Chapter 4: Four-Membered Ring Systems

74

Chapter 4 Four-Membered Ring Systems Benito Alcaide Departamento de Química Orgánica I. Facultad de Química. Universidad Complutense de Madrid, 28040-Madrid. Spain [email protected] Pedro Almendros Instituto de Química Orgánica General, CSIC, Juan de la Cierva 3, 28006-Madrid, Spain [email protected] ______________________________________________________________ 4.1

INTRODUCTION

Chemical research on four-membered heterocycles, where a non-carbon atom is part of the ring, has become a highly dynamic area of international priority and importance in many fields of Science, including Organic Chemistry, Inorganic Chemistry, Medicinal Chemistry, and Material Science. Condensing the vast amount of published material during the year 2007 to less than 20 pages is an extremely demanding task. This Chapter will focus on oxygen- and nitrogen-containing heterocycles, which dominate the field in terms of the number of publications, and will aim to highlight the current state of the art in the vast area of four-membered heterocyclic chemistry. 4.2

AZETIDINES, 3-AZETIDINONES, AND DIAZETINES

The structure-activity relationships of azetidine-based dipeptidyl peptidase IV inhibitors have been discussed in detail in a review <07MI597>. Calyciphylline C 1 and calydaphninone 2, two novel Daphniphyllum alkaloids with an unprecedented fusedhexacyclic ring system, have been isolated from the leaves of Daphniphyllum calycinum (Daphniphyllaceae), and the structures and relative stereochemistries were elucidated <07TL1587; 07OL1355>. Oxazolidinone analogs bearing substituted azetidines with a methyl group at the 3-position of the four-membered ring exhibited reduced mitochondrial protein synthesis inhibition while retaining good antibacterial potency <07BMCL5036>. Spiro[azetidine-2,2'-adamantane] and spiro[azetidine-3,2'-adamantane] have been synthesized and tested for their anti-influenza A virus properties and for trypanocidal activity <07BMCL4358>. A series of stereoisomers for the azetidine ring of penaresidin B has been synthesized and their cytotoxic and antimicrobial activities have been evaluated, showing relatively potent cytotoxic activity against A549 (lung) and HT29 (colon) tumor cells as well as antibacterial activity <07BMC4910>. It has been found that the introduction of 3substituted azetidinyl substituents onto the 4,6-diaminopyrimidine scaffold allowed improvement of phosphodiesterase inhibiting activities <07BMCL3077>. The synthesis and antimicrobial screening of 4-(3-chloro-2-oxo-4-substituted phenylazetidine-1-yl)benzoic acids have been published <07MI3607>. It has been reported that in addition to heat shock (HS), azetidine-2-carboxylic acid 3, also induces heat shock protein (HSP) synthesis

75

Four-Membered Ring Systems

<07MI1124>. A conformational study on azetidine-2-carboxylic acid 3 has been carried out using ab initio HF and density functional methods <07JPC(B)3496>. 1H NMR and FTIR studies showed the high tendency of 2-alkyl-2-carboxy-azetidines 4 to adopt -turn conformations, indicating that these azetidine-containing amino acids could serve as general -turn promoters <07TL3689>. A practical synthesis of 2'-deoxymugineic acid 5, together with a quantitative estimation of the potency of 5 in assisting specific iron(III) transport through the transporter HvYS1 have been reported <07AG(E)7060>. MeOOC

H

O

R1

CO2H Z

N

N

O

N

H N

CO2H

O

H N

N

OH

O

H 1 (Z = H) 2 (Z = OH)

N H

R2

3

CO2H

CO2H

5

4

A short and facile approach to biologically interesting N-protected alkyl 3aminoazetidine-2-carboxylic esters 6, a new class of conformationally restricted -amino esters, has been developed by reduction of anti-,-aziridino-
-(Ndiphenylmethylidene)amino esters and subsequent regioselective intramolecular ring opening of the ,-aziridine ring via nucleophilic attack of the
-amino function <07OL4399>. An optimized oxidation reaction of -(phenylseleno)alkyl arylsulfonamides, followed by the intramolecular substitution of the resulting phenylselenonyl group by the nitrogen atom have been used for the stereocontrolled synthesis of N-arenesulfonyl azetidines 7 <07OBC3510>. It has been shown that a wide range of N-arylazetidines are accessible by palladium-catalyzed cross-coupling reactions of aryl or hetaryl bromides with the parent azetidine <07S243>. A general synthetic route to various chiral 2-substituted- and 2,4-disubstituted-N-tosylazetidines (ee >99%) has been described, in good overall yields starting from chiral
-amino acids <07TL2471>. An expedient route to N-sulfonyl azetidines has been developed through the regioselective addition of 1,3-dicarbonyl dianions to N-sulfonyl aldimines <07T4779>. Ts N R1 R1

i CO2R2 N

Ph Ph

TsHN N R1 R1

CO2R2 Ph Ph

6 (33–57%)

R2 R1

SePh NHSO2Ar

ii

R2

R1 N

SO2Ar

7 (50–61%)

Key: i) (a) NaCNBH3, AcOH, RT; (b) Et3N, MeCN, 70 oC. ii) (a) m-CPBA, K2HPO4, THF, RT, then KOH, RT; (b) TsOH·H2O, MeOH, RT. The synthesis of unnatural amino acids, including azetidine 8, from serine derivatives by -fragmentation of primary alkoxyl radicals has been accomplished <07JOC7260>. It has been reported that aza-Michael adducts of chalcones and diethyl N-arylphosphoramidates undergo reductive cyclization with sodium borohydride followed by reaction with sodium hydride to afford 1,2,4-trisubstituted azetidines 9 diastereoselectively in a one-pot procedure and excellent yields <07TL8037>. -Amino alcohols with an (E)-vinylsilane moiety have been cyclized in the presence of N-bromosuccinimide to afford diastereomerically pure azetidines, and the reactivity of these azetidines towards fluorinated reagents has been studied <07EJO4517>. Enantiopure 1-phenylethylazetidine-2-carboxylates and 2-acylazetidines have been prepared and reacted with chloroformates to yield -chloro--amino butyric acid esters and ketones <07TL269>. Lewis acid mediated SN2-type nucleophilic ring opening followed

76

B. Alcaide and P. Almendros

by [4+2] cycloaddition of N-tosylazetidines with aldehydes and ketones has been reported as a strategy for the synthesis of chiral 1,3-oxazinanes and 1,3-amino alcohols <07TL4373>. A strategy for Cu(OTf)2-mediated ring-opening of 2-aryl-N-tosylazetidines in polar and coordinating solvents followed by an unprecedented rearrangement to substituted achiral and chiral (E)-allylamines (ee >99%) has been achieved <07OL5441>. An efficient synthesis of amino ethers and amino thioethers were achieved via Lewis acid promoted ring cleavage of N-tosylazetidines with alcohols or thiols <07TL5375>. O

Bz

H N

O

i

OMe

OMe

N

AcO

Ar2

O

O

3 Ar2 + (EtO)2PNHAr

Ar1 Bz

ii Ar1

OH

N

Ar3

9 (78–85%)

8 (52%)

