The oligomerization and acylation of precocene I

Abstract

The oligomerization of precocene I with Brönsted and Lewis acids has been studied. In this way, the reaction of this chromene with HCl/MeOH gave two dimers, a trimer, a linear tetramer and a mixture of pentamers, whilst with FeCl3/HOAc a dimer and six cyclic tetramers were obtained. The cyclization of linear tetramers occurs between C-4‴ and C-6 or, in lower yield, between C-4‴ and C-8. In the formation of linear tetramers the C-8 functionalization was not detected, which could indicate that it occurs during the cyclization process. Moreover, oxidative one-electron coupling reactions were also observed in the treatment of precocene I with FeCl3/HOAc. On the other hand, the reaction of precocene I with FeCl3/Ac2O produced 6-acylation leading to the natural chromene encecalin.