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

Abstract

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 and 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)-1 H-isoindole-1,3,5-triones], were isolated in equal amounts. Diene (±)- 1 cycloadded to the more reactive 4-methyl-1,2,4-triazoline-3,5-dione 14 at atmospheric pressure and room temperature. However, nothing can be said about the stereochemical outcome of this addition since the loss of the chiral auxiliary via the sulfoxide–sulfenate rearrangement gave the bicyclic α,β-unsaturated ketone 17 (2-methyl-2,3,5,6-tetrahydro-1 H-[1,2,4]triazolo[1,2- a]pyridazine-1,3,6-trione). Diels–Alder cycloaddition of enantiopure diene 2 with 10 occurred under pressure leading to a diastereomeric mixture of adducts where the trimethylsilyl enol ether moiety spontaneously evolved to the ketone function. The major enantiopure product 18a [(3a S,6 S,7a R, R S)-2-methyl-6-( p-tolylsulfinyl)perhydroisoindole-1,3,5-trione] was isolated and characterized. Cycloadditions of both dienes to dienophile 10 appear highly stereoselective.