Abstract
A new polysaccharide analogue (2) having regiospecifically a benzyloxy group and a cyano group in its repeating unit was synthesized by the cationic ring-opening polymerization of 4(a)-benzyloxy-3(e)-cyano-6,8-dioxabicyclo[3.2.1]octane (1). The bicyclic acetal 1 was prepared from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) via six reaction steps. The ring-opening polymerization of 1 proceeded, though very slowly, in dichloromethane at temperatures between −60 and 0°C in the presence of strong Lewis acids such as antimony pentafluoride and antimony pentachloride as initiators, yielding polyacetals (poly(3(e)-benzyloxy-4(a)-cyanotetrahydropyran-6,2-diyloxymethylene)) with number average molecular weights of ∼1.4×104. 1H and 13C NMR analysis disclosed that polymer 2 consisted exclusively of tetrahydropyranoside units linked in a (1→6)-α fashion according to carbohydrate chemistry terminology. On heating the polymer 2 with potassium hydroxide in an aqueous 2-methoxyethanol solution, inversion of the configuration of the carbon atom bearing the cyano group occurred to give a structural unit having the cyano group in the equatorial position. The polymerization reactivity of 1 was markedly lower than those of analogous bicyclic acetals. The retarding effect of the cyano group on the polymerization of 1 is discussed.
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