EVALUATION OF FURAN PHOTOOXYGENATION AS A DEVICE FOR CONSTRUCTION OF THE ZARAGOZIC ACID (SQUALESTATIN) CORE

A practical application of the photooxygenation chemistry of 3-substit uted furans to construction of the zaragozic acid/squalestatin backbon e is described. Although addition of 3-lithiofuran to the tartrate-der ived aldehyde 7 proceeds without chelation control to give a 1:1 mixtu re of diastereomeric alcohols 8, it is demonstrated initially that rea dy conversion to the polyfunctional intermediate 10 is possible by seq uential treatment of 9 with singlet oxygen, sodium borohydride, and tr iisopropylsilyl triflate. The actual enantiocontrolled route consisted of oxidation of 8 to the ketone and Wittig olefination of the latter in advance of asymmetric dihydroxylation with AD-mix-beta. Once this s eries of transformations had been accomplished, formation of the targe t product 30 was realized by an entirely comparable photooxygenation.