The allyl ether as a protecting group in carbohydrate chemistry. Part II

Abstract

Mercuric chloride in the presence of mercuric oxide was used to hydrolyse prop-1-enyl ethers of carbohydrates. Under these non-acidic conditions, 4,6-O-benzylidene-D-galactose and 2,3-di-O-benzyl-4,6-O-benzylidene-D-galactose were prepared from the corresponding prop-1-enyl glycosides and a mixture of the anomers of methyl 2,3,6-tri-O-benzyl-D-glucofuranoside was prepared from the corresponding 5-O-prop-1′-enyl ether. Acid-catalysed cyclisation of prop-1′-enyl 4,6-O-benzylidene-α-D-galactopyranoside gave 4,6-O-benzylidene-1,2-O-propylidene-D-galactose which was converted into 1,2-O-propylidene-D-galactose. Preferential rearrangement of the 2-O-allyl group in methyl 2,3-di-O-allyl-4,6-O-benzylidene-α-D-glucopyranoside gave methyl 3-O-allyl-4,6-O-benzylidene-2-O-prop-1′-enyl-α-D-glucopyranoside which was converted into 4,6-O-benzylidene-2-O-methyl-α-D-glucopyranoside. The rearrangement of an allyl group to a prop-1-enyl group in a carbohydrate derivative containing a benzamido-group occurred without hydrolysis of the amide linkage. The elimination of butadiene from 3-methylallyl (crotyl) ethers by the action of potassium t-butoxide in dimethyl sulphoxide suggests that the crotyl ether may provide a useful protecting group in carbohydrate chemistry. The action of potassium t-butoxide in dimethyl sulphoxide on an oxazoline derived from D-glucosamine produced a rearrangement to give an oxazole. The monoprop-1′-enyl ethers of 1,2-diols give 2′-chloromercuripropylidene acetals when treated with mercuric chloride in the presence of mercuric oxide. Reduction of these acetals with sodium borohydride regenerates monoprop-1′-enyl ethers of the glycol.