The chlorosilane-mediated dimerization of pentane-2,4-diones: a direct route to pyrans

The reaction of a pentane-2,4-dionate salt with dimethyldichlorosilane yields the pyrylium salt, 2,4-dimethyl-6-(2′-hydroxy-1′-propenyl)pyrylium chloride. The trifluoromethyl analogue yields the corresponding neutral pyranylidene complex, the structure of which was established by a variety of NMR techniques. A mechanistic rationale is proposed which accounts not only for these results but also for the isomeric products obtained by reaction of acetylacetone with MoOCl4 and WOCl4.