. Novacek J, Izzo JA, Vetticatt MJ, Waser M. Chem Eur J. 2016;22:17339. 16. Chen B-C, Zhou P, Davis FA, Ciganek E. a-hydroxylation of enolates and silyl enol ethers. Org React. 2003;62:1e356. 17. Myers E, Herrero-Gomez E, Albrecht I, et al. J Org Chem. 2014;79:9812. 18. Wilsdorf M, Lentz D, Reissig H-U. Eur J Org Chem. 2016;81:1555. 19. Roche SP, Porco Jr JA. Angew Chem Int Ed. 2011;50:4068. 20. Grandclaudon C, Toullec PY. Eur J Org Chem. 2016;81:260. 21. Zhou Z, Dixon DD, Jolit A, Titus MA. Chem Eur J. 2016;22:15929. 22. Zhang L, Zhu L, Yang J, Luo J, Hong R. J Org Chem. 2016;81:3890. 23. Steele AD, Knouse KW, Keohane CE, Wuest W. J Am Chem Soc. 2015;137:7314. 24. Cernijenko A, Risgaard R, Baran PS. J Am Chem Soc. 2016;138:9425. 25. Wehlauch R, Grendelmeier SM, Miyatake-Ondozabal H, Sandtory AH, Scherer M, Gademann K. Org Lett. 2017;19:548. 26. Nichoaou KC, Hale CRH, Nilewski C, Ioannidou HA, ElMarrouni A, Nilewski LG, Beabout K, Wang TT, Shamoo Y. J Am Chem Soc. 2014;136:12137. 27. Cheng X, Micalizio GC. J Am Chem Soc. 2016;138:1150. 28. Yang P, Yao M, Li J, Li Y, Li A. Angew Chem Int Ed. 2016;55:6964. 29. Ren J, Zhao P, Xiao X, Zeng B-B. Synthesis. 2016;48:4161. 30. Wegmann M, Bach T. Synthesis. 2017;49:209. 31. Tan SWB, Chai CLL, Moloney MG. Org Biomol Chem. 2017;15:1889. 32. Chuang KV, Xu C, Reisman SE. Science. 2016;353:912. 33. Lu H-H, Martinez MD, Shenvi RA. Nat Chem. 2015;7:604.
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34. Mori N, Kitahara T, Mori K, Watanabe H. Angew Chem Int Ed. 2015;54:14920. 35. (a) Asaba T, Katoh Y, Urabe D, Inoue M. Angew Chem Int Ed. 2015;54:14457; (b) Urabe D, Asaba T, Inoue M. Bull Chem Soc Jpn. 2016;89:1137. 36. Naganawa Y, Nishiyama H. Chem Rec. 2016;16:2573. 37. Naganawa Y, Aoyama T, Nishiyama H. Org Biomol Chem. 2015;13:11499. 38. Naganawa Y, Aoyama T, Kato K, Nishiyama H. ChemistrySelect. 2016;1:1938. 39. Turner PA, Samiullah, Geden JV, White A, Clarkson GJ, Shipman M. Tetrahedron. 2015;71:9433. 40. Henrot M, Jean A, Peixoto PA, Maddaluno J, De Paolis M. J Org Chem. 2016;81: 5190. 41. Henrot M, Richter MEA, Maddaluno J, Hertweck C, De Paolis M. Angew Chem Int Ed. 2012;51:9587. 42. Du G, Bao W, Huang J, et al. Org Lett. 2015;17:2062. 43. Trenner J, Prusov EV. Bellstein J Org Chem. 2015;11:323. 44. Davis FA, Wei J, Shepppard AC, Gubernick S. Tetrahedron Lett. 1987;28:5115. 45. Trost BM, Bringley DA, Zhang T, Cramer N. J Am Chem Soc. 2013;135:16720. 46. Liou B-S, Chou S-SP. J Chin Chem Soc. 2016;63:818. 47. Chatare VJ, Andrade RB. Angew Chem Int Ed. 2017;56:5909. 48. Knappke CEI, von Wangelin AJ. Chem Cat Chem. 2010;2:1381. 49. Lathrop SP, Pompeo M, Chamg W-TT, Movassaghi M. J Am Chem Soc. 2016;138: 7763. 50. Smith SR, Fallan C, Taylor JE, et al. Chem Eur J. 2015;21:10530. 51. Kerr RWF, Greenhalgh MD, Slawin AMZ, Arnold PL, Smith AD. Tetrahedron: Asymmetry. 2017;28:125. 52. Davis FA, Abdul-Malik NF, Awad SB, Harakal ME. Tetrahedron Lett. 1981;22:917. 53. Davis FA, Sheppard AC. J Org Chem. 1987;52:954. 54. Bian Z, Marvin CC, Martin SF. J Am Chem Soc. 2013;135:10886. 55. a) Rubottom GM, Vazquez MA, Pelegrina DR. Tetrahedron Lett. 1974:4319; b) Brook AG, Macrae DM. J Organomet Chem. 1974;77:C19eC21; c) Hassner A, Reuss RH, Pinnick HW. J Org Chem. 1975;40:3427; d) Christoffers J, Baro A, Werner T. Adv Synth Catal. 2004;346:143. 56. Davis FA, Sheppard AC. Tetrahedron Lett. 1988;35:4365. 57. Han S, Morrison KC, Hergenrother PJ, Movassaghi M. J Org Chem. 2014;79:473. 58. Litvinas ND, Brodsky BH, Du Bois J. Angew Chem Int Ed. 2009;48:4513. 59. Brodsky BH, Du Bois J. J Am Chem Soc. 2005;127:15391. 60. Adams AM, Du Bois J. Chem Sci. 2014;5:656. 61. Ghosh A, Madal S, Chattaraj PK, Baneerjee P. Org Lett. 2016;18:4940. 62. Song B, Yu C-B, Huang W-X, Chen MW, Zhou Y-G. Org Lett. 2015;17:190.
