New Applications Of The Pauson-Khand Reaction And Studies Towards The Synthesis Of The Hamigerans

Abstract

This dissertation is divided into two parts. The first part describes the dicobalt octacarbonyl mediated synthesis of medium sized rings via intramolecular Pauson-Khand (IMPK) reaction. The cyclization of 1,8-enynes has proved to be difficult via the IMPK reaction due to entropy reasons. To overcome thid limitation we attempted this cyclization in the presence of an ortho- buttressing group (t-butyl group) on the aromatic ring so as to reduce the conformationally allowed volume of the alkene arm of the substrate. The construction of the substrates started with O-alkenylation of the commercially available 3,5 di-tert-butyl-2-hydroxylbenzaldehyde and then the O-alkenylation products were alkynylated using ethynyl Grignard reagents to afford the different enynes. These enynes successfully engaged in the both thermal and oxidative IMPK reaction to afford 5,7,6-tricyclic systems. With the parent enynes we observed diketo-compounds as the major product and traces of the expected PK product. When the alkyne arm of the enynes was protected, the cyclization gave both the reduced PK product and the expected PK product. Also, the thermal mode gave a better diastereoselectivity when compared to the oxidative mode. Attempts to extend these result to a higher homolog to provide 5,8,6 tricyclic systems unfortunately failed to cyclize. Thus this work describes a new methodology for the construction of medium sized rings via IMPK reaction. A second system was investigated which examined whether highly conjested systems, related to those found in the hamigeran benzannulated terpenes, could be constructed. These experiments were successful, setting the stage for the studies described in the second section. The second part describes approach to the synthesis of the hamigerans by the application of the above developed IMPK methodology. The cyclization substrate was accessed from commercially available salicylic acid. The direct ortho metalation of the amide gave the iodo aryl amide. This was subsequently alkynylated using the Sonogashira method and then alkenylated by Grignard reaction. The enynes was the subjected to IMPK reaction to afford the tricyclic core of the hamigerans.