Synthesis and pyrolytic and kinetic behaviour of β,δ′-dioxo-stabilised phosphorus ylides:: Convenient preparation of γ,δ-alkynyl ketones and 2,3-functionalised butadienes

Both ketone- and ester-stabilised phosphorus ylides undergo Michael addition to vinyl ketones to give the beta,delta'-dioxo ylides 3 and 5, respectively, although in the latter case careful temperature control is required to avoid an undesired side-reaction. Under conditions of flash vacuum pyrolysis at 650 degreesC the ylides 3 generally undergo Ph3PO extrusion to afford the gamma,8-alkynyl ketones 14, although these are found to partly undergo a secondary fragmentation. In contrast, the ylides 5 react under the same conditions to give the synthetically useful 1,3-dienes 20 by way of cyclobutenes. Rate constants for the reaction of selected ylides 3 and 5 are reported. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.