N-PENTENYL MANNOSIDE PRECURSORS FOR SYNTHESIS OF THE NONAMANNAN COMPONENT OF HIGH-MANNOSE GLYCOPROTEINS

The high-mannose oligosaccharide 1 is present on the conserved V3 loop of the viral coat of HIV1 known as GP-120. The mannan portion of this molecule has been prepared by utilization of halogen-promoted n-pentenyl glycoside (NPG) coupling. Two advantageous properties of NPG's facilitated construction of 1, one being the ability to activate the donor, even when C2 esterified (i.e., ''disarmed''), with NIS/Et(3)SiOTf, under which all reactions are complete within the time required to take a TLC sample. The second advantage was the ''side-tracking'' strategy which allowed the pentenyl group of a glycosyl acceptor to be rendered temporarily inactive by conversion to the dibromide. After coupling, the ''side-tracked'' NPG could be reactivated by reductive elimination to serve as the glycosyl donor in a subsequent step. With the appropriately protected monosaccharide precursors in hand, the nonamannan could be assembled by a virtually iterative protocol involving deprotection-coupling- deprotection-coupling...etc. as the only synthetic manipulations.