Chromone-substituted piperidines

Polyhaloalkyl-substituted chromones and 7-pyrones react with salicylaldehydes in the presence of piperidine to give a variety of fused 277-chromenes in good yields (Scheme 58) <2006JOC4538>. Although it is conceivable that this reaction could proceed through a Baylis-Hillman reaction pathway, studies of this reaction point to the mechanism being a tandem intramolecular oxa-Michael addition and subsequent Mannich condensation. 

A review describing chromone alkaloids is available [600]. Four piperidine substituted chromones and their derivatives are described below. 

Substituted chromones are formed by heating terminal acetylenes with o-iodophenols under carbon monoxide pressure in the presence of palladium[bis(diphenylphosphino-ferrocene)] dichloride e.g. o-iodophenol and phenylacetylene afford 2-phenylchromone (equation 61) ". Acetylenic aldiminium salts undergo cyclization to six-membered nitrogen heterocycles in polar aprotic solvents. Thus heating solutions of 5-(butylamino) pent-1-yne (607), an aldehyde RCHO (R = Me, Et, Ph or C02Et) and tetrabutylammonium iodide in acetonitrile yields the piperidine derivatives 608 and similar reactions of 4-(benzylamino)but-l-yne lead to tetrahydropyridines 609. ... 

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