Franklin A. Davis was born in Des Moines, Iowa on April 1, 1939 and grew up in Hastings-on-Hudson, NY. He received his B.S. degree in chemistry from the University of Wisconsin and his Ph.D. from Syracuse University in 1966. After a Welch Postdoctoral Fellowship at the University of Texas he joined the faculty at Drexel University in 1968 where he was the George S. Sasin Professor of Chemistry. In 1995 he moved across town to Temple University where he is the Laura H. Carnell Professor of Chemistry. Davis' research interests are focused on the invention of new reagents and methodologies for the synthesis of biologically important molecules. He originated and pioneered the chemistry of N-sulfonyloxaziridines for oxidations and asymmetric oxidations and N-sulfinyl imines for chiral nonracemic amine synthesis. Davis' honors include a National Science Foundation Extension for Special Creativity in 1991, a fellowship from the Japanese Society for the Promotion of Science in 1992. He is an inaugural fellow of the American Chemical Society and a Fellow of the Royal Society. In 2006 he received the American Chemical Society Cope Scholar Award and the John Scott Medal.
Contents lists available at ScienceDirect
Tetrahedron journal homepage: www.elsevier.com/locate/tet
Tetrahedron report 1161
Recent applications of N-sulfonyloxaziridines (Davis oxaziridines) in organic synthesis Franklin A. Davis Department of Chemistry, Temple University, Philadelphia, PA, 19122, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 December 2017 Received in revised form 12 February 2018 Accepted 13 February 2018 Available online 19 February 2018
N-Sulfonyloxaziridines are the most commonly used oxaziridines in organic synthesis. Most applications of these stable, commercially available reagents involve the stereo- and regioselective oxidation of nucleophiles which have found many applications in the synthesis of architecturally complex molecules. In addition, these oxaziridines have been used in cycloaddition reactions (oxyamination), epoxidation of alkenes, silyl enol ethers and enamines, as well as C-H oxidation and amination reactions. The object of this review is to highlight recent applications of N-sulfonyloxaziridines in organic synthesis. © 2018 Published by Elsevier Ltd.
Keywords: N-sulfonyloxaziridines Davis oxaziridines Oxidation Asymmetric a-Hydroxy carbonyl compounds
Contents 1. 2.
3.
4.
5. 6. 7.
8. 9. 10. 11.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3199 Asymmetric synthesis of N-sulfonyloxaziridines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3199 2.1. Asymmetric catalytic oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3199 2.2. Kinetic resolution of racemic N-sulfonyloxaziridines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3201 a-Hydroxylation of enolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202 a-Hydroxylation of enolates in racemic syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3203 3.1. a-Hydroxylation in asymmetric syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3204 3.2. 3.3. Asymmetric a-hydroxylation of enolates using chiral catalysts and racemic N-sulfonyloxaziridines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3206 g-Hydroxylation of enolates and allylic anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3207 g-Hydroxylation of enolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3207 4.1. gHydroxylation of allylic anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3208 4.2. a-Hydroxylation of N-sulfinyl metallodienamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3210 Oxyamination reactions of N-sulfonyloxaziridines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3210 Epoxidation of alkenes, silyl enol ethers, and enamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212 7.1. Epoxidation of alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212 7.2. Epoxidation of silyl enol ethers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3212 7.3. Epoxidation of enamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 CH oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 Amination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3213
E-mail address: [email protected]. https://doi.org/10.1016/j.tet.2018.02.029 0040-4020/© 2018 Published by Elsevier Ltd.
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1. Introduction Oxaziridines are heterocyclic compounds containing oxygen, nitrogen and carbon atoms in a three-membered ring. Because of the weak NeO bond and the strained three-membered ring, these compounds function as both oxidizing and aminating reagents with nucleophiles. Regioselectivity is dependent on the group size of the nitrogen substituent where large and small groups direct attack of nucleophiles at oxygen and nitrogen, respectively. The high configurational stability of the oxaziridine nitrogen means that stable, optically active oxaziridines where the asymmetry is due only to nitrogen is possible. The chemistry of oxaziridines has been extensively explored and the subject of numerous reviews.1,2 The most widely used oxaziridines in organic synthesis are the N-sulfonyloxaziridines (Davis reagents or Davis oxaziridines), introduced by Davis and co-workers in 1977.3 As illustrated in Fig. 1, these stable, aprotic, neutral oxidizing reagents oxidize a wide variety of nucleophiles in a highly regioselective and stereoselective manner.1,2 Oxidations by this class of oxaziridines include amines to amine oxides and nitrones, sulfides to sulfoxides, disulfides to thiosulfinates, thiols to sulfenic and sulfinic acids, selenides to selenoxides, lithium and Grignard reagents to alcohols, alkenes to epoxides and enolates to a-hydroxy carbonyl compounds.1,2 With chiral N-sulfonyloxaziridines, the absolute stereochemistry of the product is predictably controlled by the configuration of the oxaziridine three-membered ring based on steric arguments.1 As shown in Fig. 2, many types of N-sulfonyloxaziridines have been used in synthesis. While they are structurally similar, they all have somewhat different reactivity and properties. For example, oxaziridine of types I and II, for steric reasons, are more reactive than the enantiomerically pure (camphorsulfonyl)oxaziridines III and V. Electron withdrawing groups on the oxaziridine 2- and 3-position can substantially increase their oxygen transfer abilities. The benzothiazine type oxaziridines IV, introduced by Du Bois, are similar to dioxiranes and capable of CH oxidations.4 Several of the oxaziridines listed in Fig. 2 are commercially available. The objective of this short review is to summarize the chemistry of N-sulfonyloxaziridines since the last, excellent review by Yoon and co-workers in 2014.2 Some overlap with earlier reviews is
Fig. 2. Types of N-sulfonyloxaziridines.
