Pyrano pyranones

Both a,p-unsaturated iminium species and enals react with 4-hydroxypyran-2-ones to give pyrano[43-h]pyranones in a formal [3+3] cycloaddition (Scheme 24) <99JOC690>. In the presence of butadienes, the malononitrile derivative 42 obtained from a 3-hydroxypyran-4-one undergoes a one-pot sequential intramolecular [S+2] pyranone - alkene cycloaddition and a Diels-Alder reaction to give the O-bridged tricyclic system 43 (Scheme 25) <99JOC966>. 

Condensation of substituted pyrano[4,3-Z>]pyridine-5-one 166 with dimethylform-amide dimethyl acetal in xylene yielded enamine 167, which underwent recyclization to 2-alkyl-5-cyano-8-phenacyl-2,7-naphthyridin-l (27/)-ones 168 on treatment with primary amines (1997IZV1079). A change in the sequence of addition of the reagents to the pyranone 168 (first, treatment with benzylamine and then condensation with dimethylformamide dimethyl acetal) resulted in 8-benzoyl-3-cyano-4-(2-dimethyl-aminovinyl)-2-phenyl-1,6-naphthyridin-5(6//)-one 169. 

Malonyl chloride enolizes and cyclizes to form a pyranone (325) which combines with dialkyl-cyanamides to yield pyrano[4,3- ][l,3]oxazinediones (328), as well as pyranoazetediones (329). Presumably, the first step is the formation of adducts (326), which undergo rearrangement to the ureas (327) and elimination of hydrogen chloride to yield ketenes. Alternate intramolecular cycloadditions of the ketenes afford the final products (Scheme 93) <85CB4707>. 

The crystal structure of the bis-sulfone 16,18-diphenyl-3,ll-dithia[3]metacyclo[3](2,6)-pyrano-phan-17-one-2,2,l 1,11-tetraoxide (32) shows that the planar pyranone and benzene rings are tilted... 

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