Photo-reduction of Pteridines

Abstract

THE recent communication by Prof. A. Albert¹ on the photo-reduction of pteridines prompts me to record another example. In the preparation of 2-amino-4-hydroxy-pteridine-6-aldehyde by the method of Waller et al. ² it was found that if the initial decomposition of pteroyl-L-glutamic acid to dihydro-2-amino-4-hydroxy-pteridine-6-aldehyde was done in the dark only a poor yield was obtained (as judged by the amount of 2-amino-4-hydroxy-pteridine-6-aldehyde formed on subsequent oxidation). If the reacting solution was irradiated with light from a 100-W. bulb then a good yield was obtained. It thus seems that the decomposition of pteroyl-L-glutamic acid to the dihydro aldehyde is a light-catalysed intramolecular hydrogen transfer from the 9 : 10 position followed by hydrolysis of the resulting anil. It has been previously reported that pteroyl-L-glutamic acid is decomposed by ultra-violet light to 2-amino-4-hydroxy-pteridine-6-aldehyde³. The position of the hydrogen atoms of the dihydro 2-amino-4-hydroxy-pteridine-6-aldehyde in the pyrazine ring has not been established; but this aldehyde would seem to be a possible intermediate in the biological synthesis of the yellow fluorescent eye pigment⁴ of Drosophila and possibly also of the red pteridine pigments⁴. Tho primary step in the degradation of pteroyl-L-glutamic acid would seem to be hydrogen transfer intramolecularly to the pyrazine ring or intermolecularly to a suitable hydrogen acceptor (for example, oxygen⁵ or methylene blue⁶).