Enantioselective epoxidation of tertiary allylic alcohols by chiral dihydroperoxides

Tetrahedron 69 (2013) 2446e2450

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Enantioselective epoxidation of tertiary allylic alcohols by chiral dihydroperoxides € rgen Hamann a, Dennis Dietz a, Ju € rgen Liebscher a, b, * Alexander Bunge a, Hans-Ju a b

€t zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany € r Chemie, Humboldt-Universita Institut fu National Institute for Research and Development of Isotopic and Molecular Technologies, 65-103 Donath, 400293 Cluj-Napoca 5, Romania

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 November 2012 Received in revised form 3 January 2013 Accepted 9 January 2013 Available online 18 January 2013

gem-Dihydroperoxides were successfully used for the enantioselective epoxidation of tertiary and primary allylic alcohols. Epoxides derived from tertiary alcohols were obtained in yields up to 71% with ee’s up to 52%. Ó 2013 Elsevier Ltd. All rights reserved.

Dedicated to Erhard Matthes on the occasion of his 90th birthday

Keywords: gem-Dihydroperoxides Enantioselective epoxidation Allylic alcohols Thujone Glycidols

1. Introduction It is a general understanding that asymmetric epoxidation of allylic alcohols is a solved problem thanks to the famous asymmetric KatsukieSharpless epoxidation.1,2 This synthesis works with tertbutylhydroperoxide as oxygen source and diethyl tartrate and titanium alkoxides as catalysts in the presence of molecular sieves.3 It provides excellent enantioselectivities and can also be performed at large scale in industries.4 However, there are still limitations due to structural restrictions in the allylic alcohols. While primary allylic alcohols are superb substrates and kinetic resolution of racemic secondary allylic alcohols can also be achieved by the Sharpless epoxidation protocol, tertiary allylic alcohols do not react under these conditions.5 There are few reports of asymmetric epoxidations of tertiary allylic alcohols. Wang et al. observed enantioselectivities of 60e87%, but substrates were limited to 1,1-disubstituted alkenes.6 Takano et al. saw good enantioselectivities in tertiary allylic alcohols that were part of 2-alkene-1,4-diol frameworks.7 Spivey et al. achieved the desymmetrization of tertiary bisallylic pentadienols using a Zr(IV)-based catalyst.8 However, none of the previous approaches addressed the 1,2-disubstituted alkene core, that is, the most common substrate framework for asymmetric epoxidation. On the other

* Corresponding author. Tel.: þ40 264 584037; fax: þ40 264 420042; e-mail address: [email protected] (J. Liebscher). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.01.032

hand there is a continuous interest to find oxygen-transferring reagents, which are more environmentally friendly, such as hydrogen peroxide.9 Since gem-dihydroperoxides (DHP) 1 can be synthesized from hydrogen peroxide and a carbonyl compound 2, gem-DHPs have been in the focus of several recent research activities concerning oxygen transfer (Scheme 1). After transferring oxygen from gemDHPs 1 to a substrate the parent carbonyl compound 2 formed can be recycled and reused. The interest in gem-dihydroperoxides 1 has led to a number of methods for their synthesis, usually starting from the respective carbonyl compound 2, a corresponding ketal or an enol ether. Some of these syntheses have been reviewed by Iskra et al.,10 others were published recently by Terent’ev,11 Das,12,13 Dussault,14 Azarifar,15,16 Wu,17 and Itoh.18 Primary aliphatic gem-DHPs were not obtained by these methods, but were reported very recently by us.19 With the synthesis protocol described there it was also possible to obtain a number of gem-dihydroperoxides from ketones, which were impossible to get by other methods previously described.20 A different approach to gem-DHPs 1 uses ozonolysis of enol ethers or alkenes 3 in the presence of ethereal hydrogen peroxide solution (Scheme 1). This method has, however, been described only sporadically in the literature.21e23 As far as the application of gem-DHPs as oxygen transferring reagents is concerned the cyclohexane-1,1-dihydroperoxide-mediated WeitzeScheffer-epoxidation of chalcones has to be

A. Bunge et al. / Tetrahedron 69 (2013) 2446e2450

R2 R1

R2

H2O2(70%), CSA R1

O 1a-i

2

R2

O3, H2O2 (eth.) 1

R

OOH OOH

1j-n

3

1 OOH

OOH HOO

OOH OOH

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2.2. Epoxidation These hydroperoxides 1 were screened in the epoxidation of 2methyl-4-phenylbut-3-en-2-ol 4d26 as an example of a tertiary allylic alcohol (Scheme 2). Reaction conditions for asymmetric epoxidation were at first chosen similar to those of the Sharpless epoxidation. The results obtained are shown in Table 1.

