Total synthesis of occidenol and synthetic and kinetic studies on substituted cyclohexadienones and pyrazoles

Abstract

Occidenol is a crystalline sesquiterpene, which was first isolated in 1960 by von Rudloff and co-workers from the heartwood of Eastern white cedar {Thuja occidentalis L). Biologically, occidenol belongs to family of compounds called phytoalexins. The structure of occidenol is unusual in its rare divinyl ether group, which is a part of a seven membered ring system (dihydrooxepine ring system). No natural product with such a divinyl ether ring system has been synthesized. The work described m this dissertation is the first total synthesis of occidenol. This synthesis proceeded from occidentalol, a related natural product, and used reversal pericyclic reactions to produce the divinyl ether fimctional group. The correct stereochemistry was determined through the comparisons of the synthesized compound with an authentic sample.

The second part is the acid catalyzed-rearrangement of carbocations. The migrating group is often hydrogen, an aryl group, or alkyl group. The migration tendencies of 3,3-disubstituted 3H-pyrazoles 5, arrangement reaction were studied. In this system, the ethyl migrates faster than the methyl by factor of about fifty times. It was predicted that the isopropyl group would migrate faster than the ethyl group by fifty-fold factor. The isopropyl pyrazole was synthesized for the first time, in addition to the methyl and the ethyl pyrazoles. The prediction was validated by kinetic studies. Two 4-methoxy- 4-R-cyclohexadienones, one with an ethynyl group and the other with a cyano group, were synthesized for the first time. The ethynyl group did migrate but further reaction occurred leading to hydration of ethyl group. The cyano group did not migrate but the methoxy group migrated instead.