Equatorial synthetic applications

Recendy, Darzens reaction was investigated for its synthetic applicability to the condensation of substituted cyclohexanes and optically active a-chloroesters (derived from (-)-phenylmenthol). In this report, it was found that reaction between chloroester 44 and cyclohexanone 43 provided an 84% yield with 78 22 selectivity for the axial glycidic ester 45 over equatorial glycidic ester 46 both having the R configuration at the epoxide stereocenter. 

The functionalization chemistry of the CDs has been extensively developed at this time.17 Synthetic methodology is available to either monofunctionalize or perfunc-tionalize either the primary or secondary hydroxyl portal of these hosts. Derivatiza-tion is still a pending subject in the chemistry of cucurbit[n]uril hosts. Kim and coworkers have reported a procedure that allows the equatorial functionalization of the periphery of CB6, but the same procedure is highly inefficient with either CB7 or CB8.18 Further developments in this area may be of great significance to extend the range of applications of this host family. 

Benzylidene acetals have found broad application in synthetic carbohydrate chemistry. A new chiral center (PhC H<) is formed during their formation, wherein the bulky phenyl substituent adopts predominantly the thermodynamically more stable equatorial orientation. Our laboratory has previously developed a simple protocol for the preparation of methyl 4,6-0-benzylidene-a-D-glucopyranoside (2) from commercially available methyl a-o-glucopyranoside (1). The purification of the product was achieved by simple precipitation without the need for chromatographic separation. Herein, we describe the application of this procedure to the synthesis of benzylidene acetals of ethyl thioglycosides 3 and 5. All product purification can be achieved by crystallization/precipitation only. Additional quantities and hence higher yields can be achieved by subsequent chromatography of the mother liquor. 

The allyl transfer chemistry is applicable to the synthesis of aryl-substituted vinylic and allylic sUanes of synthetic importance [17]. Homoallyhc alcohol bearing a ferf-butyldimethylsUyl group at the allylic position participates in the allyl transfer to yield ( )-3-aryH-alkenylsilanes (Scheme 5.17). The transition state of the retro-allylation should have the bulky silyl group at the equatorial position, accounting for the -stereoselectivity. 

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