Synthetic Approaches To Herbertenoid And Cuparenoid Sesquiterpenes

Abstract

Among Nature's creation, terpenoids are more versatile and exciting natural products. In a remarkable display of synthetic ingenuity and creativity, nature has endowed terpenes with a bewildering array of carbocyclic frameworks with unusual assemblage of rings and functionalities. This phenomenal structural diversity of terpenes makes them ideal targets for developing and testing new synthetic strategies for efficient articulation of carbocyclic frameworks. The thesis entitled “Synthetic Approaches to Herbertenoid and Cuperenoid Sesquiterpenes" describes the application of ring-closing metathesis and Claisen rearrangement based approach to some herbertenoid and cuparenoid natural products. The results are described in five different sections, viz., a) First Total Synthesis of (±)-γ-Herbertenol; b) First Total Synthesis of (±)-HM-2; c) First Total Synthesis of (±)-HM-4 and HM-3; d) First Total Synthesis of Herbertenones A and B; and e) Total Synthesis of Lagopodin A. Complete details of the experimental procedures and the spectroscopic data were provided in a different section. A brief introduction is provided wherever appropriate to keep the present work in proper perspective. The compounds are sequentially numbered (bold), references are marked sequentially as superscripts and listed in the last section of the thesis. All the spectra included in the thesis were obtained by xeroxing the original NMR spectra. To begin with, the first total synthesis of γ-herbertenol, an herbertene isolated from a non-herbertus source, has been accomplished starting from 3,5-dimethylphenol. Claisen rearrangement of 3-(3-methoxy-5-methylphenyl)but-2-en-1-ol, obtained in eight steps from 3,5-dimethylphenol, furnished a γ,δ-unsaturated ester, which was transformed into 4-aryl-4,5,5-trimethylcyclopent-2-enone employing RCM reaction as the key step, which was further transformed into (±)-γ-herbertenol, which exhibited spectral data identical to that of the natural product. An alternative RCM reaction based methodology was also developed for the synthesis of γ-herbertenol methyl ether starting from ethyl 3-aryl-3-methylpent-4-enoate, an intermediate in the first sequence. The methodology has been extended for the synthesis of the putative structure of HM-2 starting from 2,4-dimethoxy-5-methylacetophenone via the corresponding ethyl 3-aryl-3-methylpent-4-enoate. However, the spectral data of the synthetic compound was found to be different from that reported for the natural product. A new cuperenoid structure for HM-2 was proposed. Total synthesis of cuparene-1,4-diol starting from toluhydroquinone, followed by its conversion to mono methyl ether and mono acetyl derivative confirmed the structures of HM-1 and the revised structure of HM-2. In a similar manner, total synthesis of the putative structure of HM-3 starting from 4-methylresorcinol dimethyl ether proved it to be wrong. A cupereniod structure, HM-4 monoacetate was proposed for HM-3. Synthesis of HM-4, and its conversion to mono acetate confirmed the structures of HM-4 and the revised structure of HM-3. The methodology has been further extended to the first total synthesis of herbertenones A and B starting from 2,5-dimethoxybenzaldehyde. By readily identifying the similarity between lagopodin A and HM-1 and HM-2, an intermediate in the synthesis of HM-1 and HM-2 has been further transformed in to (±)lagopodin A.