Syntheses of L-vallarose (6-deoxy-3-O-methyl-L-altrose) and D-digitalose (6-deoxy-3-O-methyl-D-galactose)
The first synthesis of L-vallarose (6-deoxy-3-O-methyl-L-altrose)(8), a sugar found in certain cardiac glycosides, has been achieved by a route involving partial hydrolysis of 1,2 : 5,6-di-O-isopropylidene-3-O-methyl-α-D-galactofuranose (2) with acid to give the corresponding 1,2-acetal (3). The latter was converted into 6-O-benzoyl-1,2-O-isopropylidene-3-O-methyl-5-O-methylsulphonyl-α-D-galactofuranose(5) and, thence, on solvolysis with methanolic sodium methoxide, into 5,6-anhydro-1,2-O-isopropylidene-3-O-methyl-β-L-altrofuranose (6). Ring-opening of this anhydro-sugar with lithium aluminium hydride and hydrolysis with acid liberated L-vallarose.
A new, convenient synthesis of D-digitalose (6-deoxy-3-O-methyl-D-galactose)(13) was accomplished from 1,2-O-isopropylidene-3-O-methyl-α-D-galactofuranose (3) by ring opening of the derived epoxide (11) with lithium aluminium hydride in ether followed by hydrolysis with acid of the 6-deoxy-1,2-O-isopropylidene-3-O-methyl-α-D-galactofuranose (12) so formed.