136 JOURNAL OF COSMETIC SCIENCE thermore, the action spectrum of skin SCC and BCC carcinogenesis in the albino hairless mouse established by de Gruijl et al. (2) shows two maximum-efficiency wavelengths: the main one in the UVB and a second in the UV A region. The relative efficiency of the second maximum is approximately 10,000 times lower than that of the first, but it is increased by the higher doses of UVA received. UV A represents 98% of the total UV received at the earth's surface, and so it can be considered that the difference in the effectiveness of cancer induction between UVB and UV A is only 100 times. For these reasons, the cosmetics industry has had to improve UV A sunscreen protection (indeed, the first generation of sunscreens did not provide any protection against UV A). U nfor­ tunately, there are relatively few UV A absorbers currently available for sunscreens. One of the most commonly used, 4-tert-butyl-4' -methoxydibenzoylmethane (BM-DBM), be­ longs to the dibenzoylmethane family, as shown in Figure 1. However, it has been established that such molecules can undergo photochemical degradation (3,4). 4-tert-butyl-4' -methoxydibenzoylmethane (BM-DBM) absorbs solar energy and filters UV A rays with a maximum efficiency near 340 nm (5). Its absorbance is due to resonance throughout a large part of the molecule owing to a hydrogen bond that creates a six-bond pseudocycle between the carbonyl group and the enol (Figure 1). BM-DBM undergoes a rapid keto/enol photoisomerization by a proton transfer. The keto form absorbs UVB and UVC rays. This induces a shift in the preparation absorbance and a loss of efficiency in UVA. For longer irradiation times, the keto form undergoes photocleav­ age according to the N orrish I mechanism ( 6-8). The degradation products obtained are K 0 0 BM-DBM ClO-DBM Figure 1. Structure of the two UV filters studied (H and C are numbered for NMR assignments).

NEW LONG-CHAIN UV ABSORBER 137 well identified (3,9). The most abundant are benzaldehydes, benzoic acids, and aceto­ phenone. The importance of this phenomenon increases with the duration of the expo­ sure and is one of the reasons why it is recommended to reapply sun cream every two hours or so (other solar filters present the same drawback (10)). It is important to find a way to photostabilize preparations containing BM-DBM in order to improve UVA photoprotection. Two possibilities can be considered. The first, already widely exploited, consists in adding another molecule to the preparation. This molecule has to stabilize the BM-DBM via various mechanisms, most of which are unknown. For example, Thorel (11) added a trimellitic acid derivative, Allard and Forestier (12) incorporated a 3,5- triazine-derived compound and an alkyl (a.-cyano)-�,13'-diphenylacrylate, and Hansenne and De Chabannes (13) tested a polysaccharide alkylether in order to prepare a photo­ stable composition with a dibenzoylmethane derivative. Some research has also been done to define synergic mixtures of filters (14, 15 ). The second possibility, to our knowledge not yet explored, consists in making chemical modifications of BM-DBM itself. These modifications must keep the UV absorption capacity but must augment the resistance to photodegradation. According to the literature (3, 16), photodegradation occurs via the keto form. Our strategy was then to avoid ketonization by stabilizing the enol form. Dibenzoylmethane derivatives exist as a keto/enol mixture where the keto/enol ratio depends on the nature of the environment. Tobita et al. (16) showed that dibenzoyl­ methane exists mainly in the chelated enol form in both non-polar and polar solvents, although the enol content is higher in non-polar solvents. It seems that a non-polar environment would favor strong intramolecular hydrogen bonding. Thus a long ali­ phatic chain chemically grafted onto BM-DBM should induce the migration of the molecule to a more apolar environment in complex preparations. In this study, we present a new UV filter, the l-(4-tert-butylphenyl)-2-decanyl-3-(4' - methoxyphenyl)-propane-1,3-dione called ClO-DBM, derived from BM-DBM by graft­ ing a ten-carbon aliphatic chain on the a-carbonyl position (Figure 1). The chain length is above eight carbons, giving amphiphilic properties to the molecule (17). This filter was incorporated in a water-in-oil preparation, and the UVA absorption efficiency was tested under a 150-W xenon lamp or natural sunlight. The absorbance capacity and stability of preparations containing Cl0-DBM alone, BM-DBM alone, and a BM-DBM/ Cl0-DBM mixture were compared. MATERIALS AND METHODS CHEMICAL PRODUCTS Tetra-n-butylammonium fluoride, 75% w/w aq. soln. and 1-bromodecane 98% were obtained from Avocado (Heysham, Lancashire, England). BM-DBM (Parsol® 1789) was obtained from Givaudan-Rome (Switzerland). Dichloromethane and acetonitrile, HPLC grade, were obtained from SDS (Peypin, France), and tetrahydrofuran was from Carlo Erba RPE (Val de Reuil, France). Dodecyl sulfate sodium salt 98% was an Aldrich product (Steinheim, Germany) and used as supplied. Silica gel (0.063-0.200 mm) was obtained from Merck (Darmstadt, Germany). INSTRUMENTATION 1 H N.M.R. and 13 C were recorded with a Bruker ARX-400 MHz. Infrared spectra were