OOH

O O

OOH OOH 1d 41%

1´c 19%

1b 57% OH OOH

Ph

1f 60%

5d

Scheme 2. Asymmetric epoxidation of tertiary allylic alcohol 4d.

OOH

1e 35%

1g 55% OH

Table 1 Epoxidation of tertiary allylic alcohol 4d with various gem-dihydroperoxides 1a Entry

DHP 1

Conversion of 4d (%)

ee of 5d (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1a 1b 10 c 1d 1e 1f 1g 1h 1i 1j 1k 1l 1m 10 n

42 23 6 5 22 40 4 64 60 28 7 71 6 24

0 31 13 7 35 0 15 8 7 7 2 18 28 3

H H H

HOO HOO

H 1h 82%

H

O

H H

H 1i 31%

OH OH OH

OOH OOH 1j 62%

OOH OOH 1k 68%

OOH OOH 1l 89%

OH OOH OOH 1m 39%

OH

DCM, -20 °C, 3 d

4d

OOH

OOH

Ph

OH

OOH

OOH

HOO HOO

O

DHP 1, Ti(OiPr)4,

1a 58%

O O OOH 1´n 21%

Scheme 1. gem-Dihydroperoxides 1 (yields in parentheses) synthesized from carbonyl compounds 2 or by ozonolysis of alkenes 3.

mentioned.24 Furthermore, this gem-DHP was used by Venkateswarlu et al. in selective oxidation of sulfides to sulfoxides.25 In the course of our research we found gem-DHPs able to enantioselectively epoxidize 3-substituted 1,4-naphthoquinones,20 which often gave only low enantioselectivities in previously reported epoxidations. With this background we wanted to check the hitherto unknown suitability of gem-dihydroperoxides 1 for the epoxidation of allylic alcohols. We surmised that optically active dihydroperoxides, which proved to be successful in the enantioselective epoxidation of naphthoquinones of the vitamin K-type20 could also be used in the enantioselective transformation of allylic alcohols into corresponding epoxides, in particular tertiary members. 2. Results and discussion 2.1. Synthesis of gem-dihydroperoxides Several gem-dihydroperoxides 1aen were synthesized as epoxidizing reagents. Hydroperoxides 1aei were obtained according to previous literature from the corresponding ketones 2.20 For the synthesis of hydroperoxides 1jen we found that ozonolysis of alkenes 3jen in the presence of hydrogen peroxide (Scheme 1) can be a powerful tool. In particular, terpene-based DHPs 1 cannot be generated by alternative methods or request multiple steps starting from ketones 2. Interestingly, ozonolysis of isopulegone 3n did not give the dihydroperoxide, but the hemiacetal structure 10 n instead. 1d, 1g, 1jem, and 10 n have not yet been reported in the literature.

a b

Sign of optical rotationb þ þ þ þ    þ þ þ þ 

Reaction conditions: 20  C, 3 d, DCM, DHP:Ti(OiPr)4:4d¼1:1:1. In chloroform.

It turned out that all gem-dihydroperoxides 1 were able to epoxidize the allylic alcohol 4d. However, the yields were often unsatisfactory. Significant enantioselectivity could be observed in some cases (see entries 2, 5, and 13). As in our previous investigations in the epoxidation of naphthoquinones20 the geminal dihydroperoxide 1e performed best with respect to enantioselectivity (entry 5). It appeared that a hydroxy-group properly situated in the gemDHP 1 can have a significant positive effect on enantioselectivity, most likely due to increased rigidity of an assumed complex transition state, as can be seen by comparing substrates 1e and 1m (entry 5 and 13, favorable position) with 1j (entry 10, lack of OH group), 1k, 1l (entries 11, 12, unfavorable position). The position of a methyl group next to the gem-dihydroperoxide moiety can also strongly affect enantioselectivity (compare 1b, entry 2 with 1d, entry 4). As a result of the screening shown in Table 1 we decided to use compound 1e in further optimizations. Since the addition of molecular sieves, which is important in the Sharpless epoxidation did not result in a significant effect on the outcome of the reaction in our cases the experiments were performed without it. Conversion of the substrate 4d was quite low (Table 1, entry 5) with DHP 1e, therefore a primary allylic alcohol (4b) was used for optimization studies (Scheme 3). As can be seen in Table 2, solvents can play an important role in conversion and ee. Too nonpolar (hexane) or too polar (acetonitrile) solvents decreased both conversion and ee, while medium polar solvents gave at least comparable conversions. Although N,Ndimethylformamide (DMF) provided the highest ee (58%),

