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A stereoselective synthesis of the C16-C32 fragment of ionomycin Spino, Joseph Arthur Claude


This dissertation concerns the stereoselective synthesis of optically pure compound 196, the C₁₆-C₃₂ fragment of the polyether antibiotic ionomycin (1). Fragment 196 was synthesized by preparing and joining the two subunits 172 (C₁₆-C₂₂) and 122 (C₁₆-C₃₂). Compound 172 was synthesized by repetitive asymmetric aldol condensations of optically pure crotonyl imide 69 with aldehydes 129 and 139. The use of 69 allowed for the enantioselective construction of the two propionate units of fragment 172 having the "anti" configuration. Compound 122 was synthesized using the highly stereospecific oxidative permanganate cyclization of the 1,5-diene 93 to give racemic tetrahydrofuran 94, having the correct relative stereochemistry at the four asymmetric centres. The diene 93 was in turn prepared via the β-keto ester dianion methodology which allows for the stereospecific introduction of the isoprenoid unit. Diol 103, derived from 94 was resolved as its mono (+)- ̲O̲-acetylmandelate and was transformed into optically active fragment 122. Finally, to complete the synthesis of optically active fragment C₁₆-C₃₂ of ionomycin, compounds 172 and 122 were coupled via a Wittig reaction to afford the cis olefin 194 and the second tetrahydrofuran ring of 196 was introduced stereoselectively via an internal oxymercuration reaction.

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