Diketones, acid catalyzed pyrans

This observation led to the development of a general route to 4/f-pyrans, in which the acid-catalyzed condensation of unsaturated carbonyl compounds with 1,3-dicarbonyl compounds is used to generate 1,5-diketones, which spontaneously cyclize to the pyran (equation 3) (69JOC3169). The moderate yields are compensated by the ready availability of the... 

Hsung et al. studied a formal [3 + 3] cycloaddition reaction and found that In(OTf)3 catalyzed the reaction between a,p-unsaturated aldehyde and cyclic diketone to give pyrans [196] (Figure 8.86). However, BF3 Et20 and TiCU were found to be better suited for this reaction owing to the broader efficacies for different substrates and higher catalytic efficiencies of these Lewis acids. 

The use of A/ -heterocychc carbenes (NHC) as catalysts in the preparation of3,4-dihydro-2H-pyran-2-ones (14EJ05631),the copper-catalyzed synthesis of 3-substimted isocoumarins from o-halobenzoic acids and 1,3-diketones (14JOC8507) and the synthesis and reactivity of pyridine-flised 3-substituted 2-amino-4-arylchromenes (14H(89)1557) have been reviewed. 

Base-catalyzed cyclocondensation (a) of alkynones with malonic esters and (b) of 1,3-diketones with acetylenic acids yields esters 27 of 2H-pyran-2-one-3-carboxylic acid or 5-acyl-2H-pyran-2-ones 29, respectively. In both cases, Michael additions are the primary steps, which afford 5-oxopentenoic esters 26 or 28 providing the 2H-pyran-2-one system by enolization and enollactone formation [12] ... 

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