Efficient routes to pyranosidic homologated conjugated enals and dienes from monosaccharides

Abstract

Three new processes for the obtention of pyranosidic dienes type B, and D and homologated conjugated enals A, have been designed in the context of the preparation of useful chiral building blocks from monosaccharides. Addition of vinylmagnesium bromide onto ulose 1 , followed by acetal ring opening; benzoate protection at C-4 and C-6; and dehydration of the tertiary hydroxyl group, mediated by thionyl chloride, reveals to be a straigthforward route to diene 11 . Analogous dehydration on conformationally rigid 2 was not regioselective and afforded isomeric dienes 3 and 6 in a 1:1 ratio. Addition of 2-lithio-1,3 dithiane onto locked 2,3-anhydropyranoses 21 and 22 followed by unravelling of the S,S-acetal moiety led efficiently to homologated α-enals 26 , 27 , 31 , and 30 and hence to dienes 34 , 14 , 20 and 13 through Wittig methylenation. Finally α-alkoxyvinyl ethers, prepared either by Wittig-Horner reaction of α-alkoxy-uloses, or by hetero Diels-Alder reaction of conjugated-enals, afforded in good to excellent yields pyranosidic α-enals 37 , 42 , 43 , 44 , 50 and 51 on treatment with pyridinium hydrochloride in pyridine or a mixture acetic acid-water (4:1).