Tautomeric dibenzoylmethanes

Dibenzoylmethane (8b) has been the subject of much interest as regards the possibility that its polymorphism is associated with keto-enol tautomerism. Chemical and spectroscopic studies showed that this is not so (33a). This compound had previously been reported to be trimorphic (33b), but one form appears, in fact, to be a eutectic mixture of the other two. The molecules in these two polymorphs are both in the same state of tautomerism they differ in the torsional angle about the (CH)-(CO) bond and in the type of hydrogen bonding in which they participate. It is noteworthy that solutions prepared from these forms at low temperature have differences in chemical and spectroscopic properties that are maintained for some time. For example, such solutions prepared and held at —35° react at different rates with FeCl3. 

Other reversible transformations involve tautomeric structures. This is, for instance, the case for 1,3-dicarbonyl compounds particularly dibenzoylmethane (DBM), the parent compound of the UV A filter butylmethoxydibenzoylmeth-ane (BM-DBM). The photochemistry of DBM (Fig. 6) has been studied by Veierov (17), Markov (18,19), Yankov (20), Moriyasu (21) and others. 

Figure 6 Tautomeric equilibrium of dibenzoylmethane. Keto-enol dominates in solution. Diketo is formed upon exposure and reverts back to keto-enol mix in the absence of radiation (dark reaction).

Dibenzoylmethanes. Avobenzone (22) is the only member of this class that is approved for use in the United States There are two other dibenzoylmethanes that are approved in Europe in addition to avobenzone. Dibenzoylmethanes are substituted diketones, with a UV spectrum well into the UVA region. This unique spectrum among W filters is a result of the keto-enol tautomerism. The W A absorbance of these compounds is... 

Dibenzoylpropane (71) was inactive as an inducer probably due to its lack of an enone moiety. Dibenzoylmethane (69) was quite active with a CD value of 0.8 [iM the keto-enol tautomerization of the /3-diketone can provide the Michael reaction acceptor feature. The three natural curcuminoids showed similar CD values therefore, the methoxy group in the phenyl ring does not greatly affect the potency. However, the CD... 

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