Jc. Lopez et al., EFFICIENT ROUTES TO PYRANOSIDIC HOMOLOGATED CONJUGATED ENALS AND DIENES FROM MONOSACCHARIDES, Tetrahedron, 49(35), 1993, pp. 7701-7722
Three new processes for the obtention of pyranosidic dienes type B, an
d D and homologated conjugated enals A, have been designed in the cont
ext of the preparation of useful chiral building blocks from monosacch
arides. Addition of vinylmagnesium bromide onto ulose 1, followed by a
cetal ring opening; benzoate protection at C-4 and C-6; and dehydratio
n of the tertiary hydroxyl group, mediated by thionyl chloride, reveal
s to be a straightforward 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 alpha-enals 26, 27, 31
, and 30 and hence to dienes 34, 14, 20 and 13 through Wittig methylen
ation. Finally alpha-alkoxyvinyl ethers, prepared either by Wittig-Hor
ner reaction of alpha-alkoxy-uloses, or by hetero Diels-Alder reaction
of conjugated-enals, afforded in good to excellent yields pyranosidic
alpha-enals 37, 42, 43, 44, 50 and 51 on treatment with pyridinium hy
drochloride in pyridine or a mixture acetic acid-water (4:1).