Rearrangements of the gibbane skeleton during reactions with 2,3-di-chloro-5,6-dicyanobenzoquinone

Abstract

Gibbanes in which ring A is aromatic react with 2,3-dichloro-5,6-dicyanobenzoquinone to give a variety of products which probably arise via allylic carbonium ions; the latter are formed by abstraction of a hydride ion from position 4b to give, after elimination of a proton, a 4b(5)-ene which then loses a further hydride ion from C-6. The ions derived from 7-hydroxygibbanes such as methyl allogibberate (5) undergo Wagner–Meerwein rearrangement in which the 7,8-bond migrates to C-6 to give 7-ketones such as (1 5) in high yield. On the other hand the ions from 7-deoxygibbanes, e.g. methyl gibberate (27), give products which are mainly derived from nucleophilic attack at C-6. but which include a low yield of rearrangement product formed by migration of the 9,9a-bond to position 4b, giving 9a(10)-enes, e.g.(42). The presence of an 8-hydroxy-group in the allylic carbonium ion leads to cleavage of the 7,8-bond to give ring D seco-compounds, e.g. the ketone (49).