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 “Enantiospecific approaches to komarovispiranes” describes the utility of the monoterpene α-campholenaldehyde as chiral starting material in the enantiospecific synthesis of a variety of bi- and tricyclic compounds, and enantiospecific first total synthesis of spiro diterpenes komarovispiranes. For convenience the results are described in two different sections, viz., (a) Chiral synthons from α-campholenaldehyde; and (b) Enantiospecific synthesis of a komarovispirane. In the thesis, the compounds are sequentially numbered (bold) and the references are marked sequentially as superscripts and listed in the last section of the thesis. Complete details of the experimental procedures and the spectroscopic data were provided in the experimental section. A brief introduction is provided wherever appropriate to keep the present work in proper perspective. All the spectra included in the thesis were obtained by xeroxing the original NMR spectra. Monoterpenes are widely used as chiral auxiliaries, but their potential as chiral synthons has not been properly exploited. In the present thesis, utility of α-campholenaldehyde, which is readily available from α-pinene in two steps, as chiral synthon has been demonstrated in the enantiospecific synthesis of a few bi- and tricarbocyclic frameworks as well as spiroditerpenes komarovispiranes. To begin with, synthesis of several bi- and tricyclic compounds, namely bicyclo[3.3.0]octan-3-one; bicyclo[3.2.1]octan-2-one; bicyclo[3.2.1]octan-3-one; 3-cyclopentylcyclopentanone; bicyclo[4.3.0]non-3-one; spiro[4.4]non-2-one; tricyclo[6.3.0.02,6]undecan-6-ol; and spiro[4.5]decan-2-one; have been accomplished employing an intramolecular rhodium carbenoid C-H insertion, intramolecular type II carbonyl ene cyclisation, intramolecular acid catalysed diazoketone cyclisation reactions and ring-closing metathesis (RCM) reaction as the key steps. Komarovispirone is a tricyclic spiro diterpene isolated from Dracocephalum komarovi Lipsky, a perennial semishrub available in Uzbekistan and exhibits trypanocidal activity. The novel diterpene containing an unusual carbon framework, cyclohexane spiro fused to bicyclo[4.3.0]nonane, coupled with the potential biological activity have made komarovispirone, and its analogues interesting and challenging synthetic targets. Initially, as a model study, enantiospecific synthesis of a bis-norkomarovispirane was developed employing 7,8,8-trimethylbicyclo[3.3.0]oct-6-en-3-one as the starting material, which was readily available from campholenaldehyde. A Claisen rearrangement and RCM reaction based methodology was developed for the spiroannulation of a cyclohexane ring at the C-3 position of the bicyclo[3.3.0]octan-3-one. For the enantiospecific first total synthesis of komarovispiranes, the AB-trans ring system was generated via ring expansion of 7,8,8-trimethylbicyclo[3.3.0]oct-6-en-3-one employing ozonolytic cleavage followed by an intramolecular aldol condensation reaction of the resultant keto aldehyde. For the generation of the ABC ring system of the komarovispiranes, initially, a Claisen rearrangement and intramolecular type II carbonyl ene reaction based methodology was developed for the spiroannulation of a cyclohexane ring at the C-8 position of the bicyclo[4.3.0]nonan-8-one. Subsequently, an alternate Claisen rearrangement-RCM reaction based methodology was also developed for the spiroannulation, and extended it to the first total synthesis of a komarovispirane.

Employing a chiral pool strategy, enantiospecific syntheses of di- and triquinanes have been accomplished. α-Campholenaldehyde 95, readily available from the abundantly available monoterpene α-pinene 94, has been utilised as the chiral starting material. To begin with, enantiospecific synthesis of the diquinane 134 has been developed employing Nazarov cyclisation of the cross-conjugated dienone 132 as the key reaction (Scheme 37).71 Synthesis of the dienone 132 was accomplished by selenium dioxide mediated oxidation of the olefinic methyl group in α-campholenyl methyl ether 130, followed by further elaboration of the resultant aldehyde 131. OMe P2O5 MsOH The Nazarov cyclisation strategy has been further extended, as depicted in Scheme 38, for the synthesis of the triquinane enones 145 and 146 via the cross conjugated enone 144.71 The dienone 144 was obtained from the diquinane 136, which is readily available from campholenaldehyde 95 via an intramolecular rhodium carbenoid CH insertion reaction. Of the three methyl groups in campholenaldehyde 95, the olefinic methyl group can easily be functionalised, for example, via allylic oxidation. However, the remaining two tertiary methyl groups are difficult to functionalise, and there is no report in the literature on the utility of these two gem dimethyl groups either for functionalisation or for further elaboration, and remained only as gem dimethyl group in the products. It was conceived that it could be possible to utilise the tertiary methyl carbon for the ring construction via an intramolecular rhodium carbenoid γ-CH insertion reaction. To test the hypothesis, campho¬lenaldehyde 95 was converted into the diazoketone 165. Treatment of the diazoketone 165 with a catalytic amount of rhodium acetate furnished the diquinane 166, via a highly regio-and stereoselective insertion of the intermediate rhodium carbenoid in the CH bond of the tertiary methyl group, which is located cis with respect to the diazoketone, Scheme 39.72 As an application of the Nazarov cyclisation mediated synthesis of the diquinane 134, enantiospecific synthesis of the analogues of capnellenes, ABC and ABD ring systems of aberraranes have been carried out. A methyl cuprate reaction on the enone 134 generated the key intermediate, the ketone 169. A ring-closing metathesis (RCM) based cyclo¬pentannulation has transformed the diquinane 169 into the analogue of capnellene 175, as well as the analogue 197 of the ABC ring system of aberrarane. On the other hand, a Wacker reaction-intramolecular aldol condensation based spirocyclohexannulation transformed the diquinane 169 into an analogue 201 of the ABD ring system of aberrarane, Scheme 40.73 Finally, degradation of the two additional carbon atoms present on the A-ring furnished the ABC and ABD ring systems 235 and 238 of aberrarane, Scheme 41.(For structural formula pl refer the abstract pdf file)