Open-chain acetonides of D-galactono-1,4-lactone as starting materials for pyrrolidines, azepanes and 5-azidomethyltetrahydrofuran-2-carboxylates: monomers for polyhydroxylated nylon and for tetrahydrofuran carbopeptoids
Treatment of D-galactono-1,4-lactone with dimethoxypropane in acetone in the presence of tosic acid forms methyl 2,3:5,6-di-O-isopropylidene-D-galactonate 6 (with only the secondary hydroxy group at C-4 unprotected) and methyl 2,3:4,5-di-O-isopropylidene-D-galactonate 5 (with only the primary hydroxy group at C-6 unprotected) in yields of 20% and 78%, respectively. The value of such easily accessible intermediates is illustrated by the synthesis of 1,4-dideoxy-1,4-imino-D-glucitol 16 (a pyrrolidine), and for the efficient preparation of 6-azido-6-deoxy-D-galactono-1,4-lactone 22. The conversion of 22 to a seven-membered galactonolactam 21 (a tetrahydroxycaprolactam) may provide access to hydroxylated nylon polymers. The galactonolactam 21 has no significant inhibitory effect on a number of glycosidases. Reaction of 22 with triflic anhydride gives two epimeric 5-(azidomethyl)tetrahydrofuran-2-carboxylates which provide starting materials for two series of carbopeptoids, one of which probably has a helical structure and the other a structure reminiscent of a β-turn.