Key: i) DIB, I2, h, CH2Cl2; then 0 oC, BF3·OEt2, prop-1-en-2-yl acetate. ii) (a) NaH, C6H6, 60 oC; (b) NaBH4, t-BuOH, 60 oC; (c) NaH, t-BuOH, 60 oC. Enantiomerically pure azetidinium trifluoromethanesulfonates have been chemo- and regioselectively opened by various hydride reagents <07TL1027>. The nucleophilic opening of functionalized azetidinium ions by C-nucleophiles has been examined <07TL9180>. It has been reported that the azetidinium ylide derived from 10 effects facile epoxidation of various carbonyl compounds furnishing tri or tetrasubstituted epoxides that were unattainable via classical ammonium ylide chemistry <07CC2500>. Besides, azetidinium ylides related to 10 have shown a remarkable ability to perform the cyclopropanation of Michael acceptors <07JOC1058>. A route to 2,6-diazaspiro[3.3]heptanes 11 has been described by way of reductive amination of a readily available monocyclic aldehyde with primary amines <07SL2584>. The X-ray analysis of a zwitterionic structure containing an azetidine ring has been performed <07EJO2055>. A reactive fused azetidinium intermediate has been detected on studying the reactivity of adducts from the photocycloaddition of 2-pyridones <07S2351>. Enantiomerically pure 1-azabicyclo[3.2.0]heptane derivatives, such as 12, have been synthesized in a straightforward manner from readily available chiral sources, namely, enantiomerically pure epoxides and L-proline <07EJO5734>. The synthesis and structureactivity relationship studies of 3,6-diazabicyclo[3.2.0]heptanes 13 as novel
-4--2 nicotinic acetylcholine receptor selective agonists have been reported <07JMC5493>. CN

+

N Me + 1 R – Bn TfO 10

Bn

Me

O

i R2

N

N

O

Bn NC 71–97%

PhO

R2 R1

N 11

Ar

R2 N CO2Me 12

HN

R1

N N 13

o

Key: i) LiHMDS, THF, –78 C. Azeto[2,1-b]quinazolines 14 have been formed from bis(ketenimines) by intramolecular ketenimine-imine [2 + 2] like-cycloaddition <07JOC5863>. Functionalized 2(sulfonylimino)-4-(alkylimino)azetidine derivatives 15 were prepared in good yields via a copper-catalyzed multicomponent reaction of readily available terminal alkynes, sulfonyl azides, and carbodiimides without the assistance of a base <07OL1585>. The synthesis and thermal decomposition of stereochemically defined 1,2-diazetine N,N'-dioxides 16 have been carried out <07JOC1412; 07OL3005>. Observable azacyclobutenone ylide 17 with antiaromatic character has been generated in solution by laser flash photolysis of 2diazoacetylpyridine <07JA6210>. 3-Oxoazetidine derivatives 18 have been obtained in low yield by the reaction of dioxopiperazines with O-nucleophiles <07EJO548>. A study on

77

Four-Membered Ring Systems

polymalonamides synthesis by fast addition reaction of aliphatic diamines and azetidine-2,4dione has been carried out <07MI3591>. A 1,3-diazetidine-2-thione has been proposed as a key intermediate in the transposition reaction of 2-iminothiazoles with isothiocyanates <07TL3091>. Ph

14

Ph

–O

O–

R1N

N 15

R1

O

N N

R2

R2

O

CO2R NH

R1

R1 16

CO2R

O N –

+

N R3

Ph

NSO2R3

R2

+

N

R2

4.3

Ph

+

R1

17

NH 18

 -LACTAMS) MONOCYCLIC 2-AZETIDINONES (

A review on the use of -lactams as versatile building blocks for the synthesis of non-lactam products has appeared <07CR4437>. Recent progress on the stereoselective synthesis of acyclic quaternary -amino acids including the use of -lactams as building blocks has been reviewed <07TA569>. The synthesis of -lactams using the Kinugasa reaction has been reviewed <07SL2321>. An overview on recent developments in stereoselective syntheses using N-acylimines including -lactams has been published <07SL159>. A review on the natural occurrence, syntheses, and applications of cyclopropylgroup-containing
-amino acids including -lactams containing 2,3-methanoamino acid fragments has appeared <07CRV4493>. The catalytic, asymmetric alkylation of imines including -lactam formation has been reviewed <07T9581>. An overview on metal-assisted multicomponent reactions involving carbon monoxide, towards heterocycle synthesis including -lactams, has been published <07AG(E)3615>. The enantioselective insertion of metal carbenes into N–H bonds as a potentially versatile route to chiral amine derivatives including -lactams has been reviewed <07AG(E)9148>. A review on the domino approach to heterocycles including -lactams has appeared <07T5341>. The synthesis and biological activity of several azetidinone derivatives as potential human leucocyte elastase inhibitors have been reported <07MI484>. The organocatalytic diastereoselective Henry reaction of enantiopure 4-oxoazetidine-2-carbaldehydes as well as the organocatalytic direct aldol and nitroaldol reactions between azetidine-2,3-diones and ketones or nitromethane have been achieved <07ARK285; 07T3102>. Enantiopure 2-azetidinone-tethered furans 19 can be prepared by Pd(II)-catalyzed dimerization reaction of terminal
-allenones <07EJO2844>. Nonthermal microwave effects in Staudinger reactions have been investigated using diastereoselectivity as a probe <07T9387>. Base-induced cyclization of 1aminodioxolanones afforded the corresponding chiral tetra-substituted 3-hydroxy--lactams <07TL5081>. The reaction of 4-acetoxy-2-azetidinones with organoindium reagents generated in situ from indium and 1,4-dibromo-2-butyne in the presence of LiCl in DMF produced 2-azetidinones 20 selectively which contain a 1,3-butadien-2-yl group at the 4position in good yields <07CEJ8877>. The chemoselective and stereoselective synthesis of gem-difluoro--lactams 21 has been investigated from ethyl bromodifluoroacetate and imines during a Reformatsky reaction <07T12352>. The Vilsmeier reagent is an efficient acid activator for the synthesis of -lactams <07TL8713>. The Lewis acid-mediated reaction of substituted cyclopropanone acetals with alkyl azides has been found to generate N-substituted 2-azetidinones, arising from azide addition to the carbonyl group followed by ring expansion <07JOC9439>. The chemoenzymatic preparation of fluorine-substituted -lactam

78

B. Alcaide and P. Almendros

enantiomers exploiting Burkholderia cepacia lipase has been achieved <07TA2468>. Enantioselective acylation of alcohols with fluorinated -phenyl--lactams in the presence of Burkholderia cepacia lipase has been accomplished <07TA1567>. The intramolecular carbenoid C–H insertion of 2-diazoacetoacetamides leading to -lactams has been catalyzed effectively by dinuclear Ru complexes of the type [Ru2(μ-L1)2(CO)4L22] <07T12174>. The electrocyclization of 4-substituted 3-silyloxy-2-azadienes to -lactams has been studied at the MP2/6-31G* level of theory and the effect of the substituents on the reactivity of the azadiene and on the stereochemistry of the cyclic products has been evaluated <07EJO3533>. R2

O R1

O

N

R1 N

R3

R3

O

R H H OAc

R2

O

RH H

N

H

O

19

N Br

O

Br

F

F

In, LiCl, DMF, 90 oC

R2

N H

O

20 (76–77%)

