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

Abstract

Base treatment of O-benzyl protected C-2- or C-4-ulopyranosyl compounds ( 4α, 4β, and 11) by either 10% Et 3N or 1% K 2CO 3 in MeOH initiated a β elimination to afford α,β-unsaturated C-ulopyranosyl compounds ( 5α, 5β, and 12), which further rearranged in a stereocontrolled manner to multifuctionalized α,β-cyclopentenones ( 6 and 14) in 70–80% yield. Both C-α- and C-β-2-ulosides ( 5α and 5β) produced the same cyclopentenone 6, indicating that a 1,2-enolate is formed prior to the cleavage of the C-5O bond. Because 6 is racemic, it was probably formed by the intramolecular cycloaldolization of two equally populated enantiomeric intermediates. When treated with 90% Et 3N in MeOH, 5α yielded almost exclusively 15 (isomer of 6), which was formed by a migration of the double bond in 5α during the previously described rearrangement. Thus either 6 or 15 was the major product, depending on the base used.