Synthesis of Medium Ring Compounds via Sigmatropic Rearrangements

Abstract

The present review surveys recent application of sigmatropic rearrangements to ring enlargement reactions leading to medium ring compounds. The rearrangements concerned are thermally-allowed [2, 3] - and [3, 3] sigmatropic rearrangements as well as related [1, 3] sigmatropic shift.
The 1st chapter deals with a general strategy for applications of these rearrangements to synthesis of medium ring compounds via two- or more-carbon ring enlargements.
The 2nd chapter describes various methods for ring enlargement reactions via [3, 3] sigmatropic rearrangements. The sigmatropic shifts concerned include Cope, oxy-Cope and its variants, Claisen rearrangements, and tandem sigmatropic rearrangements. In particular, applications of these approaches to the synthesis of germacrene-type sesquiterpenoids are discussed.
The 3rd chapter deals mainly with ring enlargement reactions via [1, 3] sigmatropic shifts which are closely related oxy-Cope rearrangements. Ring enlargements via photochemical [1, 3] acyl shifts are also described.
The 4th chapter describes various methodologies via [2, 3] sigmatropic rearrangements. The rearrangements. concerned include Sommelet-Hauser type rearrangements in both N- and S-ylide systems and [2, 3] Wittig rearrangements. In particular, repeatable [2, 3] sigmatropic rearragements of S-ylides are shown to be quite useful for ring-growing reactions.