Disarming, non-participating 2- O-protecting groups in manno- and rhamnopyranosylation: scope and limitations of sulfonates, vinylogous esters, phosphates, cyanates, and nitrates

Abstract

A series of d-mannopyranosyl and l-rhamnopyranosyl thioglycosides protected with electron-withdrawing, non-participating protecting groups on O-2 have been prepared and investigated for their potential as β-glycosyl donors. Both α- and β-thioglycosides were investigated and the latter preferred on the grounds of enhanced stability at room temperature. A 2- O-nitro- l-rhamnosyl fluoride was also prepared and investigated. Moderate β-selectivities were observed with some of these donors. With the more powerfully electron-withdrawing groups reduced donor reactivity leads to a requirement for higher reaction temperatures and reduced selectivities. Decomposition temperatures of the intermediate glycosyl triflates were determined by variable temperature NMR spectroscopy and generally correlate with the disarming propensity of the protecting group system.