Reactions of 4-hydroxycoumarin with some α,β-unsaturated carbonyls and 1,3-dicarbonyls : trapping of 4-hydroxycoumarin tautomers; formation of a pimelic acid derivative and a novel bicyclo compound

Centre of Advanced Studies on Natural Products, Department of Chemistry, University of Calcutta, University College of Science, 92, Acharya Praful la Chandra Road, Kolkata-700 009, India E-mail : talapatrask@yahoo.com Manuscript received 31 August 2001 4-Hydroxycoumarin (1) being endowed with both nucleophilic and electrophilic properties furnishes the dimeric coumarin derivative 3 in a pyridine catalyzed self-condensation process; while its attempted morpholine catalyzed self-condensation yields no dimeric coumarin derivative, but leads to the formation of the enamine derivative 2, establishing the existence of the less stable 2,4-chromandione tautomer (I') of 4-hydroxycoumarin. Pyridine catalyzed condensation between 4- hydroxycoumarin and acrylic acid affords 4-salicyloylpimelic acid (4) via the formation of 3,3-disubstituted-4-hydroxycoumarin and its subsequent hydrolysis and decarboxylation. But the condensation under similar condition with crotonic acid and with mesityl oxide separately leads to the isolation of the pyranocoumarin derivatives 5 and 6, respectively, along with a little amount 3 in either case. However, when a β,β-disubstituted-α,β-unsaturated acid like β,β-dimethylacrylic acid, and α,β-un­saturated acids with a bulky β-substitutent like cinnamic, p-chlorocinnamic and 3, 4-methylenedioxycinnamic acids are used, no condensation product excepting the same (Enteric coumarin derivative 3 is formed apparently due to the reduced elec­trophilicity of the β-carbon and steric crowding in the transition state. Condensation of 4-hydroxycoumarin with ethyl acetoacetate produces the a-pyranocoumarin 7, while with acetylacetone leads to the formation of a novel bicyclo compound 8 having two 4-hydroxycoumarin moieties with one acetylacetone moiety bridging them. These condensations using other bases like sodium bicarbonate, piperidine and morpholine afford the same products with varied rates of their formation. Refluxing 4-hydroxycoumarin (1) or its enamine 2 with acetic anhydride in acetic acid gives 3-acetyl-2-hydroxy-4-chromanone (9), stabilized by intramolecular H-bonding and thus indicating the existence of the least stable tautomeric form 1" (2-hy­droxy-4-chromenone) of 4-hydroxycoumarin (1)