unavoidable and necessary for continuity. 2. Asymmetric synthesis of N-sulfonyloxaziridines Baeyer-Villiger type oxidation of N-sulfonylimines with buffered potassium peroxymonosulfate (Oxone) is the method of choice for the preparation of N-sulfonyloxaziridines (equation (1)).5 Yields are excellent and reactions can be carried out on multi- kilogram scale.
(1)
2.1. Asymmetric catalytic oxidation Recent efforts have been focused on the asymmetric synthesis of enantiomerically enriched N-sulfonyloxaziridines using chiral catalysts.2 These methods include the oxidation of N-sulfonylimines
Fig. 1. Oxygen transfer reactions of N-sulfonyloxaziridines.
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Scheme 1.
with m-chloroperbenzoic acid (m-CPBA) using cinchona alkaloid based organocatalysts and metal-catalyzed oxidations (equation (2)). Kinetic resolution of racemic N-sulfonyloxaziridines has also been described. Good yields and excellent levels of stereoinduction for the trans-2-arylsulfonyl-3-aryloxaziridines are realized.6
Scheme 2.
(2)
Earlier studies by Jin and co-workers explored the use of cinchona alkaloid sulfide 2 (R ¼ TBS) catalyst for oxaziridination of N-tosyl arylaldimines 1 with ee's up to 95% for meta- and para-aryl substitution (Scheme 1).7 In continuing, studies this group examined a series of cinchona alkaloid ester derivatives 2 for asymmetric oxaziridination of 1 with m-CPBA to give (R,R)-()-3.8 The highest ee's were found for the 4-nitrophenyl ester 2, whereas lower ee's were noted when R was acyl. For oxaziridine 3, best ee's >90% were generally found for meta aryl substitution and for Ph and 4-t-BuPh (Scheme 1). In an attempt to make (S,S)-3, the use of pseudoenantiomer of 2 resulted in racemic material. A series of thiourea organocatalysts were evaluated in the catalytic oxaziridination of N-tosyl arylaldimines 1 with m-CPBA at 40 C in toluene (Scheme 2).9 The highest ee's were observed for thiourea organocatalyst 4 affording the (S,S)-oxaziridines 5 in good yield and good to excellent ee's. Oxidation of the p-nitrophenylsulfonyl (Ns) and methanesulfonyl (Ms) benzaldimines resulted in much lower ee's for oxaziridines 6
Scheme 3.
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Scheme 4.
and 7 (Scheme 2). Using basic P-spiro chiral triaminoiminophosphorane organocatalyst 8 (Ar ¼ 3,5-Cl2C6H3-), Ooi and co-workers reported that the asymmetric oxidation of N-sulfonylimines 9 (R2 ¼ H) gave (R,R)oxaziridines 10 (R2 ¼ H, Scheme 3).10 The oxidizing reagent utilizes hydrogen peroxide and trichloroacetonitrile in a Payne-type oxidation. The enantioselectivity and yields are uniformly excellent for a wide range of substrates including aromatic, heteroaromatic, and aliphatic N-sulfonylimines 9 (R2 ¼ H). Importantly, epoxidation of the alkene in oxaziridine 11 was not observed under these conditions.10b N-Sulfonyl a-imino ester derived oxaziridines, 10 (R2 ¼ CO2Me), were prepared from N-sulfonylimino ester 9 (R1 ¼ 4-MePh, R2 ¼ CO2Me, Ar ¼ Ph).11 Interestingly higher ee's for 10 (R2 ¼ CO2Me) were noted in the absence of trichloroacetonitrile, 97 vs 69 %ee, respectively. Bicyclic N-sulfonylketimine 12 on oxidation with m-CPBA and C3-symmetric chiral tris(imidazoline) organocatalyst 13 afforded
the (R,R)-bicyclic oxaziridines 14 with ee's up to 87% (Scheme 4).12 Importantly, a single recrystallization of 14a and 14b from hexane/ CH2Cl2 gave enantiomerically pure materials. 2.2. Kinetic resolution of racemic N-sulfonyloxaziridines Making use of the fact that oxaziridines readily isomerize to amides in the presence of transition metal catalysts, Yoon and co-workers devised an iron (II)-catalyzed kinetic resolution of racemic N-sulfonyloxaziridines.13 With bis(oxazoline) ligand 16 and 5% FeCl2, oxaziridine (±)-15 gave N-sulfonyloxaziridines 17 in high ee and amide 18 (Scheme 5). The reaction is remarkably rapid and complete within a few minutes i.e. ()-19 and 20. This procedure works well for meta- and para-substitution, but lower ee's for C-alkyl substituents. Overall the reaction is scalable affording gram quantities of 17 (R ¼ Ar ¼ Ph). Mixing two equivalents of oxaziridine (±)-21 with one
Scheme 5.
