Spinosyn G: Proof of structure by semisynthesis

Article abstract of DOI:10.1021/jo048173k

(Chemical Equation Presented) 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 ¹³C 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-ossaminide (14). This gave an ～2:1 mixture of anomeric products 17 with the desired equatorial glycoside predominating, which was then converted to 3 by N-deprotection and methylation.