Détermination De La Structure Du 3‐OXO‐3,3a‐Dihydroguaiazulène, Produit De La Réaction De Dakin Sur Le 3‐Formylguaiazulène. Un Exemple Inhabituel De Tautomérie Cétoénolique

The Dakin reaction on 3‐formylguaiazulene (I) did not lead to the expected 3‐hydroxyguaiazulene (II). The structural study of the pale‐yellow crystalline compound, isolated in 60% yield, proves it to be 3‐oxo‐3,3a‐dihydroguaiazulene (IIIa), the ketonic tautomer of 3‐hydroxyguaiazulene. Chemical (reduction, Grignard reaction and hydrogenation) and spectroscopic investigations (IR, UV and NMR) are confirmatory to this conclusion. The NMR spectra have shown two structures to be present in solution. These are probably the two ketonic tautomers (IIIa and IIIb) possible for 3‐hydroxyguaiazulene. Preponderance of a ketonic non azulenic structure over its hydroxyazulenic tautomer is the first example of such a tautomery.