Zwitterion-accelerated [3,3]-sigmatropic rearrangements and [2,3]sigmatropic rearrangements of sulphoxides and amine oxides

Abstract

The effect of a zwitterionic moiety on [3,3]-sigmatropic rearrangements has been studied. The allyl vinyl sulphoxide 5 underwent a [3,3]-sigmatropic rearrangement at 23 °C under neutral conditions to give the thione S-oxide 6(96%). This rearrangement (k₁= 1.56 ± 0.04 ± 10^–1 h^–1) was 45 times faster than conversion of the corresponding sulphide 3 into the thione 4(k₁= 3.5 ± 0.1 × 10^–3 h^–1). At 23 °C, the sulphoxide 7 also rearranged to give a mixture of (E)- and (Z)-thial S-oxide 8(90%). These experiments showed that the accelerating effect of the charges in the sulphoxide, a zwitterionic moiety, did not cancel out. Instead, the sulphoxide moiety significantly facilitated the [3,3]-sigmatropic rearrangement.

In the thermolysis of hexa-1,5-dienes with a zwitterionic moiety attached to the C-3 position (e.g., 9 and 24), [2,3]-sigmatropic rearrangements occurred. The conversion of the allyl sulphoxide 21 into the corresponding sulphenate ester 22 by a [2,3] process was used as the key step in a total synthesis of yomogi alcohol 23, a biologically active monoterpenoid.