cis/trans-Isochromanones. DMAP induced cycloaddition of homophthalic anhydride and aldehydes

Homophthalic anhydride (1) reacts with wide variety of aromatic aldehydes, in the presence of chloroform and DMAP (N,N-dimethyl-4-amino-pyridine) at room temperature, to give in high yields cis- and trans-1-oxo-isochroman-4-carboxylic acids. Under these conditions, the trans-isomer is predominant and formation of Perkin-type products was not observed in contrast to the reaction carried out in the presence of pyridine. The unexpected trans-6-oxo-11-thiophen-2-yl-11,12-dihydro-6H-dibenzoc,h]chromene-12-carboxylic acid methyl ester (8) was isolated when the reaction between 1 and thiophene-2-carbaldehyde was carried out in pyridine.