ELECTROPHILE-MEDIATED CYCLIZATIONS INVOLVING THE ALLENE PI-SYSTEM - STEREOSELECTIVITY AND SYNTHETIC UTILITY OF PD(II)-CATALYZED HETEROATOM CYCLIZATION REACTIONS - X-RAY MOLECULAR-STRUCTURE OF METHYL 2-[TRANS-3-PHENYL-N-(PARA-TOLYLSULFONYL)PYRROLIDIN-2-YL]ACRYLATE

Pd(II)-Mediated cyclisation and methoxycarbonylation of the phenyl-substituted allenic sulfonamides 4, 5 and 6 gave the corresponding N-sulfonyl 2,3-, 2,4- and 2,5-disubstituted pyrrolidines 9, 10 and 11, respectively. With the exception of compound 4, cyclisations were not highly selective and similar trends were observed with the alpha-amino allenic esters 7 and 8. Some improvement, both in yield and diastereoselectivity, was apparent when cyclisations were carried out in the presence of an excess of Et3N. The isolation of acyclic by-products 14 and 15 from alpha-amino allenic ester 7 suggests that chloropalladiation may play a key role in the mechanism of this cyclisation sequence and similar by-products were obtained from cyclisation of the unsubstituted allenic sulfonamides 16 and 20 leading to 6- and 7-membered rings, respectively. Other synthetic aspects of this palladium-based chemistry, including efforts directed towards coupling of the cyclisation step with a Heck-type olefination, are also described.