Multifunctionalized alpha,beta-cyclopentenones from C-2 and C-4-ulopyranosyl compounds: a stereospecific rearrangement initiated by base

Base treatment of O-benzyl protected C-2- or C-4-ulopyranosyl compounds (4 alpha, 4 beta, and 11) by either 10% Et3N or 1% K2CO3 in MeOH initiated a b eta elimination to afford alpha,beta-unsaturated C-ulopyranosyl compounds ( 5 alpha, 5 beta, and 12), which further rearranged in a stereocontrolled ma nner to multifuctionalized alpha,beta-cyclopentenones (6 and 14) in 70-80% yield. Both C-alpha- and C-beta-2-ulosides (5 alpha and 5 beta) produced th e same cyclopentenone 6, indicating that a 1,2-enolate is formed prior to t he cleavage of the C-5-O bond. Because 6 is racemic, it was probably formed by the intramolecular cycloaldolization of two equally populated enantiome ric intermediates. When treated with 90% Et3N in MeOH, 5 alpha yielded almo st exclusively 15 (isomer of 6), which was formed by a migration of the dou ble bond in 5 alpha during the previously described rearrangement. Thus eit her 6 or 15 was the major product, depending on the base used. (C) 2001 Els evier Science Ltd. All rights reserved.