R1

21

Versatile and efficient solid-supported synthesis of C3-anchored monocyclic -lactam derivatives has been reported <07JCO189>. The 2-oxoazetidinylacetate sodium salt, which has been synthesized as a model of a minimal -lactam drug, and the monobactam aztreonam have been assayed as metallo--lactamase substrates <07BMCL5171>. It has been reported that second-order rate constants for the alkaline hydrolysis and elastase inactivation by Ncarbamoyl monobactams are independent of the pKa of the leaving group at C4 <07OBC2617>. Enantiomerically pure ethylene glycol derived -lactams have been synthesized from Ley’s BDA-protected L-glyceraldehyde imines via Staudinger reaction followed by deprotection <07SL3180>. Hexamethyldisilazane sodium salt has been found to be a highly active Lewis base catalyst for the Staudinger reaction with disubstituted ketenes and imines <07SL3032>. An efficient synthesis of azetidine-2,3-diones from L-(+)-diethyl tartrate has been described <07SL2242>. Synthesis of (–)-tabtoxinine--lactam 22 and its (3R)-isomer, the cause of tobacco wildfire disease, has been achieved from L-serine using a zinc-mediated coupling reaction, Sharpless asymmetric dihydroxylation and lactamization of -mesyloxy benzylhydroxamate amide as the key steps <07S2471>. 2-Azetidinone has been converted steroselectively into enamides 23 or 24 through olefination or hydroamination reactions <07OL1631; 07OL5609>. An efficient and general ionic liquid supported synthesis of a -lactam library via multistep reactions has been successfully carried out in a single ionic liquid batch <07TL5143>. 4-Cyano- or 4-carbamoyl--lactams have been isolated from the reaction of -lactam carbenes with alkyl isonitriles <07T9359>. The X-ray analysis of 4-(4nitrophenyl)-3-phenoxyazetidin-2-one has been reported <07AXE3729>.
-Silylmethylene-lactams have been synthesized through a silylcarbocyclisation process by the reaction of tosyl amides with a hydrosilane in the presence of catalytic amounts of 1,8diazabicyclo[5.4.0]undec-7-ene <07T6843>. NH2

OH

HO O

N O 22

i N H

O 23 (43%)

CO2Et

ii NH O

N O

n-C5H11

24 (68%)

Key: i) (a) acetic formic anhydride, 60 oC; (b) ethyl triphenylphosphonoacetate, benzene, 80 o C. ii) 1-heptyne, 5 mol% Re2(CO)10, toluene, reflux.

79

Four-Membered Ring Systems

Approaches to the synthesis of enantiopure
-hydroxy--lactams with functionalized side-chains have been described <07TA2021>. Functionalized gem-difluoro -lactams 25 have been synthesized through a novel intramolecular hydroamination reaction of difluoropropargyl amides via a Baldwin disfavored 4-exo-digonal cyclization using palladium acetate as the catalyst <07OL4251>. The deprotonation of a suitable bromoamide by electrogenerated cyanomethyl anion has formed -lactams 26 predominantly as the cisisomer in good yields by a 4-exo-tet cyclization process <07ASC2177>. The native strain of Arthrobacter sp. (MTCC 5125) bearing a lipase has been found to be the most effective in the kinetic resolution of racemic arylazetidinones 27 <07TA1059>. The stereoselective synthesis of trans-disubstituted--lactams from N-phenylsulfenylimines has been carried out <07TL4301>. A highly regio- and stereoselective transformation of non-conventional lactam-containing epoxides into the corresponding cyclic 1,3-dioxolanes and oxazolines has been reported, using microwave irradiation as an efficient source of energy, in the presence of stoichiometric or catalytic amounts of Lewis acids, without a solvent <07ASC1256>. Various monocyclic -lactams, both cis and trans, have been successfully prepared via Kinugasa reaction mimicking click chemistry conditions <07SL1585>. The asymmetric intermolecular Kinugasa reaction using HETPHOX ligands, the first class of P,N ligands to be employed in this copper-catalysed transformation, has been studied <07TA199>. The synthesis of a new family of halogenated -lactams by oxidative substitution of vinylic hydrogen in conjugated double bonds of 4-alkylidenazetidinones has been reported <07EJO2526>.
-Haloenol acetates have been proved to be versatile reactants for oxetan-2-one, azetidin-2-one and isoxazolidin-5-one synthesis <07EJO101>. The solvent-free, one-pot synthesis of -lactams by Sc(OTf)3-catalyzed reaction of silyl ketene thiocetals with imines has been documented <07EJO2865>. O

F F

N H

R2 i

F N O

R1

R2

R1 R2

25 (33–63%)

R3

R1

N

ii

R1

Br O

AcO

O

O

F

R3 N

O

R2

26 (69–79%)

Ar N

O OMe 27

Key: i) 10 mol% Pd(OAc)2, THF, Et3N, RT. ii) Pt, MeCN–Et4NPF6, 0 oC. A study on the stereodivergent behavior observed in the Staudinger reaction between methoxyketene and (E)-N-benzylidenearyl amines has been documented <07AG(E)3028>. A notable and obvious ketene substituent-dependent effect of temperature on the stereoselectivity in the Staudinger reaction has been observed <07JOC990>. A theoretical investigation of the Co(CO)4–-catalyzed carbonylative ring expansion of N-benzoyl-2methylaziridine to -lactams has been performed <07JOC3259>. The reaction of phenoxyacetyl chloride with aryl imines in the presence of catalytic quantities of a silyl cinchona alkaloid and an achiral Lewis acid has afforded
-phenoxy--aryl--lactams <07TL1657>. An efficient synthesis of multi-functionalized lactams, including -lactams through intramolecular cyanoamidation of unsaturated cyanoformamides catalyzed by palladium has been described <07T2978>. The synthesis of chiral 2,2,3-3-amino-2hydroxyalkanoates and 3-alkyl-3-hydroxy--lactams by double asymmetric induction has been published <07T7949>. The asymmetric synthesis of -lactams by [2+2] cycloaddition using 1,4:3,6-dianhydro-D-glucitol (isosorbide) derived chiral pools has been reported <07T3380>. The stereoselective synthesis of 3-alkylidene/alkylazetidin-2-ones from azetidin2,3-dione has been achieved <07T2524>. Selected N-sulfenylimines have acted as good nucleophilic partners in the Staudinger reaction with methoxy- and benzyloxy-ketenes