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Scheme 6.
equivalent of indanone b-ketoester 22 in the presence of 5 mol% of chiral bifunctional guanidine catalyst 23 afforded (S,S)-()-N-sulfonyloxaziridines 24 and a-hydroxy b-ketoester (R)-()-25 (Scheme 6).14 The enantioselectivities of ()-24 and ()-25 are excellent, 92e99% ee, with good product yields. The kinetic resolution worked best with catalyst 23 and indanone 22 both having the bulky adamantyl (Ad) group. While racemic oxaziridines 21 having a variety of substitution patterns in the C-aryl work well, much poorer ee's were noted for C-naphthyl, 2-MePh and cyclohexyl groups. In a study aimed at developing catalysts for the asymmetric a-hydroxylation of b-ketoesters, motifs found in natural products and pharmaceutical lead structures, Waser and co-workers explored the use of various catalysts and oxidizing reagents.15 Of the oxidants explored (H2O2, m-CPBA, t-BuOOH), racemic N-sulfonyloxaziridine 26 gave highest enantioselectivities of (R,R)-28 when used with chiral bifunctional urea ammonium salt (S,S)-25 (Scheme 7). At 40 C with 2 equiv. of (±)-26 and 1 equiv. of 27 oxaziridine (R,R)-28 and a-hydroxy b-ketoester (R)-29 were obtained in 88 and 97% ee after 3 h. When 1.5 equiv. of (±)-26 was used the ee of N-sulfonyloxaziridine (R,R)-28 improved to 97%. A variety of enantiopure a-hydroxy b-ketoesters and N-sulfonyloxaziridines were prepared in this manner.
3. a-Hydroxylation of enolates The a-hydroxy carbonyl moiety is a common feature found in many biologically active materials such as pheromones, sugars, antibiotics, terpenes and alkaloids. Chiral a-hydroxy carbonyl compounds are also versatile synthons for the asymmetric construction of natural products and are useful stereodirecting groups for the synthesis of enantiomeric pure materials. For these reasons, the most widely used application of N-sulfonyloxaziridines is the ahydroxylation, of metal enolates because of their aprotic nature and the simplicity for installing this key functional group (eq (3)).1,16 A transition state controlled by steric factors is generally recognized as determining the stereochemistry.1c,f
Scheme 7.
(3)
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Scheme 8.
3.1. a-Hydroxylation of enolates in racemic syntheses Trichodermatide A is a member of a family of natural products isolated from marine-fungi having interesting bioactivities. In a racemic synthesis of trichodermatide A, the bis-lithium enolate of hexahydroxanthene-dione 30 was treated with (camphorsulfonyl) oxaziridine (þ)-31 to give the di-hydroxylated product 32 as the
major diastereoisomer (Scheme 8).17 It was noted that the bulky side chain controls the stereochemistry of the enolate hydroxylations, countering the reagent control of the enantiopure oxaziridine. This phenomenon is often observed in enolate hydroxylations. Steric considerations also play a role in the regioselectivity of enolate hydroxylations by N-sulfonyloxaziridines. Attempts to
Scheme 9.
Scheme 10.
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by treatment of b-ketoester 41 with NaHMDS followed by hydroxylation with 2-(4-toluenesulfonyl)-3-phenyloxaziridine (43) (Scheme 11).21 3.2. a-Hydroxylation in asymmetric syntheses
Scheme 11.
a-hydroxylate the C-1 position in ketone 33 with (camphorsulfonyl) oxaziridine (þ)-31 failed.18 The sterically less demanding 2phenylsulfonyl-3-phenyloxazidine (34) gave only low yields of 35 (Scheme 9). However with LiHMDS/HMPA, 33 gave the C-3 hydroxylation product 36 in good yield. The authors attribute the regioselectivity of 35 and 36 to thermodynamic versus kinetic enolate formation where shielding of the former enolate by the two bulky ortho groups inhibits C-1 hydroxylation. The dearomative ortho-hydroxylation of phenolic substrates is a useful method for the synthesis of highly functionalized polycyclic natural products.19 Grandclaudon and Toulle explored the phasetransfer (benzylcinchonidinium chloride) catalyzed oxaziridinemediated ortho-hyroxylation of substituted phenols and naphthols.20 In the dearomatization of phenol 37, they found that oxaziridines 38a and 38b gave the best yields of dimer 40, resulting in hydroxy cyclohexanone intermediate 39 (Scheme 10) (see Scheme 11). Rocaglamide is a member of a class of compounds that exhibits potent cytostatic and anti-inflammatory activity. C-3 Alcohol 42, an intermediate in the synthesis of racemic rocaglamide, was prepared
In this section, recent applications of N-sulfonyloxaziridines for the a-hydroxylation of chiral nonracemic enolates for the synthesis of natural products and other applications are summarized. Hong and co-workers described the a-hydroxylation of amides 44 derived from Oppolzer's (1S)-()-2,10-camphorsultam auxiliary using oxaziridine 34 (Scheme 12).22 The dr was >20:1, affording the a-hydroxy amide (S)-45 in good to excellent yields. The auxiliary was removed without epimerization with in situ generated MeOMgBr and the sultam recycled. Diastereoselective oxidation of the Evans oxazolidinone of 5-hexenoic acid (R)-46 with 34 gave a-hydroxy amide (R)-47.23 This amide was an intermediate in the synthesis of ()-promysalin, a natural product that exhibits antivirulence phenotypes against pathogenic bacteria (Scheme 12). The Baran group a-acetoxylated ketone 48 with oxaziridine 34 to give 49 as 2:1 mixture of diastereoisomers on a 30 g scale (Scheme 12).24 This ketone was a key intermediate in the total synthesis of the diterpene ()-maoecrystal V. Bicyclic lactenone 52 is a key intermediate in the synthesis of the alkaloid secu'amamine E.25 It was prepared by a-hydroxylation of enone 50 with N-sulfonyloxaziridine 34 followed by treatment with the Bestmann ylide 51 (Scheme 13).25 Many diastereoselective a-hydroxylations of enolates in the synthesis of natural products are performed with racemic 2-phenylsulfonyl-3-phenyloxaziridine (34) because it is more reactive than the enantiomerically pure camphor derived N-sulfonyloxaziridines. Recent examples are given in Table 1.
