Diastereoselective Michael Additions of <i>N</i>-Boc-2-(<i>tert</i>-butyldimethylsiloxy)pyrrole to 2-(Arylsulfinyl)-1,4-benzoquinones

Abstract

Reactions of 2-(arylsulfinyl)-1,4-benzoquinones 1a-c with N-(tert-butoxycarbonyl)-2-(tert-butyldimethylsiloxy)pyrrole 2 have been studied under different catalytic conditions. Under BF(3).OEt(2) catalysis, reactions were completely stereoselective leading to the Michael-type adducts 3, whereas in the presence of SnCl(4) diastereomeric mixtures of the pyrrolo[3,2-b]benzofuranes 5 and 6 were obtained. The latter products result from a tandem process involving the Michael reaction followed by an intramolecular cyclization of the intermediates 3 and 4, the first step being hardly stereoselective. Reactions catalyzed by Eu(fod)(3) evolved in a highly stereoselective manner yielding the products 3 or 5 depending on the electronic density of the arylsulfinyl group.