80

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<07T3205>. The base-promoted cyclization of optically pure N-(p-methoxybenzyl)-N-(2chloro)propionyl amino acid derivatives has resulted in a diastereo- and enantioselective approach to valuable 1,3,4,4-tetrasubstituted -lactams <07OL1593>. Ring opening of
substituted
-methoxycarbonyl-N-nosylaziridines provides a practical access to enantiopure
,
'-disubstituted -lactam scaffolds, novel types of ditopic reverse turn surrogates <07OL101>. The synthesis and cellular profiling of diverse organosilicon small molecules, including -lactams, has been reported <07JA1020>. The reaction of -lactam carbenes with aryl isonitriles proceeds in a novel [2+2] fashion to give high yields of 2azetidinonylidene indoles, which undergo an unprecedented rearrangement to furnish carboline-2,4-diones 28 in good yields <07JOC2625>. Conformationally restricted, orthogonally protected 2,4-diaminocarboxylates with a cyclopentane skeleton have been efficiently synthesized from a bicyclic -lactam <07JOC8786>. The regio- and stereoselective synthesis of the enantiomers of monoterpene-based -amino acid derivatives has been accomplished from cis--pinene-fused -lactams <07TA2442>. Candida antarctica lipase B has catalyzed the ring opening of 4-arylalkyl-substituted -lactams <07TA2841>. It has been reported that the kinetic resolution of racemic cis-4-phenyl- and cis-4-tert-butyl-3hydroxy--lactam derivatives with 7-O-triethylsilylbaccatin III yielded paclitaxel and butitaxel analogues with high diastereoselectivity <07JOC756>. A single-step catalytic ring expansion approach from 4-oxoazetidine-2-carbaldehydes to enantiopure succinimides 29 has been achieved by the use of a base (DBU) and a thiazolium salt precatalyst <07CC4788; 07OL3519>. 4-Oxoazetidine-2-carbaldehydes have proved to be useful building blocks for the formal synthesis of xylo-(2S,3R,4R)-phytosphingosine, threo-(2S,3S)-sphingosine, and D(+)-biotin <07S2631; 07SL1159>. Stereocontrolled access to orthogonally protected anti,anti-4-aminopiperidine-3,5-diols, including fully orthogonally protected sialidase inhibitors, through chemoselective reduction of enantiopure -lactam cyanohydrins has been described <07JOC7980>. The asymmetric total synthesis of (–)-cribrostatin 4 as well as the construction of the pentacyclic frameworks of the ecteinascidin–saframycin class of tetrahydroisoquinoline antitumor antibiotics have been achieved using as key step the reductive opening/elimination of the -lactam ring <07AG(E)1517; 07TL3719>. A new method for the synthesis of cobactin analogs involving single-step coupling reactions between an amine of a cyclic (L)-lysine-derived hydroxamic acid with cyanide-activated lactams has been presented <07TL5103>. 2-Oxazolidinones, including the cytokine modulator (–)-cytoxazone, have been prepared in stereomerically pure form from 3-hydroxy -lactams by a ring-opening-cyclization isomerization process <07OL575>. Substituted amino esters have been prepared by -lactam nucleus isomerization followed by N1–C2 bond cleavage and methanol Michael addition <07EJO356>. Y Bn

Me N R

R O

N N

N O

Ar

O

i

Y

N

X

R

CHO N

R O

N H

28 (41–77%)

O

Ar

X = O, N

Me

Cl S (5 mol%)

DBU (10 mol%), MeCN, reflux

X

O

N Ar

O

29 (62–88%)

Key: i) (a) p-YC6H4NC, dioxane, reflux; (b) HCl (aq.), dioxane, reflux; (c) TsOH, ClCH2CHCl2, reflux.

81

Four-Membered Ring Systems

4.4

FUSED AND SPIROCYCLIC  -LACTAMS

The use of chemo-differentiating multicomponent reactions in diversity-oriented molecular construction, including spiro -lactams, has been reviewed <07CSR484>. The synthesis of oxetane spiro--lactams 30 by epoxide ring-opening has been accomplished <07EJO3199>. Some new mono- and bis-spiro--lactams of benzylisatin have been prepared by Staudinger’s ketene–imine [2+2] cycloaddition reaction <07TL7140>. A rigid bicyclic ketene has been used to generate highly constrained polycyclic spiro--lactams through the Staudinger reaction <07EJO4594>. A concise synthesis of 2,6-diazaspiro[3.3]heptan-1-ones 31 and their subsequent conversion into 2,6-diazaspiro[3.3]heptanes has been reported <07SL2587>. The synthesis of spirocyclic selenoazetidin-2-ones has been described <07T3195>. The stereoselective synthesis of spiro--lactams using D-(+)-glucose has been achieved <07T9179>. A diversity oriented preparation of enantiopure spirocyclic 2azetidinones such as 32 and 33 from
-oxo--lactams through Barbier-type reactions followed by metal-promoted cyclizations has been reported <07ASC749>. R2 O

R2 N

Ph

O H

O

R2 OH

O

O

i

Ph

N O

N

R1

O

30

R1

N

R2 O

31

OH

O H

O

OH

ii N

R1

O

N

R2 O

R1

32 (35–41%)

R1

33 (quantitative)




o

Key: i) 5 mol % Pd(PPh3)4, Ag2CO3 (2 equiv), toluene, sealed tube, 180 C. ii) AgNO3 (1 equiv), acetone–H2O (1:1), reflux. A study of some third-generation cephalosporins, their synthetic intermediates and reaction byproducts has been carried out by 13C NMR spectroscopy <07MRC236>. A simple and specific assay for real-time colorimetric visualization of -lactamase activity by using gold nanoparticles has been developed <07AG(E)8799>. The design and evaluation of the new bioluminogenic cephalosporin substrate 34 for imaging the -lactamase activity in vivo have been described <07AG(E)7031>. The apo crystal structure of CTX-M-9 -lactamase has been determined to 0.88 Å at pH 8.8 <07JA5378>. The role of zinc content on the catalytic efficiency of B1 metallo -lactamases has been studied <07JA2808>. The rational design and synthesis of a -lactam conjugate with a cephalosporin core, which transforms into a hydrogelator by catalysis of a -lactamase and yields a supramolecular hydrogel has been reported <07JA266>. A novel approach to racemic and enantiopure non-conventional fused bi- and tricyclic -lactams 35 and 36 has been developed by using regio- and stereocontrolled intramolecular free radical reactions in monocyclic 2-azetidinone-tethered allenynes and haloallenes <07JOC1604>. The reductive radical cyclisation of - and epoxynitrile-2-azetidinones to afford bi- and tricyclic -lactams containing an aryl group fused to a seven-membered ring has been achieved using titanocene monochloride <07T3017>. O S

H N

Ph O

N

O

O CO2H 34

HO R1

R2 CH3

S N

N

CO2H

R1

N

H H N

O

OH

R2 CH3

O

S

Ph3Sn 35

36




82

B. Alcaide and P. Almendros

The penam nucleus has been modified to behave as a -lactamase-dependent prodrug by incorporation of a vinyl ester side-chain at the 6-position <07OBC160>. It has been observed that the aminolysis of N-aroyl -lactams occurs by a concerted mechanism <07OBC485>. The synthesis of novel selenium-containing -lactam 37, including selenapenams, selenacephems and selenazepines, has been described using a 2(trimethylsilyl)ethyl protection approach <07OL4455>. Diamino-substituted bisketenes with both free and tethered amino substituents have been generated by using laser flash photolysis for ring opening of the corresponding cyclobutenediones and have been trapped to generate -lactams <07JOC1951>. A route to two epimeric 2-piperidinylglycine derivatives, as precursors of carbocyclic -lactam derivatives, has been developed <07TL6113>. An efficient metal-controlled regiodivergent preparation of 2-azetidinone-fused tetrahydrofurans 38 and tetrahydrooxepines 39 starting from enantiopure -allenols has been developed from the observation that a (methoxymethyl)oxy protecting group not only masks an hydroxyl functionality, but also exerts directing effects as a controlling unit in a regioselectivity reversal <07AG(E)6684>. The synthesis of 4-allenyl- and 4-propargyl-2-azetidinones via Znmediated Barbier-type reactions and Pt-catalyzed intramolecular amidation to carbapenem skeletons has been achieved <07TL7942>. The stereoselective dimerization of 3arylisoxazoles to cage-shaped bis--lactams syn-2,6-diaryl-3,7-diazatricyclo[4.2.0.02,5]octan4,8-diones induced by hindered lithium amides has been reported <07T12388>. The photochemical behaviour of some 5-alkylidene-2,5-dihydroisoxazoles to yield cis-4,5dihydrofuroazetidinones has been investigated <07T1584>. An approach to substituted methylcarbapenems and benzocarbacephems by radical cyclization using titanocene monochloride has been reported <07SL1243>. Structure-based design, synthesis, and biological activity of 4-substituted trinems as broad spectrum -lactamase inhibitors has been published <07JMC4113>. R2 HO