Scheme 12.
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Scheme 13.
In the eight step gram scale synthesis of ()-jiadifenolide (54), a neurotrophic trace metabolite, the lithium trianion of 53 was treated with CBr4 to give an intermediate a-bromolactone.33 The bromolactone (not shown) on reaction with NaHMDS and
oxaziridne 34 afforded ()-jiadifenolide (54) in 50% yield for the two steps (Scheme 14). The formal synthesis of azadirachtin, an insect antifeedant, involved reaction of 55 with TBSOTf and Et3N to give an intermediate silyl ketal acetal followed by a-hydroxylation
Table 1 Hydroxylation of metal enolates to a-hydroxy carbonyl compounds using racemic 2-phenylsulfonyl-3-phenyloxaziridine (34). Ketone/ester
Base
% Yield
(Ref.)
KHMDS
Product
20%
(26)a
NaHMDS
54%
(27)b
KHMDS
69%
(28)c
KHMDS
87%
(29)d
NaH
66%
(30)e
KHMDS
57%
(31)f
KHMDS
42-50%
(32)g
Synthesis of (a) Viridicatumtoxin B. (b) ()-Jiadifenin. (c) Rubriflordilactone B. (d) Anisodorin 5. (e) ()-Enterocin. (f) mimic of Pramanicin. (g) (þ)-Ryanodol.
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Scheme 14.
Scheme 15.
with N-sulfonyloxaziridine 34 to give 56.34 In the total synthesis of crotophorbolone, hydroxylation of the C-4 position of tetracyclic ketone 57 was challenging because of its steric congestion and its proximity to oxidizable olefins.35 Oxidation of the sodium enolate of 57 with (camphorsulfonyl)oxaziridine (þ)-31 resulted in formation of the undesired isomer 58 (Scheme 15). Hydroxylation of a less congested fragment, 59 gave the desired C-4 hydroxylation product 60 but was highly dependent on the oxaziridine structure as summarized in Table 2.35b
Enantiomeric (camphorsulfonyl)oxaziridine ent-()-31 gave no yield and 2-phenylsulfonyl-3-phenyloxaziridine (34) gave lower yields (Table 2). 3.3. Asymmetric a-hydroxylation of enolates using chiral catalysts and racemic N-sulfonyloxaziridines Using a Cu(II) complex of chiral phenanthroline ligand (S)-63, Naganawa and co-workers explored the asymmetric a-hydroxylation
Table 2
Oxaziridine
% Yield of 60
(þ)-31 ent-()-31 (±)-34
43 0 27
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Scheme 16.
of various oxindole derivatives 61 with racemic N-sulfonyloxaziridine 43 (Scheme 16).36,37 Yields and enantioselectivities of the (R)-3hydroxy-2-oxindole derivatives 62 were excellent. In a related study, this group studied the a-hydroxylation of various b-ketoesters with Cu(II) and ligand (S)-63. Best enantioselectivities for (S)-64 were noted using racemic bicyclic oxaziridine 65 (Scheme 16).38 Attempted kinetic resolution of the 43 with 2 equivalents of 61 gave racemic material.37 It was suggested that the stereoinduction of 62 results from the chiral environment of the prochiral Cu(II) enolate of 61. This result is in contrast to the kinetic resolution of (±)-43 in the
a-hydroxylation of indanone b-ketoesters with chiral bifunctional guanidine catalyst 23 where the ee's for the oxaziridines (S,S)-()-43 and (R)-()-25 were excellent (see Scheme 6).13 4. g-Hydroxylation of enolates and allylic anions 4.1. g-Hydroxylation of enolates On treatment with oxaziridine 34, the potassium dianion of (R)tetrahydroxanthone 66 resulted in a 1:1 mixture of cis/trans
Scheme 17.
Scheme 18.
