Cyclopentane stereoselective annulations

The 4,5-threo-thiobutenolide 91 thus obtained was then reduced and shortened by one carbon atom to arrive at aldehyde 92 onto which a highly stereoselective cyclopentane-forming annulation was performed via a TBSOTf/DIPEA-promoted silylative protocol. In the event, a major 2,3-cA-configured bicycle educt 94 was isolated in a 82% yield, accompanied by only 7% of fram-isomer 93. Completion of the synthesis entailed the reductive cleavage of the thiolactone bond within 93 and 94, followed by acidic deprotection, giving l-deoxy-l-thio-4a-carba-[ -D-... 

The procedure described here serves to illustrate a general [3+2] annulation method for the synthesis of cyclopentane derivatives. A unique feature of this one-step annulation is its capacity to generate regio-specifically five-raembered rings substituted at each position, functionally equipped for further synthetic elaboration. As formulated in the following equation, the reaction proceeds with remarkably high stereoselectivity via the effective suprafacial addition of the three-carbon allene component to an electron-deficient olefin ("allenophile"). ... 

In a similar manner, the corresponding cyclopentane annulated spirocyclobutanones 40 were obtained in high yield. Clearly, initial formation of a terminal oxonium ion which undergoes stereoselective addition to the vinyl bond induces the cyclopropylmethyl to cyclobutyl cationic rearrangement. 

V. Nair, B. P. Babu, S. Vellalath, V. Varghese, A. E. Raveendran, E. Suresh, Org. Lett. 2009, 11, 2507-2510. Nucleophihc heterocyclic carbene catalyzed annulation of enals to chalcones in methanol a stereoselective synthesis of highly functionalized cyclopentanes. 

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