H H

Se

H N

N O

O 37

OP1 H

O

R1

38 (57–58%)

i P2 = H

P2O

H H

i N

O

OP1

R1

P2 = MOM

OP1

O H

H N

O

R1

39 (45–58%)




Key: i) 5 mol% AuCl3, CH2Cl2, RT. 4.5 OXETANES, DIOXETANES, OXETANEDIONES AND 2-OXETANONES ( LACTONES) A review on the synthetic uses of ynolates including -lactones preparation has appeared <07T10>. The structures of oxygen heterocyclic sugar amino acids, including 4membered rings, has been reviewed <07TA2001>. An overview on the carbonylation of heterocycles by homogeneous catalysts, included the preparation of -lactones from epoxides <07CC657>. A review on applications of total synthesis toward the discovery of clinically useful anticancer agents, including taxol has been published <07CSR1207>. The preparation of paclitaxel from primary taxanes using organozirconium chemistry has been reviewed <07JOC3981>. Examples showing the versatility of lipases for organic chemists, including the industrial biocatalysis process of taxol 2-vinyladipate, have been discussed in a review <07T1721>.The synthetic strategies of lactacystin and salinosporamide A have been reviewed <07CAJ20>. The chemiluminescent caspase-3 substrate 40 bearing a 1,2-dioxetane

83

Four-Membered Ring Systems

subunit has been prepared <07OL4853>. Splitting of chemiluminescence efficiency for charge-transfer-induced decomposition of optically active bicyclic dioxetanes bearing a 2hydroxy-1,1-binaphth-4-yl moiety under chiral recognition has been reported <07TL491>. Rotational-echo double-resonance NMR distance measurements for the tubulin-bound Taxol (paclitaxel, 41) conformation have been carried out <07JA361>. An oxetane-pregnane derivative has been identified <07TL4987>. The acid-catalyzed rearrangement of fused alkylideneoxetanols 42 has afforded aza-fused tricyclic lactones <07OL4681>. A bisfunctionalized (2-aminoethyl)glycine-derivative modified with a flavin and an oxetane moiety has been synthesized <07EJO681>. MeO

O O AcO

O

OH O H OAc PhOCO

O OH

N H

R1

N O

31

30

OH R2

O

O

Ph

O Peptide sequence

NH

Ph

O

O OH

32

Double [2+2] cycloaddition by irradiation to 1:2 mixed crystals between polymethylenedioxy-di-2-pyrones and benzophenone gives bis-oxetanes 43 <07TL12367>. Mechanistic studies on the -selective glycosidation of trimethylene oxide using mannosyl iodides have been described <07JOC4663>.
-Mesyloxy oxetanes undergo stereospecific elimination to 2-alkylidene oxetanes upon treatment with potassium t-butoxide <07TL8354>. The treatment of norbornenoxetanes with LDA or RLi reagents results in an unusual ringexpansion reaction leading to the corresponding 2,3-dihydro-4-trifluoromethyl-5-X-furans (X = H, F, Cl, n-C4H9) <07S1349>. The uncommon 4-exo ring closure which occurred in the copper-catalyzed intramolecular O-vinylation of -bromohomoallylic alcohols has been demostrated to be an efficient process leading to the convenient synthesis of 2methyleneoxetanes 44 <07JA8092>. It has been reported that under the conditions of flash vacuum thermolysis, loss of acetone from the keto-form of a pyrrolidinylidene derivative of Meldrum's acid affords the isolable oxetanedione 45 via an activation barrier of 37 kcal mol–1 <07OBC1438>. Ph Ph H

O Me

O

O

( )n

H

O

Ph O Ph

O

Br

O O Me

R1

R R2

O

R3

OH

R2

i 3

R1

43

O

44 (46–98%)

O

N Me

O 45

Key: i) CuI, NaOt-Bu, 1,10-phenanthroline, MeCN, reflux. A new stereocontrolled synthetic route to the potent proteasome inhibitor omuralide 46 has been developed from methyl pyroglutamate <07JOC10108>. The development of a new convertible isocyanide, indole-isocyanide, for ready access to pyroglutamic acids has culminated in the formal total synthesis of omuralide featuring a stereocontrolled Ugi reaction <07OL3631>. A novel enantioselective total synthesis of 20S proteasome inhibitor salinosporamide A 47 included, as a key feature, an intramolecular aldol cyclization <07OL2289>. A concise total synthesis of (±)-salinosporamide A, (±)-cinnabaramide A, and derivatives via a bis-cyclization process has been documented <07OL2143>. The

84

B. Alcaide and P. Almendros

stereoselective formal synthesis of the proteasome inhibitor salinosporamide A has been reported <07TL381>. Feeding experiments with stable isotopes established that salinosporamide A and B are biosynthesized in the marine bacterium Salinispora tropica from three biosynthetic building blocks, namely, acetate, -hydroxy-2'-cyclohexenylalanine, and either butyrate or a tetrose-derived chlorinated molecule <07OL845>. The stereoselective enzymatic reduction of keto-salinosporamide to (
)-salinosporamide A has been carried out <07TL2537>. The stereogenic evolution of clasto-lactacystin -lactone 48 from L-serine has been studied <07EJO37>. The first synthesis of the pyrrolidinone core of the polyene lactone antibiotic KSM-2690 B has been described, using an ammonia-free Birch reductive aldol reaction utilizing acetaldehyde as one of the key steps <07OL421>. The enantioselective synthesis of belactosin C 49 and derivatives has been accomplished in a concise manner employing the tandem Mukaiyama aldol-lactonizaton process <07OL1537>. A concise stereoselective total synthesis of belactosin C and synthetic variants has been achieved using an (S)-proline catalyzed crossed-aldol reaction as the key step <07TL1707>. O

