Cationic ring expansion reactions

Abstract

The novel methods for the cationic ring expansion of cyclic ketone adducts have been developed. Palladium(0) catalyzed ring expansion of vinyl oxaspirohexanes occurred smoothly in the presence of 1 equiv of p-nitrophenol in tetrahydrofuran, yielding the ring-expanded cyclopentanones in high yields via $\pi$-allylpalladium cationic species. On the other hand, trimethylsilyl ethers of 1-alkenyl-l-cycloalkanols readily rearrange to the ring expanded $\beta$-mercurio cycloalkanones via mercurinium ions and $\beta$-thallium cycloalkanones via thallium ions. For unsymmetrical substrates, the more substituted alkyl group migrates preferentially. $\alpha$-Methylenecycloalkanones were obtained in good yields by elimination of the $\beta$-mercurio or $\beta$-thallium moiety with bases. Also, $\beta$-mercuriocycloalkanines undergo further ring expansion involving free radical reactions in the course of demercuration with $Bu_3SnH$ in methylene chloride. As a synthetic utility, hexahydro-1(2H)-anthracenone was produced via cationic ring expansion and intramolecular nucleophilic participation of a phenyl ring.