Approaches to a synthesis of galbonolide B
An approach to the C(7)–C(15) fragment of galbonolide B 2 has been completed in which the diene fragment 51 was assembled from (R)-3-tert-butyldimethylsilyloxypentan-2-one 29 by conversion into the unsaturated ester 30, acylation of the sulfone 47 using this ester, reductive desulfurisation, methylenation using a Wittig reaction and deprotection. Following model studies, the aldehyde 62, prepared by oxidation of the alcohol 51, was converted into a mixture of the epimeric alcohols 63 and these were converted into the di(methylene)tridecadienoic acid 65 using a phosphine catalysed Ireland–Claisen rearrangement. Sharpless epoxidations of the alcohol 67 using either L-(+)- or D-(–)-diethyl tartrate were highly stereoselective and gave the epoxides 68 and 69 which were clearly distinguishable. Model studies using the heptadiene monoepoxide 70 led to a synthesis of the monoprotected dihydroxy aldehyde 76 so establishing a protocol for the introduction into the vicinal diol of the galbonolides. Finally, aldol addition of tert-butyl acetate to the aldehyde 78 followed by selective protection, deprotection and cyclisation completed a synthesis of the macrolide 85.