Ring contraction by carbon participation in a hexopyranoside ring: Formation of benzyl 2-0-benzyl-3,5-dideoxy-3-C-benzyloxymethyl-α-L--pentofuranoside

A facile formation of 2,5-anhydro sugars by ring contraction in methyl hexopyranoside 2-triflates under conditions of nucleophilic displacement

Reaction of methyl 3,4-O-isopropylidene-2-O-(trifluoromethylsulfonyl)-alpha-L-fucopyr anoside and its beta anomer with a variety of nucleophilic reagents under mild conditions led to displacement of the triflic ester group, with migration of the ring-oxygen atom to C-2 and entry of the nucleophile at C-1, to give good yields of 2,5-anhydro sugar derivatives. Reagents employed were hydrogen fluoride-triethylamine complex, methanol in the presence of sodium hydrogencarbonate, sodium benzoate, sodium azide, potassium thiocyanate, and sodium borohydride. The same type of substitutive ring-contraction also occurred in methyl 4-O-benzyl-3-O-(4-methoxybenzyl)-2-O-(trifluoromethylsulfonyl)-alp ha-L- fucopyranoside and methyl 4,6-dideoxy-3-O-(4-methoxybenzyl)-2-O-(trifluoromethylsulfonyl)-al pha-L-xylo- hexopyranoside. The reaction is discussed in light of a literature survey of the chemical behavior of hexopyranoside 2-sulfonates in general, and 2-triflates in particular. Direct SN2 displacements, eliminations, and such different kinds of rearrangement as have previously been observed with structural analogs were not encountered in the present study. However, there is some precedent, in hexopyranoside 2-triflates, for the facile rearrangement that the four representatives here investigated have been found to undergo. The synthesis of these triflates from L-fucose is described.