Unexpected Formation of A 4,6-O-Propylidene Derivative from Galactose and Allyl Alcohol in the Presence of AmberlystR15

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Abstract
Allyl ethers are convenient and widely used protecting groups in synthetic carbohydrate chemistry.2 One of the attractive reactions of the allyl ether group is its ready isomerization into a prop-1-enyl ether function under basic or metal-mediated conditions.2-4 In our research projects to extend galactosyltransferase reactions to the enzymic syntheses of βGal 1,1-linked sugars,5 1-O-allyl galactopyranoside was needed as a key synthetic intermediate. Anhydrous cation exchange resin (AmberlystR15:Rohm & Haas Co.) was chosen as the acidic catalyst owing to its ease of handling and removal. In this paper we report an unexpected result of this reaction which gave allyl 4,6-O-propylidene galactopyranoside from galactose and Amberlyst in refluxing allyl alcohol. On treatment of galactose (200 mg) and Amberlyst (100 mg) in allyl alcohol (5 mL) at 100–110 °C, TLC analysis (silica gel, acetonitrile:water, 10:1) indicated that the allyl glycosylation was completed within 15 min, affording allyl D-galactopyranoside.