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A. Bunge et al. / Tetrahedron 69 (2013) 2446e2450

These conditions were now applied in the epoxidation of various primary and tertiary allylic alcohols (Scheme 4).

OH OOH OOH

OH

1e

solvent

Cl

O

, catalyst,

OH Cl

4b

5b

Scheme 3. Variation of reaction conditions in the asymmetric epoxidation of 4b with gem-dihydroperoxide 1e.

Scheme 4. Asymmetric epoxidation of allylic alcohols 4 using DHP 1e.

Table 2 Dependence of conversion and ee of epoxidation of 4b on solventsa Solvent

Conversion of 4b (%)

ee (5b) (%)

Hexane Toluene Diethyl ether THF Ethyl acetate DCM Acetonitrile DMF

21 54 61 14 50 68 28 28

14 26 31 20 20 35 27 58

a

Reaction conditions: 20  C, 3 d, 1e: Ti(OiPr)4:4b¼1:1:1.

dichloromethane (DCM) turned out to be an optimum regarding speed of reaction, conversion, and ee. Apparently complexation of DMF with the titanium catalyst gives rise to a particularly favorable transition state. To our knowledge nothing similar has been observed for the Sharpless epoxidation or similar reaction types. Variation of reaction time in DCM resulted in a slight decrease in enantioselectivities (from 39% after 1 h to 35% after 72 h), while the conversion grew to 82% at 24 h (37% ee) with partial decomposition occurring at this time. Choice of reaction temperature is crucial, because conducting the reaction at 0  C yielded only racemic product with notably increased decomposition, whereas at room temperature no epoxide 5b was detected anymore. Performing the reaction at 78  C gave only a minor increase of ee to 39% but drastically reduced conversions. Without the application of Ti(OiPr)4 product 5 was not observed. Change of the amount of Ti(OiPr)4 from 1 equiv to 2 or 0.5 equiv yielded worse results in both conversion and ee. Decrease of the ratio of oxidant to 50% was unfavorable while doubling of the amount of oxidant afforded a significant increase of conversion (93%), whereas enantioselectivity remained almost unchanged (36%). Testing of some metal catalysts different from Ti(OiPr)4 did not result in an improvement of the epoxidation of 4b. Ti(OiBu)4 gave worse results, while others (VO(acac)2, MoO2(acac)2) blocked the reaction or gave racemates (H4WO5). We assume that Ti(OiPr)4 plays a similar rule like in the Sharpless epoxidation, namely it complexes the allylic alcohol and the gem-dihydroperoxide in a rigid transition state. Since gem-dihydroperoxides have two hydroperoxy moieties capable of complexation with titanium it is essential that one of the both is more exposed in such a way that it can easily bind to titanium. This would also explain to some extent the effect of the configuration of the a-methyl group in gem-DHPs 1b and 1d. The additional hydroxyl group found in the gem-DHP 1e is likely also to be complexed to the titanium thus covering the reaction site more selectively from one direction. To resume, best reaction conditions for primary allylic alcohol 4b were found to be DMF as solvent, 2 equiv of the oxidant 1e, 1 equiv of Ti(OiPr)4, 20  C, and 24 h reaction time. Unfortunately, DMF gave no conversion of tertiary allylic alcohols, so that DCM was used as solvent in those cases.

