Allylic mono- and di-hydroxylation of isolated double bonds with selenium dioxide–tert-butyl hydroperoxide. NMR characterization of long-chain enols, allylic and saturated 1,4-diols, and enones

Abstract

Selenium dioxide with tert-butyl hydroperoxide as re-oxidant was used in the allylic hydroxylation of isolated double bonds in straight-chain hydrocarbons. This was shown for mono-unsaturated fatty acids, esters and alcohols. Either allylic position was hydroxylated individually or both positions reacted to give dihydroxy isomers, affording numerous novel hydroxy compounds. Yields of monohydroxy compounds in which the OH group is between the double bond and C-1 were usually higher than those in which the OH group is between the double bond and the methyl terminus. Monohydroxy products were used as starting material in subsequent allylic hydroxylation reactions to increase the yield of dihydroxy product, although this reaction is slow. Coinciding with the known mechanism, cis double bonds of starting materials isomerized nearly quantitatively to trans double bonds in the products while trans double bonds did not isomerize. Resonance differences of the olefinic carbons in ¹³C NMR of the unsaturated monohydroxy compounds show on which side of the double bond the hydroxy group is located. The magnitude of these differences depends on the nature of the group at C-1 and the distance of the double bond from C-1. Corresponding saturated hydroxy fatty acids were synthesized with the hydrazine–air system. ¹³C NMR of the saturated compounds showed that the dihydroxy products were erythro/threo diastereoisomers. With this assignment, ¹H NMR of the unsaturated allylic dihydroxy compounds may be used to distinguish these diastereoisomers. The olefinic protons of the erthryo dihydroxy diastereoisomer resonate downfield from those in the threo form. The threo diastereoisomers are formed in higher yields than their erythro counterparts. Compounds with allylic keto group (enones) analogous to the monohydroxy products arose as side products. The ¹³C NMR spectra of these enones are discussed.