Reactions of boroxazolidones with aromatic aldehydes : An easy route to derivatives of isoquinoline and iso-indolinone

Boroxazolidones 1 derived from glycine and phenylalanine react with aromatic aldehydes to form the corresponding imines. The product 3 from 1a with o-carboxybenzaldehyde is converted into 4-hydroxyisoquinoline-3-carboxylic acid 6 by dimethyl sulfate, followed by t-BuOK, and aqueous acid.

With o-phthalaldehyde and 1a,b the isoindolinones 11a,b are obtained. These reactions proceed via carboxylic acids 9. Compound 9a was also prepared from 3 by catalytic, hydrogenation. Salicylaldehyde and 1a gave polymeric material, but the preformed Schiff's base 13 can be transformed into the p-nitrobenzyl ester 15 by treatment successively with dicyclohexylamine, triethylborane and p-nitrobenzyl bromide.