4-Hydroxycoumarin 1, 4,5-dihydroxycoumarin 2, and 4,7-dihydroxycoumari
n 3 undergo H-D exchange at the C(3) atom of the lactone ring. Althoug
h only the 4-hydroxy-2-chromenone tautomeric forms are seen in the H-1
and C-13 NMR spectra of compounds 13, the equilibrium between the 4-h
ydroxy-2-chromenone and 2,4-chromandione forms is suggested to be the
key step in the H-D exchange reaction. 4,5-Dihydroxycoumarin shows the
highest rate of the reaction, since H-bonding between 5-hydroxyl and
4-keto functional groups can provide relative stability to the 5-hydro
xy-2,4-chromandione tautomeric form, a probable intermediate of the ex
change. NMR spectra and tautomeric transformations of 3-(4-methoxyphen
ylazo)-4-hydroxycoumarin 4 and 3-acetyl-4-hydroxycoumarin 5 are also d
iscussed. The stabilities of different tautomeric forms of compounds 1
-5 have been evaluated by MNDO calculations.