Epoxidations with iodine tris

Applications of Organoborates. Details have appeared of reactions of tri-alkylethenylborates with aldehydes, epoxides, and iodine, respectively useful for syntheses of 1,3-diols, 1,4-diols, and alk-l-enes. Trialkylalkynylborates cannot normally be acylated successfully because of cyclization and further rearrangement of. the initial intermediates. However, the trialkylalkynylborates... 

The key step of the synthesis is the rearrangement of the a-acetylenic alcohol 97 into the unsaturated carbonyl compound 124. This rearrangement was carried out with tris(triphenylsilyl)vanadate, triphenylsilanol and benzoic acid to give a mixture of the isomers 124 and 125. The latter was converted by iodine catalysis into the desired isomer 124. This key intermediate was afterwards transformed into the Cis-phosphonium salt 123 by standard procedures. The Wittig olefination of the Cio-dial 45 first with the fucoxanthin end group 123 and then with the peridinin end group 122 gave, in five steps, the C4o-carotenoid 126. Finally the epoxidation of this compound resulted in optically active fucoxanthin (121) and its (5S,6R)-isomer (Scheme 28). 

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