H

"OH

Scheme 72

uv irradiation (as well as hematoporphyrin photosensitisation) of methotrexate (261) leads to benzylic cleavage. The products include 2,4-diaminopteridincaboxyladehyde (262) and the corresponding acid (263) as well as 4-aminobenzoylglutamic acid (264) (in turn oxidised to the azo derivative by sensitisation by the pteridinaldehyde, see Scheme 73).240"242

Doxorubicin (265) and daunorubicin (266) are photolabile.243' 244 In particular, the latter drug undergoes homolytic deacylation.245 The antitumor antibiotic hedamycin (267) reacts photochemically via initial intramolecular hydrogen abstraction from the tetrahydropyranyl ring in position 8; in the presence of oxygen the final product is the corresponding dihydropyranyl derivative (268), while in nitrogen-flushed solution this process is accompanied by reductive cleavage of an epoxide ring of the chain at C-2 to give products (269) and (270) (see Scheme 74); the related derivative kidamycin reacts similarly.246 The glycopeptides bleomycins247"249 and the protein zinostatin,250 both used as antineoplastics, are photoreactive.

Scheme 73

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