DIHYDROSELENAPYRANS BY [4+2] CYCLOADDITION OF DIARYL SELENOKETONES

The phosphorus ylides Ph3P=CAr1Ar2 5 [Ar1 = Ph, Ar2 = 4-CH3C6H4 (c); Ar1 = Ar2 = ph (d); 4-ClC6H4 (e); 4-FC6H4 (f); 3-CF3C6H4 (g); Ar1 = Ph, Ar2 = 4-ClC6H4 (h)] were allowed to react with elemental selenium at ca. 75-degrees-C in toluene in the presence of an excess of 2,3-dimethylbutadiene. The diaryl selenoketones 1 thus generated in situ by means of the ''Staudinger chalcogenation' reaction were trapped by the added conjugated diene to give the 2,2-diaryl-3,6-dihydro-4,5-dimethyl-2H-selenapyrans 7 in high yield. Similarly, the ylides 5c-f and Sh were treated with selenium, and the resulting diaryl selenoketones 1 added to 1,3-butadiene to give the corresponding 2,2-diaryl-3,6-dihydro-2H-selenapyrans 8. Selenobenzophenone, synthesized analogously, was employed in a [4 + 2] cycloaddition reaction with 2,3-dimethoxybutadiene to yield 3,6-dihydro-4,5-dimethoxy-2,2-diphenyl-2H-selenapyran (9), which was characterized by an X-ray crystal structure analysis. Compound 9 crystallizes in the space group P2(1)/n. In the crystal the dihydro-2H-selenapyran adopts a distorted half-chair conformation.