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DOI: 10.1055/sos-SD-047-00004

Schobert, R.; Hölzel, C.; Barnickel, B.Science of Synthesis, (20104769.

For a carbo- or heterocycle to be formed by a ring-closing Wittig alkenation, an ω-carbonylated phosphonium ylide is required as the immediate precursor.[‌481‌,‌482‌] Small and normal-sized rings may be built up by any possible combination of carbonyl (aldehyde, ketone, ester, etc.) and ylide (unstabilized, stabilized, moderated) reactivities in this precursor. Medium and large rings can only come from intramolecular alkenations of unstabilized ω-formylated ylides. Even strained ring systems, including annulated and bicyclic bridgehead cycloalkenes, can be formed as the phosphine oxide elimination step is strongly exothermic.[‌483‌] There are several protocols for the in situ generation of the required ω-carbonyl ylides from more stable precursors: (1) deprotonation of ω-carbonylated phosphonium salts; (2) Michael addition of ω-carbonylated anions to (vinyl)phosphonium or (buta-1,3-dienyl)phosphonium salts; (3) partial oxidation of diylides; and (4) intermolecular double condensation between a diylide and a reactant containing two carbonyl groups.[‌27‌‌32‌,‌484‌]