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Novel cyclopentanoid annulation sequences : an approach to the synthesis of c19-oxygenated cyathanes Cook, Katherine Louise

Abstract

Two cyclopentanoid annulation sequences were developed. In the first sequence, (Z)-lbromo- 3-iodo-2-butene (9) was employed in the preparation of the series of angular allylic alcohols 28-30. Upon treatment with an oxochromium(VI) amine reagent, the allylic alcohols 29 and 30 underwent oxidative rearrangement to provide the bicyclic enones 32 and 33, respectively. Similar treatment of the allylic alcohol 28 resulted in the formation of a mixture of the epoxides 70 and 71. In the second sequence, (Z)- and (£)-5-iodo-3-trimethylstannyl-2-pentene (10 and 11, respectively) were employed in the preparation of the series of angular allylic alcohols 138-143. [2,3]-Wittig rearrangement of either of the allylic alcohols 138 or 141 according to the Still-Mitra protocol provided the homoallylic alcohol 152. Similarly, rearrangement of either of the allylic alcohols 139 or 140 provided the homoallylic alcohol 155. Rearrangement of 142 and 143 provided 162 and 165, respectively. The success of the second sequence, in particular of the conversion of 143 into 165, was extended to the preparation of an advanced intermediate (236) in an approach to the total synthesis of (±)-sarcodonin G (40). (F)-5-iodo-3-trimethylgermyl-2-pentene (247) was employed in the formation of the five-membered ring of the angular allylic alcohol 237. [2,3]-Wittig rearrangement of 237 according to the Still-Mitra protocol then provided the homoallylic alcohol 236. Compound 236 possesses the complete AB ring system of sarcodonin G (40), with the correct relative configuration at all four stereocenters.

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