Scheme 14 The Ogawa synthesis of the HA trisaccharide.
was not selective at the 2-deoxy-2-amino position. Consequently, addition of sodium methoxide in methanol was required to produce the desired disaccharide II.39 (65%).
The preparation of the trisaccharide II.40 was carried out by condensation of imidate II.45 with acceptor II.43 in the presence of TMSOTf to afford disaccharide II.50 in 87% yield (Scheme 14). Conversion of II.50 into glycosyl acceptor II.51 was achieved by de-isopropylidination and acetylation followed by removal of the allyloxycarbonyl group. Glycosyl acceptor II.51 was subsequently condensed with imidate II.44 in the presence of BF3Et2O to give the trisaccharide derivative II.52 in 81% yield. The deprotection, oxidation, and N-acetylation sequence was achieved as described for II.39 to afford II.40 in 66% yield over four steps.
The preparation of tetrasaccharide II.41 is outlined in Scheme 15. Boron tri-fluoride etherate mediated glycosylation of II.51 with imidate II.46 produced the corresponding trisaccharide (88%), which was readily converted to the glycosyl acceptor II.53 upon removal of the allyloxycarbonyl group. Under TMSOTf-catalyzed
HO^ OTo1 PhthN
OTol NPhth Cnw COAr,
^O^A OTol PhthN
NPhth QW C0Ac
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