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Scheme 19.
diacetates 67 and 68 resulting from g-hydroxylation of the extended enolate of 66 (Scheme 17).39 In a related study, g-hydroxylation of the lithium enolate of deoxyaureothin 69 with (þ)-(camphorsulfonyl)oxaziridine 31 afforded alcohol 70 in 35% ee and 76% yield (Scheme 17).40 Alcohol 70 was a precursor to the polyketide metabolite (þ)-aureothin, a potent antiproliferative agent.41 A g-hydroxylation strategy was used in the enantioselective synthesis of the ABC-tricyclic core of phomactin A by Lee and co-workers.42 Enone 71 on treatment with t-BuLi followed by oxidation with 2-phenylsulfonyl-3-phenyloxaziridine (34), gave the ABC-tricyclic furanochroman core 72 of phomactin A in 63% yield (Scheme 18). 5-Hydroxy-3-acyltetramic acid 74 is a structural unit found in natural products such as delaminomycin A. Tetramic acid 73 on treatment with excess LDA, followed by hydroxylation with
oxaziridines (þ)-31 or 34 gave 74 in 27e28% yield (Scheme 19).43 Oxaziridine 34 also gave product 75 in 15% yield. Addition of the enolate to the N-sulfonylimine by-product (PhSO2N¼CHPh) is avoided using bulky (camphorsulfonyl)oxaziridines such as (þ)-32.44 4.2. gHydroxylation of allylic anions A step in the first asymmetric synthesis of ()-marcfortine C, a complex indole alkaloid, was the hydroxylation of the allylic cyanide anion derived from cycloadduct ()-76. The allylic anion of ()-76 was prepared by reaction with LiOt-BuO/n-BuLi and quenched with oxaziridine 34 to give ()-77 in 64% yield (Scheme 20).45 Bromide 78 gave 79 in 29% yield on treatment with 4 equiv. of (camphorsulfonyl)oxaziridine (þ)-31 and 2 equiv. of indium at
Scheme 20.
Scheme 21.
F.A. Davis / Tetrahedron 74 (2018) 3198e3214
Scheme 22.
Scheme 23.
Scheme 24.
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Scheme 25.
Scheme 26.
70 C in a sealed tube for two days (Scheme 20).46
6. Oxyamination reactions of N-sulfonyloxaziridines
5. a-Hydroxylation of N-sulfinyl metallodienamines The first example of the a-hydroxylation of an N-sulfinyl metallodienamine was described by Chatare and Andrade in their total synthesis of the macrolactone antibiotic ()-albocycline.47 Reaction of 80 with LiHMDS, followed by regioselective hydroxylation with oxaziridine 34, gave N-sulfinyl imine 81 as an inseparable mixture of isomers (4:1 dr) in 73% yield, which was elaborated in four steps to ()-albocycline (Scheme 21).
The cycloaddition reactions of N-sulfonyloxaziridines with alkenes involving NO (oxyaminations) and CeO bond cleavages (1,3-dipolar cycloadditions) has been extensively explored and recently reviewed.2,48 An oxamination step was used to prepare a key intermediate in the total synthesis of ()-communesin F by the Movassaghi group.49 Indole ()-82 with stoichiometric copper (II) and 3,3dimethyl-2-(4-nitrophenylsulfonyl)-1,2-oxaziridine (83) afforded oxazoline ()-84 in 68% yield (dr 89:11) (Scheme 22). Formal [3þ2] cycloadditions involving NeO bond cleavage of N-
Scheme 27.
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3211
Scheme 28.
sulfonyloxaziridines have been used to prepare stereodefined oxazolidin-4-ones.2 Smith and co-workers reported that the reaction of a series of homoanhydrides 85 with racemic oxaziridine 43 and 10 mol% of Lewis-base (2R,3R)-HyperBMT 86 afforded mixtures of anti- and syn-oxazolidin-4-ones 87 in good yield (Scheme 23).50 The ee's of the individual oxazolidin-4-ones were excellent (78e99% ee). When (R,R)-43 (94% ee) was used anti-87 (R ¼ Ph) was obtained in 81% yield and >99% ee (Scheme 23). The mismatched pair (2R,3S)-HyperBMT-86 and (R,R)-43 gave syn-87 (80:20 d.r. syn/ anti) with 98% ee. The authors suggest that these results are more consistent with a-oxidation of an intermediate ammonium enolate
than formation of a transient epoxide by the oxaziridine. With (R,R)-43 (95% ee) and indole 88 adduct 89 was isolated in 76% yield with a dr of 1.2:1 (95%ee/94% ee).10a Much better control of the diastereo- and enantioselectivity in the formal [2þ3] cycloaddition of oxaziridines with a-aroyloxyaldehydes in the synthesis of oxazolindin-4-ones was found using a N-heterocyclic carbene (NHC) catalyst.51 With (R,R)-43 (98% ee) aaryloxyaldehydes 90 with 10 mol% of catalyst (þ)-91 gave antioxazolindin-4-ones 92 in excellent ee and diastereoselectivity (Scheme 24).
Scheme 29.
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Scheme 30.
Scheme 31.
7. Epoxidation of alkenes, silyl enol ethers, and enamines
7.1. Epoxidation of alkenes
On heating, N-sulfonyloxaziridines epoxidize alkenes in a synstereospecific manner similar to peracids.1,2,52 With electron rich silyl enol ethers and enamines, reaction rates are much faster and the elusive a-silyl and a-amino epoxides can often be isolated.53,54
Heating oxaziridine (þ)-92 (96% ee) at 60 C for 77 h resulted in intramolecular oxygen transfer to give imine epoxide 93 (Scheme 25).10a Lewis acid mediated reduction resulted in cyclization affording the pyrrolidine methanol derivative ()-94 in 74% yield for the two steps. The more reactive N-sulfonyl a-imino ester oxaziridines are capable of oxidizing non-activated alkenes with high ee's under milder conditions.11 For example stirring (þ)-95 (96% ee) with alkene 96 for 3e7 days at 0 C afforded the corresponding epoxides (þ)-97 in excellent ee and good yield (Scheme 26). 7.2. Epoxidation of silyl enol ethers The oxidation of silyl enol ethers, the Rubottom reaction, is a widely-used method for the synthesis of a-hydroxy carbonyl compounds (eq (4)).55 The intermediate siloxy epoxides are seldom isolated or even detected because trace acid results in facile rearrangement to the a-hydroxy carbonyl compounds. Silyloxy epoxides have been isolated using aprotic oxidizing reagents such as N-sulfonyloxaziridines.53 With chiral N-sulfonyloxaziridines, the ee's are poor (7e11% ee).
