Tandem radical cyclisation of enamides mediated by tin hydride; pyrrolizidinone or indolizidinone ring formation

S R Baker, K I Burton, A F Parsons, J F Pons, M Wilson

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The tin hydride-mediated cyclisation of a variety of enamides under mild, neutral reaction conditions has been investigated. Enamides derived from pyruvate were reacted with R3SnH-AIBN to generate alpha-carbamoylmethyl radicals which underwent 5-endo cyclisation to give Cyclic a-amino ester radicals. Subsequent 6-endo or 5-exo cyclisation was observed leading to the formation of indolizidinone and pyrrolizidinone products respectively. The reaction pathway was governed by the alkene substitution. If an electron rich double bond was used the cyclisation was under thermodynamic control and a 6-membered ring was observed. Five-membered rings were formed on reaction of the alpha-amino ester radical with unsaturated esters or ketones. The ester or ketone functional group can stabilise the radical produced on 5-exo cyclisation and pyrrolizidinones could be prepared in up to 66% yield. Stannane by-products were also isolated from some reactions. These were thought to be derived from a competitive Michael-type addition of the tin radical to the enamide double bond.

Original languageEnglish
Pages (from-to)427-436
Number of pages10
JournalJournal of the Chemical Society-Perkin Transactions 1
Issue number4
Publication statusPublished - 21 Feb 1999



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