This process occurs through a radical pathway,100-101 and causes reduction to (95), substitution to give (96) as well as dimerisation and/or polymerisation. In the presence of oxygen, oxidation to the sulfoxide (97) and the vV-oxide (98) takes place.102"104 It has been proved that singlet oxygen is formed under these conditions.105 Thioridazine (99) is likewise S and N-oxidised (as well as iV-dealkylated) upon irradiation in aqueous solution.106 Several other phenothiazines of this type have been investigated.107 The relative photoreactivity has been found to depend on the ring substituent X and its liability to substitution.108 B. D. Glass has carried out an in depth study of various phenothiazines, and has established a photoreactivity versus activity relationship (p. 134).

2.2.4 Other Psychotropic Agents. The tricyclic antidepressants doxepin (100) and dothiepin (101) are photodegraded upon exposure to natural, and more rapidly, to artificial light. Besides E/Z isomerisation, the processes occurring for the former drug are oxidation of the side-chain to give (102) and (103).109 For the latter compound either cleavage and rearrangement to give (104) or S-oxidation to (105) are observed according to conditions (see Scheme 31).110 The related dibenzocycloheptene protriptyline, also an antidepressant, reacts at the double bond C^-Cn: the epoxide is formed by irradiation of the hydrochloride in water in the presence of oxygen,111 and a [2+2] dimer by irradiation in water or ethanol under nitrogen.112

The solid-state photodegradation of carbamazepine (106), an anticonvulsant drug, has been found to depend on the polymorf considered.113 Another anticonvulsant, phenyltoin (107), has been found to undergo cleavage of the heterocyclic ring upon irradiation in methanol, and to give benzophenone (108) and benzyl (109) (Scheme 32).114

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