Spinosyn G:Proof of Structure by Semisynthesis

摘要

Spinosyn G was isolated in the late 1980s as a minor component from the broth of our potent,fermentation-derived insecticide spinosad.Its structure was then tentatively identified as 5"-epispinosyn A (3) on the basis of ~1H and ~(13)C NMR data,but the 4"-epi compound 4 could not be conclusively ruled out with the data available.Described herein are unambiguous syntheses of both 3 and 4,whereby 3 was proved identical to the natural product.Compound 4 was prepared from intact Spinosyn A by a novel F-TEDA-promoted oxidative deamination to the 4"-ketone 5,stereoselective reduction to the equatorial alcohol 6,and nitrogen incorporation via the axial azide 7.Compound 3 was obtained by coupling Spinosyn A 17-pseudoaglycone (9) with the N-protected dihydropyran 16 derived from methyl L-ossarninide (14).This gave an approx 2:l mixture of anomeric products 17 with the desired equatorial glycoside predominating,which was then converted to 3 by N-deprotection and methylation.