O Cl

NH Me

O NH

O

NH

H

Me OH

O O 46

OH

O O 47

OH

O O 48

O O

N H

H N HO2C

NH2 O

49

The direct synthesis of chlorinated -lactones 50 from unsaturated acids with sodium hypochlorite and Lewis acids has been described <07TL1749>. A Lewis acid-Lewis base bifunctional catalyst from a new mixed ligand has been used for the enantioselective synthesis of -lactones <07OL567>. The practical enantioselective synthesis of -lactones catalyzed by aluminum bis-sulfonamide complexes has been achieved <07ASC1647>. The highly reactive indanedioneketene, resulting from the thermal decomposition of phenyliodonium ylide of 2-hydroxy-1,4-naphthoquinone, in the absence of nucleophiles dimerizes to the corresponding tetraoxo spiro oxetanone 51 in quantitative yield <07JOC502>. Fused tricyclic chlorinated lactones have been proposed as intermediates for the benzannulation reaction of phenyl trichloroacetates <07AG(E)1869>. The enantioselective synthesis of
,
-disubstituted cyclopentenes, involving a fused bicyclic lactone intermediate, by an intramolecular aldol reaction of achiral tricarbonyl compounds catalyzed by chiral N-heterocyclic carbenes has been reported <07JA10098>. Experimental and computational evidence for
-lactone intermediates in the addition of aqueous bromine to disodium 2,3-dimethylmaleate or -fumarate has been documented <07OBC4001>. The lactone nucleus has been used as a synthon for the preparation of various compounds. Indeed, the development of a diastereoselective, three-step strategy for the construction of substituted tetrahydrofurans from alkenyl aldehydes based on the tandem Mukaiyama aldol-lactonization process and Mead reductive cyclization of keto -lactones has been reported <07JOC9053>. Azide ring opening, reduction, and ester hydrolysis of disubstituted -lactones, formed by cinchona alkaloid-catalyzed reaction of ethyl glyoxylate with substituted ketenes, allows access to chiral -alkyl aspartates 52 <07JOC8091>. An efficient synthesis of optically pure
-alkyl--azido- and
-alkyl--aminoalanines via ring opening of 3-amino-3-alkyl-2oxetanones has been achieved <07TL6794>. A two-step strategy for conversion of -lactones into -lactones and 3(2H)-furanones has been developed involving initial acyl C–O cleavage leading to -hydroxy-
-diazo--ketoesters and -ketophosphonates <07JOC8939>. The insecticidal tetrahydroisocoumarin (3R,4S,4aR)-4,8-dihydroxy-3-methyl-3,4,4a,5-tetrahydro1H-2-benzopyran-1-one, (
)-mellein, (+)-ramulosin, and related natural products have been

85

Four-Membered Ring Systems

synthesized in optically active forms using one-pot esterification–Michael addition–aldol reaction of a -hydroxy-
,-unsaturated aldehyde and 4-methyleneoxetan-2-one as a key step <07T1074; 07T1281>. One-pot synthesis of functionalized furamides involved a threecomponent reaction between an amine, 4-methyleneoxetan-2-one and dibenzoylacetylene in the presence of triphenylphosphine <07T8083>. Spirocyclic imidazolidines derived from Nheterocyclic carbenes are initiators for the zwitterionic ring-expansion polymerization of lactones to generate cyclic polyesters of well-defined molecular weight <07JA8415>. A study has explained the surprising failure of the ring-opening polymerization for the reactive butyrolactone <07JOC9656>. Novel structural motifs consisting of chiral thiazolines which have been evaluated for chiral recognition and antitumor activity, have been prepared using -lactone ring opening with thiobenzoic acid <07CEJ3026>. Highly diastereoselective Cu(I)mediated, bicyclic -lactone ring cleavage reactions with either alkyl or aryl cuprates proceeded with inversion of stereochemistry to give optically active trans-substituted cyclopentanes and cyclohexanes, including a Merck new drug intermediate for an anti-HIV CCR5 antagonist <07OL2111>.

R

i

R

OO

Cl

O OH

R

O

O

O O

O

CO2Et O

O

NH2 ii

R

O

50 (up to 93%)

CO2H

EtO2C

52 (61–74%) dr> 9:1

51

Key: i) NaOCl, CeCl3·7H2O, CH2Cl2, RT. ii) (a) NaN3, DMSO; (b) Pd/C, H2, MeOH. 4.6

THIETANES,  -SULTAMS, AND RELATED SYSTEMS

The use of Lawesson’s reagent in organic synthesis has been reviewed <07CRV5210>. A review on the synthesis and reactions of 2-alkylidene thietanes has appeared <07S2755>. Substituted 2-thiobenzamidomethylindole derivatives have been prepared by the reaction of 2-aminomethylindole with substituted benzoyl chlorides, followed by sulfurization using Lawesson's reagent <07T4983>. Thietane nucleosides such as 53 have been synthesized by the glycosidation of glycosyl fluoride with a nucleobase <07T11622>. Sulfurization of a -dithiolactone followed by reaction with ethylenebis(triphenylphosphine)platinum or tetrakis(triphenylphosphine)palladium afforded the corresponding thietane dithiolato-metal complexes <07T11748>. A thietanium salt intermediate has been proposed for the synthesis of solvatofluorescent push-pull thiophenes <07EJO5404>. Deprotonation of a triphenylisothiazolium perchlorate led to the corresponding 2-imino-2H-thiete 54 via rearrangement, with an energy barrier of about 1 kcal mol-1 of an isothiazole carbene intermediate <07AG(E)6922>. OH O

OH

N S

HN O 53

Key: i) t-BuOK, THF, RT.

Ph

Ph S N Ph

Ph H

ClO4

i Ph

S N Ph

Ph

N Ph S

Ph 54 (91%)

86

B. Alcaide and P. Almendros

The formation of thioketene S,S-dioxides through [3,3] sigmatropic rearrangement of alkynyl propargyl sulfones has been confirmed by trapping experiments using cyclohexene or allyltrimethylsilane affording the corresponding [2+2] cycloadducts 55 in good yields <07TL1915>. Diallenic
-disulfones have been found to rearrange and cyclize to a novel bicyclic multifunctional product containing a four-membered sulfone ring <07TL6713>. The synthesis of four-membered thiolato sulfinato-platinum and palladium complexes have been reported <07T5161>. -Sultams (1,2-thiazetidine 1,1-dioxides) 56, sulfonyl analogues of lactams, have been prepared enantio- and diastereoselectively by quinine-catalyzed [2+2] cycloaddition reactions <07OL2007>. Reactivity and selectivity in the inhibition of elastase by 3-oxo--sultams 57, which are both -sultams and -lactams, and in their hydrolysis have been described <07OBC3993>. The formation of a four-membered ring azanickelacyclecontaining sulfone has been reported <07AG(E)4930>. Sulfenes, the highly reactive sulfonyl equivalents of ketenes, have been used for the first time as substrates in asymmetric catalysis for the enantio- and diastereoselective preparation of -sultones 58 <07AG(E)2685>. Me3Si

O S O

O

R

R2

O S N Ts O

O S N 1 R O

56

57

55

R2

R2

R1

EWG

O Me

O O i S Cl + H CCl3

Me

CCl3

O S O O 58 (up to 87%)

Key: i) 9 mol% dihydroquinine-2,5-diphenyl-4,6-pyrimidinediyl diether, 18 mol% Bi(OTf)3, CH2Cl2, –15 oC. 4.7

SILICON AND PHOSPHORUS HETEROCYCLES. MISCELLANEOUS

An overview on stable silyl, germyl, and stannyl cations, radicals, and anions, including four-membered heterocycles has appeared <07ACR410>. The chemistry of pnictogen(III)–nitrogen ring systems, including four-membered ring has been reviewed <07CSR650>. The nickel-catalyzed regio- and stereoselective silylation of terminal alkenes with silacyclobutanes 59 gives easy access to vinylsilanes <07JA6094>. The synthesis of substituted alkylidene silacyclohexadiene derivatives, conjugated dienes and trienes utilizing alkylidene silacyclobutenes 60 has been described <07JOC3137; 07TL3671>. A series of novel organosilicon based spiro compounds (such as 61 and 62) that combine two organic photoactive subunits at one silicon center have been synthesized, fully characterized, and their photoluminescence spectra have been recorded <07CEJ7204>. The synthesis and structures of heteroleptic silylenes 63 have been described <07JA12050>. [1]Molybdarenophanes with aluminum, gallium, and silicon in bridging positions have been prepared from silicon-based four-membered rings <07JA9314>. R Si R1 R1 59

