Kinetics and mechanism of the acid-induced rearrangements of α-santonin, 6-epi-α-santonin, and related compounds

Abstract

The well known rearrangements of α-santonin (2) to α-desmotroposantonin (3) in aqueous sulphuric acid, and to its acetate in acetic anhydride, have been reinvestigated. No evidence was found for the formation of isomeric products apart from the known β-desmotroposantonin (4). Kinetic and basicity measurements, allied to the use of acidity functions, allow the conclusion that, in >45% H₂SO₄, α-santonin (2) rearranges slowly to trans-desmotroposantonin, which rapidly epimerises to (3). In more dilute acid there is evidence of a change in mechanism. The cation of 6-epi-α-santonin (5) rearranges about 1 000 times faster than those of α-santonin and a number of analogues, including the lactone-free analogue (6). This is attributed to a favourable interaction between the lactone ring of (5) and the dienone ring over which it can be closely folded.