2-Hydroxybenzophenone derivatives

The capability of 2-hydroxybenzophenone derivatives to dissipate light energy has been ascribed to rapid deactivation of the excited singlet state by intramolecular interaction between the carbonyl and hydroxyl groups, possibly involving reversible H-transfer. These proposals are outlined in Scheme I, where P and PP represent the polymer and photoproduct, respectively. 

Several pharmacological studies were earned out to substantiate the proposed prodrug concept for (/< )-a-methylhistamine (12). The parent compound of this series of azomethines is the 2-hydroxybenzophenone derivative of (ft)-a-methylhistamine which is designated BP 2.94 (36). To evaluate the desired protection against HMT metabolization, BP 2.94 (36) was tested as a possible substrate in the presence of S-adenosylmethionine in comparison to histamine and ( )-a-methylhistamine (12). As a result no methylation of the prodrug 36 was observed, whereas both primary amines displayed similar Km- and Fmax-values [6],... 

The cyclization of 2-hydroxybenzophenones is a well-documented procedure for the synthesis of xanthones < 1984CHEC, 1996CHEC-II>. Xanthone 799 is a key intermediate during the synthesis of gambogin and can be accessed in high yield via cyclization of the corresponding 2-hydroxybenzophenone derivative (Equation 332) <2005AGE756>. 

It can be concluded that the photostability of 2-hydroxybenzophenone derivatives is mainly due to the internal hydrogen bonding which favours the radiationless S - S0 deactivation process. Although the intersystem crossing St ->T1 occurs to some extent in polar hydrogen bonding solvents, the triplet state is nevertheless non-reactive because of the reversal of n—n and n—n levels. 

These compounds have a lower absorption coefficient in the solar ultraviolet region than 2-hydroxybenzophenone derivatives but, after exposure to sunlight for a time, they show an increased absorption in the 290—400 nm region due to the occurrence of the photo-Fries rearrangement which transforms them into ultraviolet absorbers of the first family (see Fig. 26) [129] for example,... 

UV absorbers have been found to be quite effective for stabilization of polymers and are very much in demand. They function by the absorption and harmless dissipation of the sunlight or UV-rich artificial radiation, which would have otherwise initiated degradation of a polymer material. Meyer and Geurhart reported, for the first time in 1945 [10], the use of UV absorber in a polymer. They found that the outdoor life of cellulose acetate film was greatly prolonged by adding phenyl salicylate (salol) [10]. After that, resorcinol monobenzoate, a much more effective absorber, was introduced in 1951 [11] for stabilization of PP, but salol continued to be the only important commercial stabilizer for several years. The 2,4-dihydroxybenzophenone was marketed in 1953, followed shortly by 2-hydroxy-4-methoxybenzophenone and other derivatives. Of the more commonly known UV absorbers, the 2-hydroxybenzophenones, 2-hy-droxy-phenyl-triazines, derivatives of phenol salicylates, its metal chelates, and hindered amine light stabilizers (HALS) are widely used in the polymer industry. 

Figure 3. Transient absorption spectroscopy on 2-hydroxybenzophenone and its derivative copolymer (A) 2-hydroxybenzophenone in EtOH solution and 2-hy-droxybenzophenone in CHtClt solution (B) copolymer in CHuCU. solution...

Each of the derivatives may be regarded as a substituted styrene, and classical styrene syntheses have been employed. Radical polymerization of the phenolic monomers (salicylate esters, 2-hydroxybenzophenones and hydroxyphenylbenzotriazoles) proceeds normally with AIBN as initiator, at least when oxygen is carefully excluded. It is expected that polymeric ultraviolet stabilizers, perhaps in combination with conventional stabilizer will make an important contribution to photostabilization technology. 

Many UV absorbers are based upon either 2-hydroxybenzophenone or 2-hydroxytriazole derivatives, (the corresponding 3-hydroxy-compounds are not effective). These work via the following mechanisms ... 

The furanoid ring in benzo[6 ]furan is susceptible to attack by oxidants. Permanganate and chromic acid give derivatives of 2-hydroxybenzoic acid with compounds unsubstituted at the 3-position, but compounds with a 3-methyl or a 3-aryl substituent give derivatives of 2-hydroxyacetophenone or 2-hydroxybenzophenone. Ozonolysis of benzo[6]furan affords 2-hydroxybenzoic acid, 2-hydroxybenzaldehyde and some catechol produced via its diformate. Before the advent of NMR spectroscopy these methods were used in structural elucidation of benzofuranoid natural products, as in the case of O-methyleuparin (Scheme 26). 

An anion derived from a ketone such as acetophenone cleaves the 1,2-bond of pyran-4-one the resulting dione (406) is cyclized by mineral acid to 4-hydroxybenzophenone (407). 

Commonly used ultraviolet absorbers such as hydroxybenzophenones, hydroxybenzotriazoles, and salicylic acid derivatives transfer absorbed light energy rapidly from their excited states, giving off thermal energy and longer wavelength light quanta (18). 

Several molecules which can undergo intramolecular proton transfer in the excited state have been found to be unusually photostable. Thus, for example, 2-hydroxybenzophenone is used as a photostabilizer in polymers while benzophenone itself is photoactive (Kysel, 1969). In crystalline 2-(2 -hydroxyphenyl)benzothiazole and its derivatives [3] a proton is transferred in the excited state from an... 

Thermal dehydration of o- (acylamino)phenols is the method of choice for the preparation of benzoxazoles (equation 96) and other annulated oxazoles. 0,iV-Diacyl derivatives of o-aminophenols cyclize at lower temperatures than do the monoacyl compounds. The synthesis is often carried out by heating the aminophenol with the carboxylic acid or a derivative, such as the acid chloride, anhydride, an ester, amide or nitrile. The Beckmann rearrangement of oximes of o-hydroxybenzophenones leads directly to benzoxazoles (equation 97). 

Polyfunctional polycondensates were prepared from formaldehyde and a mixture of two different low molecular weight stabilizers, e.g. of 2-hydroxybenzophenone or its derivatives with various 4-alkylphenols [170], e.g. stabilizer 133 (a combination of LS and AO), of a phenol with benzoxazole [171] or 4-nonylphenol and... 

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