Electrophile-mediated cyclisations involving the allene π-system. Stereoselectivity and synthetic utility of PdII-catalysed heteroatom cyclisation reactions. X-Ray molecular structure of methyl 2-[trans-3-phenyl-N-(p-tolylsulfonyl)pyrrolidin-2-yl]acrylate

Abstract

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 α-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 α-amino allenic ester 7 suggests that chloropalladiation may play a key role in the mechanism of this cyclisation sequence and similar byproducts 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.