(4) Scheme 32.
F.A. Davis / Tetrahedron 74 (2018) 3198e3214
Silyl enol ether 98 was transformed into a-hydroxy carbonyl compound (þ)-99 on reaction with oxaziridine (þ)-95 (Scheme 27).11 Yields and ee's were excellent. A catalytic one-pot method employing the N-sulfonyl a-imino ester corresponding to (þ)-95, (4-MePhSO2N¼C(4-ClPh)COt2Bu) and chiral iminophosphorane organocatalyst 8 (see Scheme 3) was also use to prepare (þ)-99. 7.3. Epoxidation of enamines Disubstituted and trisubstituted enamines are oxidized by N-sulfonyloxaziridines to a-hydroxy and a-amino ketones, respectively.56 Oxidations of indoles by oxaziridines have found numerous applications in the synthesis of architecturally complex alkaloids.2 Recent examples include the asymmetric synthesis of ()-citrinadin A and the ()-trigonoliimines by Martin54 and Movassaghi,57 respectively. In the synthesis of ()-citrinadin A, indole 100 was first treated with pyridinium p-toluene sulfonate (PPTS) to prevent oxidation of the amino groups and followed by reaction with bicyclic oxaziridine 101 (Scheme 28).54 A semipinacol rearrangement gave spirooxindole 103 on treatment of the moderately stable epoxide 102 with AcOH. As part of a model study in the synthesis of the trigonoliimines, homodimeric 2,20 -bistryptamine 104 was oxidized with oxaziridine 101 to give hydroxyindolenine 105 in quantitative yield (Scheme 29).57 Bistryptamine 106 with (þ)-(8,8-dichlorocamphorsulfonyl)oxaziridine (107) afforded hydroxyindolenines (þ)-108 and (þ)-109 (108:109 ¼ 2.2:1) in 95% yield and 96% ee (Scheme 29). 8. C¡H oxidation Du Bois and co-workers, in a series of papers, demonstrated that N-sulfonyloxaziridine 111 derived from benzoxathiazine heterocycle 110 is capable of hydroxylating unactivated 3 CeH bonds similar to dioxiranes.2,58 In addition to stoichiometric oxidations, organocatalytic C-H hydroxylations using H2O2 to generate 111 were reported.59 However this system was limited to a few substrates and required long reaction times (>96 h). Recently this group reported that Oxone and an aqueous hexafluoroisopropanol (HFIP) gave much improved results for 3 CsH oxidation (Scheme 30).60 9. Amination Oxaziridines with large groups on the nitrogen atom, such as N-sulfonyloxaziridine, react almost exclusively at oxygen with nucleophiles.1,2 However, the Banerjee group recently reported that N-sulfonyloxaziridines react at nitrogen with donor acceptor cyclopropanes to give azetidine analogs in the presence of Lewis acids such as MgI2 (Scheme 31).61 Thus, treatment of oxaziridine 43 with cyclopropane 112 and 40 mol% MgI2 afforded azetidines 113 in good yield (Scheme 31). In the presence of the Lewis acid, the mechanism is envisioned as nucleophilic attack of the ring-opened cyclopropane at the N-atom of the MgI2-activated oxaziridine. 10. Reduction Racemic bicyclic oxaziridines 114a-b on hydrogenolysis (H2, 600 psi), in the presence of Pd(OCOCF3)2/(S,S0 ,R,R0 )-115 and ()-camphorsulfonic acid (L-CSA) provide novel sultams 116a-b with ee's up to 97% (Scheme 32).62 Mechanistic studies suggest a mechanism involving NeO bond cleavage and dehydration followed by asymmetric hydrogenation.
