“Plasmalogen-type” cyclic acetals: formation and conformation of the 1,3-dioxanes and 1,3-dioxolanes from 1-O-cis-alk-1′-enyl-sn-glycerols

Abstract

Acid-catalyzed cyclization of 1-O-cis-alk-1'-enyl-sn-glycerol produced four structurally and geometrically isomeric long-chain cyclic acetals of glycerol. The isomers were isolated by adsorption and gas-liquid chromatography and were identified as cis-2-alkyl-5-hydroxy-1,3-dioxane (Ia), trans-2-alkyl-5-hydroxy-1,3-dioxane (IIa), cis-2-alkyl-4-hydroxymethyl-1,3-dioxolane (IIIa), and trans-2-alkyl-4-hydroxymethyl-1,3-dioxolane (IVa). The structure of each isomer was established by chemical and spectroscopic methods. Cyclization with p-toluenesulfonic acid in boiling benzene led to a thermodynamically equilibrated mixture of isomers Ia-IVa in which the cis isomers predominated. Cyclization in acetic acid was found to be kinetically controlled, and formation of the trans isomers was relatively favored. Rearrangement of the cyclic acetal isomers did not occur in acetic acid; hence, optically active five-membered ring acetals were prepared.