Highest enantioselectivities were observed with the primary cinnamyl alcohols 4a, b and c (Table 3, entries 1, 2, 3). However, these substrates were epoxidized in much higher enantioselectivities before under Sharpless conditions.29,31,32 In contrast, none of the tertiary allylic alcohols 4 (R1¼Me) has yet been described to be epoxidized enantioselectively, neither by Sharpless nor by any other epoxidation method. Allylic alcohols 4 and their corresponding epoxides 5 were sometimes difficult to separate by column chromatography. Thus, in case of 5e, the yield could only be determined from the mixture of 4e and 5e by 1H NMR measurements. It turned out that no side reactions occurred in the case of tertiary allylic alcohols, while some decomposition took place with primary allylic alcohols explaining the low yields, as also described for Sharpless epoxidations.33 Table 3 Enantioselective epoxidation of various allylic alcohols 4a with DHP 1eb Entry

Epoxide 5c

R

R1

Yield (%)

ee (%)

1 2 3 4 5 6 7 8

5a 5b 5c 5d 5e 5f 5g 5h

Ph 4-CleC6H4 1-C10H7 Ph 4-CleC6H4 1-C10H7 Phe(CH2)2e H

H H H Me Me Me Me Me

43 35 43 64 21g 50 22 71h

60d,e 57d,e 68d,f 46e 32 40f 50e 52

a b c d e f g h

Substrates 4 were obtained according or analogous to known syntheses.26e28 Reaction conditions: 20  C, 1 d, 1e: Ti(OiPr)4:4¼1:1:1. Solvent for 4aec: DMF, for 4deh: DCM. Configuration of major enantiomer (2R, 3R).29,30 Sign of optical rotation in CHCl3 þ. Sign of optical rotation in CHCl3 . Only isolated as mixture. Product highly volatile, not isolated, yield determined by GC.

To simplify the reaction procedure a combination of DHP preparation and epoxidation was attempted circumventing any purification of the DHP 1e by column chromatography (Scheme 5).

OH O

OH O

+

1. H2O2, H

O OH

+

2. 4d, Ti(OiPr)4, DCM, molecular sieves 2e

5d

2e

Scheme 5. Simplified epoxidation of 4d.

Since this reaction mixture still contained hydrogen peroxide, molecular sieves were added to remove it.20 It turned out that 5d was obtained in 55% yield with only a slightly lower enantioselectivity of

A. Bunge et al. / Tetrahedron 69 (2013) 2446e2450

44% as compared with the application of separately prepared 1e. It was possible to re-obtain ketone 2e in 70% yield, proving the possibility of recycling and reusing the ketone. 3. Conclusion In summary, we have demonstrated that gem-dihydroperoxides 1 can be successfully used in enantioselective epoxidation of allylic alcohols. This methodology represents the best way to epoxidize tertiary allylic alcohols affording ee’s of up to 52%. For primary allylic alcohols it cannot compete with the superb Sharpless epoxidation. Combination of the synthesis of the gem-dihydroperoxides 1 from corresponding ketones 2 with subsequent application in epoxidation of allylic alcohols is possible with recycling of the starting ketone. 4. Experimental 4.1. General TLC analysis was performed on Merck silica gel 60 F254 plates and visualized with UV illumination or stained with phosphomolybdic acid in EtOH (5%, v/v) or N,N0 -dimethyl-4-phenylendiamoniumdichloride-solution in MeOH/H2O/HOAc (peroxides). Column chromatography was conducted with Merck silica gel 60 (400e639 mesh). 1H NMR and 13C NMR spectra were recorded at 300 (500) and 75 (125) MHz, respectively, on a Bruker AMX 300, DPX 300 or Avance III 500. Chemicals used were purchased from Aldrich, Acros or Merck and were used without further purification. Hydrogen peroxide was donated by Solvay Interox GmbH. Essential oil of Thuja occidentalis was purchased from Baccararose, Sonsbeck (Germany). (þ)-b-Thujone 2d34 (from hydroxymethylthujone20), hydroperoxides 1aei,20 ()-pinocamphone,26,35 ()-isopulegone 3n,26 (þ)-neoisopulegol 3m,36 the allyl alcohols 4b,c27 (via DIBAL reduction of the unsaturated ethyl esters), and 4deg27,28 (via MeMgIaddition to the unsaturated ethyl esters) as well as the racemic epoxides 5beg26 were synthesized according or analogous to known procedures. Asymmetric epoxidations were carried out under an argon atmosphere. NMR-spectra of the hydroperoxides should be measured as fast as possible after dissolving the samples in the deuterated solvent, for we found them to at least partially decompose quite often. In some cases, benzene-d6 or CD3CN as a solvent helped to prevent the decomposition. CDCl3 should be taken from a relatively fresh bottle, as the decomposition products of the solvent seem to promote decomposition of the samples as well. HPLC: HPLC was done on a system consisting of high pressure gradient system 322 (Kontron), UV-detector DAD K-2800 (Knauer), chiral detector IBZ Messtechnik, injection ventile 7125 (10 mL, Rheodyne). HRMS spectra were measured either on an ESI-MS-device LTQFT-ICR-MS (Thermo Finnigan (HU-Berlin)) or on a Waters Acquity UPLC/MS (LCT Premier XE Mass spectrometer). Optical rotation was measured on a PerkineElmer 241 Polarimeter, at 589 nm wavelength. The determination of absolute configuration was taken from the literature by comparing optical rotation. Caution: 70% hydrogen peroxide and peroxidic compounds are potentially explosive and should be handled with precautions (shields, fume hoods, avoidance of transition metal salts). 4.2. General procedure for preparation of gem-DHPs 1d,g The corresponding ketone 2 (10 mmol) was dissolved in diethyl ether (2 mL) and cooled in an ice bath. After addition of 70%