Ph Ph

Ph Ph

Si Ph 60

Ph 61

R

t-Bu

Ph Si

Si

N

Ph

Si

Ph

N Ph 62

Ph

t-Bu 63

Phosphacyclobutene P-oxides 64 are produced by the reaction of titanacyclobutenes with dichlorophenylphosphine <07CEJ4077>. The large instability of heterophosphete compounds 65, composed of a four-membered unsaturated ring with a pentavalent phosphorus atom and a heteroatom, has been identified and characterised by first-principle molecular orbital computations, at several levels of theory; showing that strong electron-

87

Four-Membered Ring Systems

withdrawing groups bound to the phosphorus atom are able to stabilize the system <07EJO1759>. The photochemical isomerizations of bicyclo[1.1.0]butanes into fourmembered rings 66 have been reported <07AG(E)5741>. Quantitative displacement of triflate groups in 1,3-ditriflato-2,4-bis(2,6-dimethylphenyl)cyclodiphospha-2,4-diazane by 4dimethylaminopyridine or Me3P gives dicationic complexes containing bifunctional diphosphorus Lewis acceptors 67 <07CC4671>. The Pd-catalyzed dynamic kinetic enantioselective arylation of silylphosphines has been catalyzed by the chiral phosphetane derivative 68 <07JA15122>. The use of chiral phosphines, including phosphetane-based compounds, as catalysts improves previous results for the enantioselective [3+2] cyclisation reactions between allenic esters and N-tosylimines, both in terms of conversion rate and enantioselectivity <07T11921>. The reaction of Na[cyclo-(P5t-Bu4)] with complexes of Ni, Pd, or Pt can lead to the rearrangement of the cyclo-(P5t-Bu4) ligand to several tetraphosphetane derivatives <07CEJ7974>. New reaction patterns between an allenylidene complex and alcohols involving sequential and selective hydrogenation of the C
–C and M– C
double bonds of an allenylidene ligand coordinated to osmium have generated a fourmembered heterocycle containing phosphorus and osmium <07JA8856>. O Ph P Ph

XX Y P X Me

64

X = F, Cl, CN Y = O, NH, S 65

R1 Ph B P R2 P B R1 R2 Ph 66

Et Dmp N P P N

P Dmp

Et

Et

Fe P

Dmp = 2,6-dimethylphenyl 67 68

Et

Organotitanium species generated by the reaction of -chloroallyl sulfides with the titanocene(II) reagent [Cp2Ti{P(OEt)3}2] reveal a dual reactivity: acting as both titanacyclobutenes 69 and vinyl carbene complexes <07CEJ4077>. Titanocene alkylidene complexes, generated by desulfurizative titanation of thioacetals with Cp2Ti[P(OEt)3]2, reacted with alkynyl methyl sulfones to produce titanacyclobutene species, which gave terthomopropargyl alcohols with high diastereoselectivity on treatment with aromatic and
,unsaturated ketones <07OL2875>. Oxatitanacyclobutanes have been postulated as intermediates for the titanocene(II)-promoted carbonyl cyclopropylidenation utilizing 1,1dichlorocyclopropanes <07TL3521>. Woollins’ reagent 70 acts as a deoxygenation reagent for a wide range of sulfoxides affording the corresponding sulfides in good to excellent yields under mild conditions <07TL3677>. It has been reported that refluxing a toluene solution of 2,2,7,7-tetramethyl-3,5-octadiynebutadiyne with two equivalents of Woollins’ reagent resulted in two unusual products, the four-membered heterocycle 71 and a bisselenaheterocycle with two fused five-membered rings <07CC1465>. The phosphonium alkylidene [(NHC)Cl2Ru=CH(PCy3)]+[B(C6F5)4]– (NHC = N-heterocyclic carbene, Cy = cyclohexyl, C6H11) has reacted with 2.2 equiv of ethylene at –50 oC to form a 14-electron ruthenacyclobutane (NHC)Cl2Ru(CH2CH2CH2), which has a C2v symmetric structure with a flat, kite shaped ruthenacyclobutane ring with significant C
–C agostic interactions with the Ru center to judge by NMR spectroscopic data <07JA1698>. Four-membered metallacycles with an ethenylidene-metallacyclobutane-3-one framework, have been obtained from bis(alkynyl)-ruthenium compounds by coupling of the alkynyl groups and concurrent incorporation of water molecules, under protic conditions <07CEJ4024>. Reactions of cationic terminal aminoborylene complexes towards dicyclohexylcarbodiimide proceed by unprecedented insertion chemistry for terminal borylene complexes into the Fe=B and B=N bonds, forming four-membered rings linked at a spirocyclic boronium center 72

88

B. Alcaide and P. Almendros

<07AG(E)2043>. The formation of azazirconacycles has been detected on studying C–H bond activation reactions by a Zr=N complex <07AG(E)5580>. A four-membered azapalladacycle intermediate has been postulated to facilitate the nucleophilic addition during the intramolecular palladium-catalyzed acylation of -(2-haloanilino)esters to give dihydroquinolin-4-ones <07AG(E)7270>. The encapsulation of a silver(I) cluster bearing azatitana four-membered rings has been described <07CC2983>. The reaction of tricarbonylmanganese(I)–lysozyme complex with a Ni complex has yielded a four-membered sulfanickelacycle <07CC2805>. The synthesis and characterization of 1,3digermacyclobutanes bearing a new type of bis(methylenethioxophosphorane) has been reported <07CC4149>. The catalytic dehydrogenation of amineborane R2NHBH3 (R = Me, H) promoted by a family of bis(cyclopentadienyl)titanium and bis(indenyl)zirconium compounds has been reported to yield the corresponding diazadibora four-membered heterocycle <07CC3297>. It has been reported that the reaction between cyclodiphosphazane, cis-[t-Bu(H)NP(μ-t-BuN)]2 and paraformaldehyde in 1:1 and 1:2 ratio produces four-membered ring mono- and bis-
-aminophosphonates in good yields <07TL5227>. The reaction pathway for the formation of a four-membered thorium metallacycle has been proposed <07CC1029>. The solid-state molecular structure of a fourmembered plutonium ring has been reported <07CC1659>. The design, structure, function, and reaction mechanism of a four-membered aluminum ate base have been described <07JA1921>. A tin-based four-membered heterocycle has been used as catalyst for the synthesis of a ring-expanded bryostatin analogue <07JA2206>. The synthesis of an azadialuminacyclobutane has been achieved <07JA2234>. N- versus P-coordination in bis(amino)cyclodiphosphazane complexes of aluminum has been observed in the P2N2 heterocycle <07CEJ7410>. Se Se P Ph Ph P Se Se

R3 Cp2Ti

R1

R2

Cp = cyclopentadienyl

Se Se P Ph

B OC CO

70

69 71

4.8

Cy N

Cy N

N Cy

N Cy

+ R N R

72

REFERENCES

07ACR410 07AG(E)1517 07AG(E)1869 07AG(E)2043 07AG(E)2685 07AG(E)3028 07AG(E)3615 07AG(E)4930 07AG(E)5580 07AG(E)5741 07AG(E)6684 07AG(E)6922 07AG(E)7031 07AG(E)7060 07AG(E)7270

