L. D. Buyck, Ann Verhue
Sep 1, 2010
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Journal
Bulletin des Sociétés Chimiques Belges
Abstract
Asymmetric acetals of α-chloro and α,α-dichloroaldehydes in acetic anhydride containing sulfuric acid afforded hemiacetal acetates at convenient rates at room temperature resp. at 127°C. The main product involved cleavage to the better stabilized α-alkoxycarbenium ion intermediate, with a typical ratio 87:13 for ethyl:methyl and 94:6 for propyl:2-chloroethyl. For 2-alkoxytetrahydrofuran and -tetrahydropyran substrates (alkyl = methyl, ethyl, 2-chloroethyl) ring cleavage was predominant (89-98%). The kinetics were affected by several factors: the concentration of strong acid and of acetic anhydride (ionizing power of the medium), the ring size, nature and conformation of the 2-alkoxygroup and by a minor side-reaction that consumes acid by alkylation of its anion (OMe > OEt; eq. OMe > ax. OMe).