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hydrogen peroxide (2 mL) and camphorsulfonic acid monohydrate (25 mg, 0.1 mmol) the biphasic system was left to stir over night (16 h). The mixture was diluted with water (30 mL), extracted with diethyl ether (320 mL), and the combined organic layers washed with saturated NaHCO3-solution (20 mL), dried (Na2SO4), and the solvent evaporated under reduced pressure. After purification by column chromatography (silica gel, DCM/MeOH, 99:1 or CyH/ EtOAc, 7:3) the desired DHP 1 was obtained. 4.3. General procedure for preparation of gem-dihydroperoxides 1jen by ozonolysis The corresponding alkene 3 (3 mmol) was dissolved in 20 mL of an ethereal solution of hydrogen peroxide (ca. 3 M, 60 mmol) and a small quantity of Sudan red was added. At 78  C an ozone stream was blown through the solution until the color disappeared. The reaction mixture was washed with brine (ice cold, 20 mL), dried (Na2SO4), and the solvent carefully evaporated under vacuum. Purification was done by column chromatography (CyH/EtOAc or DCM/MeOH). 4.4. General procedure for epoxidation (analytical scale) Dihydroperoxide 1 (0.02 mmol) was dissolved in dry solvent (DCM, 5 mL) under argon. After cooling to 40 to 50  C a 0.2 M solution of (Ti(OiPr)4) (100 mL, 0.02 mmol) in DCM was added. After stirring at this temperature for 30 min a 0.2 M DCM solution of the allylic alcohol 4 (100 mL, 0.02 mmol) was added. The solution was stirred for additional 60 min at this temperature and afterward stored in a freezer at 30  C. After 72 h, the reaction mixture was poured onto water (20 mL) and extracted with DCM (220 mL); the combined organic phases were dried (Na2SO4) and the solvent was evaporated. The reaction mixture was analyzed by HPLC on a chiral column (See Supplementary data). 4.5. General procedure for epoxidation (preparative scale) 3,3-Dihydroperoxy-4-(hydroxymethyl)-b-thujane 1e (93 mg, 0.4 mmol) in dry DCM or DMF (20 mL) were combined with Ti(OiPr)4 (56.8 mg, 0.2 mmol) and the allylic alcohol 4 (0.2 mmol) in dry DCM (1 mL) under the same conditions as in the analytical scale (vs). The solution was stirred for additional 60 min and then stirring continued in a cryostat (20  C). After keeping the reaction mixture at 20  C for 24 h it was diluted with DCM (40 mL), washed with a saturated NaHCO3-solution (60 mL), and the aqueous phase reextracted with DCM (40 mL). The combined organic layers were washed again with a saturated solution of NaHCO3 (40 mL), dried (Na2SO4), and the solvent was evaporated. The residue was purified by column chromatography (15 g silica gel, CyH/ethyl acetate, 9:1 or DCM/ethyl acetate, 99.5:0.5), in some cases several times. Acknowledgements We wish to thank Dipl.-Ing. Angela Thiesies for NMR measurements. We further thank Solvay Interox GmbH for donation of hydrogen peroxide. Supplementary data Characterization of hitherto unknown gem-DHP 1, preparation and characterization of allylic alcohols 2 and their epoxides 5, combined procedure for synthesis of 5d. Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.tet.2013.01.032. These data include MOL files and InChiKeys of the most important compounds described in this article.

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