Synthesis of homogalacturonan fragments

Glycosylation of the D-galacturonic acid ester derivatives 15 and 17, which are prepared directly from D-galacturonic acid, with the thioglycosides 28 and 32, derived from the same sugar, provides alpha(1-->4)-linked dimers. The formation of the glycosidic linkage between the galacturonic acid moiet ies is best achieved by iodonium di-sym-collidine perchlorate promotion. Th us, the 4'-O-p-methoxybenzyl dimer 38 can be obtained in 64% yield. Partial deprotection of the 4'-O-position provided the glycosyl acceptor 36, which was coupled with the donor 32 to yield the alpha(1 "-->4')-linked trimer 3 9 (48%). Approximately 8% of the beta(1 "-->4')-coupled isomer was observed in the C-13 NMR spectrum of the reaction mixture.