From 1,3-Dicarbonyl Compounds and Ketones

The acid-catalysed condensation of a ketone with a 1,3-dicarbonyl compound, with dehydration in situ produces pyrylium salts. 

Aldol condensation between a 1,3-dicarbonyl component and a ketone with an a-methylene, under acidic, dehydrating conditions, prodnces pyrylinm salts.It is likely that the initial condensation is followed by a dehydration before the cyclic hemiacetal formation and loss of a second water molecule. The use of the bis-acetal of malondialdehyde, as a synthon for malondialdehyde, is one of the few ways available for preparing a-unsubstitnted pyrylinms.  

Variations on this theme include the use, as synthons for the 1,3-dicarbonyl component, of P-chloro-a,P-unsaturated ketones, or of conjugated alkynyl aldehydes.  

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