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11. Summary N-Sulfonyloxaziridines (commonly known as Davis oxaziridines or Davis reagents), are the most widely used oxaziridines in organic synthesis. Most applications of these stable, commercially available reagents involved the stereo- and regioselective oxidation of nucleophiles. They have found numerous uses in the synthesis of architecturally complex molecules. Most recent applications of these oxaziridines include cycloadditions (oxyamination), epoxidation of alkenes, silyl enol ethers, enolates, N-sulfinyl dienamines, enamines, CH oxidations, aminations and stereoselective reductions. It is anticipated that these applications, as well as new uses of these novel reagents, will find increasing utility in organic synthesis. Acknowledgements I am indebted to Dr. Rodrigo B. Andrade, Temple University, for stimulating conversations and for critiquing this manuscript. The comments and suggestions of the reviewers are greatly appreciated. References 1. (a) Davis FA, Sheppard AC. Tetrahedron. 1989;45:5703; (b) Andreae S, Schmitz E. Synthesis. 1991:327; (c) Davis FA, Chen B-C. Chem Rev. 1992;92:919; (d) Petrov VA, Resnati G. Chem Rev. 1996;96:1809; (e) Davis FA, Reddy RT. In: Padwa A, ed. Comprehensive Heterocyclic Chemistry II. vol. 1. Oxford: Permagon Press; 1996:365; (f) Davis FA. J Org Chem. 2006;71:8993; (g) Davis FA, Chen B-C, Zhou P. In: Katritzky AR, Ramsden CA, Scriven EFV, Taylor RJK, eds. Comprehensive Heterocyclic Chemistry III. vol. 1. Oxford: Elsevier; 2008:559; (h) Kumar KM. Synlett. 2012:2572. 2. Williamson KA, Michaels DJ, Yoon TP. Chem Rev. 2014;114:8016. 3. Davis FA, Nadir UK, Kluger EWJ. Chem Soc Chem Commun. 1977:25. 4. (a) Brodsky BH, Du Bois J. J Am Chem Soc. 2005;127:15391; (b) Litvinas ND, Brodsky BH, Du Bois J. Angew Chem Int Ed. 2009;48:4513. 5. Davis FA, Chattopadhyay S, Towson JC, Lal S, Reddy T. J Org Chem. 1988;53: 2087. 6. For a recent review on the asymmetric catalytic synthesis of N-sulfonyloxaziridines see Sala DG, Lattanzi A. ACS Catal. 2014;4:1234. 7. Zhang T, He W, Zhao X, Jin Y. Tetrahedron. 2013;69:7416. 8. Jin Y, Zhang T, Zhang W, Chang S, Feng B. Chirality. 2014;26:150. 9. Ji N, Yuan J, Xue S, Zhang J, Wei H. Tetrahedron. 2016;72:512. 10. (a) Uraguchi D, Tsutsumi R, Ooi T. Tetrahedron. 2014;70:1691; (b) Tsutsumi R, Kim S, Uraguchi D, Ooi T. Synthesis. 2014:871. 11. Tanaka N, Tsutsumi R, Uraguchi D, Ooi T. Chem Commun. 2017;55:6999. 12. Takizawa S, Kishi K, Abozeid MA, Murai K, Fujioka H, Sasai H. Org Biomol Chem. 2016;14:761. 13. Williamson KS, Swicki JW, Yoon TP. Chem Sci. 2014;5:3524. 14. Lin X, Ruan S, Yao Q, et al. Org Lett. 2016;18:3602. 15. Novacek J, Izzo JA, Vetticatt MJ, Waser M. Chem Eur J. 2016;22:17339. 16. Chen B-C, Zhou P, Davis FA, Ciganek E. a-hydroxylation of enolates and silyl enol ethers. Org React. 2003;62:1e356. 17. Myers E, Herrero-Gomez E, Albrecht I, et al. J Org Chem. 2014;79:9812. 18. Wilsdorf M, Lentz D, Reissig H-U. Eur J Org Chem. 2016;81:1555. 19. Roche SP, Porco Jr JA. Angew Chem Int Ed. 2011;50:4068. 20. Grandclaudon C, Toullec PY. Eur J Org Chem. 2016;81:260. 21. Zhou Z, Dixon DD, Jolit A, Titus MA. Chem Eur J. 2016;22:15929. 22. Zhang L, Zhu L, Yang J, Luo J, Hong R. J Org Chem. 2016;81:3890. 23. Steele AD, Knouse KW, Keohane CE, Wuest W. J Am Chem Soc. 2015;137:7314. 24. Cernijenko A, Risgaard R, Baran PS. J Am Chem Soc. 2016;138:9425. 25. Wehlauch R, Grendelmeier SM, Miyatake-Ondozabal H, Sandtory AH, Scherer M, Gademann K. Org Lett. 2017;19:548. 26. Nichoaou KC, Hale CRH, Nilewski C, Ioannidou HA, ElMarrouni A, Nilewski LG, Beabout K, Wang TT, Shamoo Y. J Am Chem Soc. 2014;136:12137. 27. Cheng X, Micalizio GC. J Am Chem Soc. 2016;138:1150. 28. Yang P, Yao M, Li J, Li Y, Li A. Angew Chem Int Ed. 2016;55:6964. 29. Ren J, Zhao P, Xiao X, Zeng B-B. Synthesis. 2016;48:4161. 30. Wegmann M, Bach T. Synthesis. 2017;49:209. 31. Tan SWB, Chai CLL, Moloney MG. Org Biomol Chem. 2017;15:1889. 32. Chuang KV, Xu C, Reisman SE. Science. 2016;353:912. 33. Lu H-H, Martinez MD, Shenvi RA. Nat Chem. 2015;7:604.
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Franklin A. Davis was born in Des Moines, Iowa on April 1, 1939 and grew up in Hastings-on-Hudson, NY. He received his B.S. degree in chemistry from the University of Wisconsin and his Ph.D. from Syracuse University in 1966. After a Welch Postdoctoral Fellowship at the University of Texas he joined the faculty at Drexel University in 1968 where he was the George S. Sasin Professor of Chemistry. In 1995 he moved across town to Temple University where he is the Laura H. Carnell Professor of Chemistry. Davis' research interests are focused on the invention of new reagents and methodologies for the synthesis of biologically important molecules. He originated and pioneered the chemistry of N-sulfonyloxaziridines for oxidations and asymmetric oxidations and N-sulfinyl imines for chiral nonracemic amine synthesis. Davis' honors include a National Science Foundation Extension for Special Creativity in 1991, a fellowship from the Japanese Society for the Promotion of Science in 1992. He is an inaugural fellow of the American Chemical Society and a Fellow of the Royal Society. In 2006 he received the American Chemical Society Cope Scholar Award and the John Scott Medal.