Synthesis of chiral 1-and 2-(p-tolylsulfinyl)-3-trimethylsilyloxybuta-1,3-dienes and their behaviour in Diels-Alder cycloadditions

The synthesis of (+/-)-(E)-1-(p-tolylsulfinyl)-3-trimethylsilyloxybuta-1,3- diene 1 and (S-S)-2-(p-tolylsulfinyl)-3-trimethylsilyloxybuta-1,3-diene 2 a nd their reactions with cyclic dienophiles are described. Cycloaddition of diene 1 with N-methylmaleimide 10 was performed under pressure affording a complex reaction mixture from which two epimers at C-3', (+/-)-11a and (+/- )-11b [2,3,3a,4,5,7a-hexahydro-2-methyl-7a-(1-methylsuccinimide-3-yl)-1H-is oindole-1,3,5-triones], were isolated in equal amounts. Diene (+/-)-1 cyclo added to the more reactive 4-methyl-1,2,4-triazoline-3,5-dione 14 at atmosp heric pressure and room temperature. However, nothing can be said about the stereochemical outcome of this addition since the loss of the chiral auxil iary via the sulfoxide-sulfenate rearrangement gave the bicyclic a,P-unsatu rated ketone 17 (2-methyl-2,3,5,6-tetrahydro-1H-[1,2,4]triazolo[1,2-a]pyrid azine-1,3,6-trione). Diels-Alder cycloaddition of enantiopure diene 2 with 10 occurred under pressure leading to a diastereomeric mixture of adducts w here the trimethylsilyl enol ether moiety spontaneously evolved to the keto ne function. The major enantiopure product 18a [(3aS,6S,7aR,R-S)-2-methyl-6 -(p-tolylsulfinyl)perhydroisoindole-1,3,5-trione] was isolated and characte rized. Cycloadditions of both dienes to dienophile 10 appear highly stereos elective. (C) 2000 Elsevier Science Ltd. All rights reserved.