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Four-Membered Ring Systems 07AG(E)8799 07AG(E)9148 07ARK285 07ASC749 07ASC1256 07ASC1647 07ASC2177 07AXE3729 07BMC4910 07BMCL3077 07BMCL4358 07BMCL5036 07BMCL5171 07CAJ20 07CC657 07CC1029 07CC1465 07CC1659 07CC2500 07CC2805 07CC2983 07CC3297 07CC4149 07CC4671 07CC4788 07CEJ3026 07CEJ4024 07CEJ4077 07CEJ7204 07CEJ7410 07CEJ7974 07CEJ8877 07CSR484 07CSR650 07CSR1207 07CRV4437 07CRV4493 07CRV5210 07EJO37 07EJO101 07EJO356 07EJO548 07EJO681 07EJO1759 07EJO2055 07EJO2526

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B. Alcaide and P. Almendros S.Yudha S., Y. Kuninobu, K. Takai, Org. Lett. 2007, 9, 5609. B. Witulski, S. Senft, J. Bonet, O. Jost, Synthesis 2007, 243. V. Petrov, W. Marshall, Synthesis 2007, 1349. P. Chen, Synthesis 2007, 2351. H. Kiyota, T. Takai, Y. Shimasaki, M. Saitoh, O. Nakayama, T. Takada, S. Kuwahara, Synthesis 2007, 2471. A.S. Kale, P.S. Sakle, V.K. Gumaste, A.R.A. S. Deshmukh, Synthesis 2007, 2631. S.K. Richardson, A.H. Howell, Synthesis 2007, 2755. M. Petrini, E. Torregiani, Synlett 2007, 159. A.S. Kale, V.G. Puranik, A.R.A. S. Deshmukh, Synlett 2007, 1159. L.M. Monleón, M. Grande, J. Anaya, Synlett 2007, 1243. A. Basak, K. Chandra, R. Pal, S. C. Ghosh, Synlett 2007, 1585. P.M. Chincholkar, V.G. Puranik, A.R.A.S. Deshmukh, Synlett 2007, 2242. R. Pal, S.C. Ghosh, K. Chandra, A. Basat, Synlett 2007, 2321. D. Hamza, M.J. Stocks, A. Décor, G. Pairaudeau, J.P. Stonehouse, Synlett 2007, 2584. M.J. Stocks, D. Hamza, G. Pairaudeau, J.P. Stonehouse, P.V. Thorne, Synlett 2007, 2587. O. Sereda, R. Wilhelm, Synlett 2007, 3032. P. Areces, E. Carrasco, M.E. Light, M. Santos, J. Plumet, Synlett 2007, 3180. M. Shindo, Tetrahedron 2007, 63, 10. M.S. Islam, K. Ishigami, H. Watanabe, Tetrahedron 2007, 63, 1074. K. Uchida, K. Ishigami, H. Watanabe, T. Kitahara, Tetrahedron 2007, 63, 1281. D. Donati, S. Fusi, F. Ponticelli, R.R. Paccani, M.F.A. Adamo, Tetrahedron 2007, 63, 1584. A. Ghanem, Tetrahedron 2007, 63, 1721. Y. Kobayashi, H. Kamisaki, H. Takeda, Y. Yasui, R. Yanada, Y. Takemoto, Tetrahedron 2007, 63, 2978. D.K. Tiwari, A.Y. Shaikh, L.S. Pavase, V.K. Gumaste, A.R.A.S. Deshmukh, Tetrahedron 2007, 63, 2524. L.M. Monleón, M. Grande, J. Anaya, Tetrahedron 2007, 63, 3017. B. Alcaide, P. Almendros, A. Luna, Tetrahedron 2007, 63, 3102. A. Bhalla, P. Venugopalan, K.K. Bhasin, S.S. Bari, Tetrahedron 2007, 63, 3195. S. Coantic, D. Mouysset, S. Mignani, M. Tabart, L. Stella, Tetrahedron 2007, 63, 3205. A.L. Shaikh, A.S. Kale, Md.A. Shaikh, V.G. Puranik, A.R.A.S. Deshmukh, Tetrahedron 2007, 63, 3380. M.K. Ghorai, A. Kumar, S. Halder, Tetrahedron 2007, 63, 4779. P. Csomós, L. Fodor, I. Mándity, G. Bernáth, Tetrahedron 2007, 63, 4983. T. Shigetomi, K. Shioji, K. Okuma, T. Inoue, Y. Yokomori, Tetrahedron 2007, 63, 5161. A. Padwa, S.K. Bur, Tetrahedron 2007, 63, 5341. L.A. Aronica, G. Valentini, A.M. Caporusso, P. Salvadori, Tetrahedron 2007, 63, 6843. A. Guerrini, G. Varchi, R. Daniele, C. Samorì, A. Battaglia, Tetrahedron 2007, 63, 7949. A. Alizadeh, N. Zohreh, S. Rostamnia, Tetrahedron 2007, 63, 8083. P.M. Chincholkar, V.G. Puranik, A.R.A.S. Deshmukh, Tetrahedron 2007, 63, 9179. L.-Q. Cheng, Y. Cheng, Tetrahedron 2007, 63, 9359. L. Hu, Y. Wang, B. Li, D.-M. Du, J. Xu, Tetrahedron 2007, 63, 9387. D. Ferraris, Tetrahedron 2007, 63, 9581. N. Nishizono, M. Sugo, M. Machida, K. Oda, Tetrahedron 2007, 63, 11622. K. Okuma, A. Nojima, T. Shigetomi, Y. Yokomori, Tetrahedron 2007, 63, 11748. N. Fleury-Brégeot, L. Jean, P. Retailleau, A. Marinetti, Tetrahedron 2007, 63, 11921. M. Grohmann, G. Maas, Tetrahedron 2007, 63, 12174. Nicolas B., P. Gloanec, G. De Nanteuil, P. Jubault, J.-C. Quirion, Tetrahedron 2007, 63, 12352. L. Di Nunno, P. Vitale, A. Scilimati, L. Simone, F. Capitelli, Tetrahedron 2007, 63, 12388. A.G. Coyne, H. Müller-Bunz, P.J. Guiry, Tetrahedron: Asymmetry 2007, 18, 199. C. Cativiela, M.D. Díaz-de-Villegas, Tetrahedron: Asymmetry 2007, 18, 569. N. Anand, M. Kapoor, K. Ahmad, S. Koul, R. Parshad, K.S. Manhas, R.L. Sharma, G.N. Qazi, S.C. Taneja, Tetrahedron: Asymmetry 2007, 18, 1059. X.-G. Li, M. Lähitie, M. Päiviö, L.T. Kanerva, Tetrahedron: Asymmetry 2007, 18, 1567. M.D.P. Risseeuw, M. Overhand, G.W.J. Fleet, M.I. Simone, Tetrahedron: Asymmetry 2007, 18, 2001. Y. Yang, J. Wang, M. Kayser, Tetrahedron: Asymmetry 2007, 18, 2021. Z. Szakonyi, T.A. Martinek, R. Sillanpää, F. Fülöp, Tetrahedron: Asymmetry 2007, 18, 2442.

Four-Membered Ring Systems 07TA2468 07TA2841 07TL269 07TL381 07TL491 07TL1027 07TL1587 07TL1657 07TL1707 07TL1749 07TL1915 07TL2471 07TL2537 07TL3091 07TL3521 07TL3677 07TL3671 07TL3689 07TL3719 07TL4301 07TL4373 07TL4987 07TL5081 07TL5103 07TL5143 07TL5227 07TL5375 07TL6113 07TL6713 07TL6794 07TL7140 07TL7942 07TL8037 07TL8354 07TL8713 07TL9180 07TL12367

93

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