Tributyltin hydride-mediated cyclisations of cinnamic enamides to piperidin-2-ones or pyrrolidin-2-ones. Indolizidinone ring formation by tandem radical cyclisation

Efficient 6-endo-trig free radical cyclisations of various 3-phenyl-acryl enamides to piperidin-2-ones, by using Bu3SnH and AIBN in boiling toluene, are reported. A different result has been observed for related enamide system without phenyl substituent at the 3-position of an acrylic moiety, as such a reaction of 3-methyl-butenoic acid cyclohex-1-enyl-methyl-amide afforded only the 5-exo-trig product. The difference in the cyclisation mode has been explained in terms of a reversible process leading to the thermodynamically more stable product. An application of this method for the tandem cyclisation permits to obtain idolizidine derivatives.