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A convergent synthesis of the C(11)–C(25) fragment of the aglycone of avermectin A2b

✍ Scribed by Eric Menager; Eric Merifield; Mark Smallridge; Eric J. Thomas

Publisher
Elsevier Science
Year
1997
Tongue
French
Weight
958 KB
Volume
53
Category
Article
ISSN
0040-4020

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✦ Synopsis

Oxidation of the mixture of products obtained by treatment of (S)-2-methylbutanal 8 with the E-but-2-enyldi-isopinocampheylborane prepared from (+)-pinene gave a mixture of homoallylic alcohols from which the major isomer 9 was isolated by chromatography. Oxidation with vanadylacetoacetate and ten-butyl hydroperoxide gave the epoxides 12 and 13, ratio 80 : 20. Following procedures developed in a synthesis of the model spiroacetal 19, 1,3-dithiane was alkylated, firstly using the epoxide 12, and then, after protection of the product 21 so obtained as its acetonide 23, using the epoxide 25, to give the 2,2-dialkylated 1,3-dithiane 25. Deprotection was accompanied by cyclisation to give the spiroacetal 28. Spiroacetal 29 was similarly prepared and taken through to the C(11)-C(25) fragment 38 of the aglycone of avermectin A2b 3.

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