Guanosine labeled with 15N at N1, amino, and N7 and 13C at either C2 or C8 was oxidized by Rose Bengal photosensitization (singlet oxygen) in buffered aqueous solution. At pH > 7, spiroiminodihydantoin was the major product, while at pH < 7, guanidinohydantoin (Gh) was the principal product. 15N and 13C NMR studies confirmed that Gh was formed as a mixture of slowly equilibrating diastereomers. Experiments conducted in H218O indicated that Gh and Sp each contained one oxygen atom derived from O2 and one from H2O. Tandem mass spectrometry was used to identify the C4 carbonyl of Gh as the one labeled with 18O, supporting a mechanism involving attack of water at C5 of a dehydro-8-oxoguanosine intermediate.

Chemical investigation of the Philippine ascidian Perophora namei has resulted in the isolation of a novel polycyclic alkaloid, perophoramidine (1). The structure of 1 was determined by the interpretation of 1D/2D NMR and MS data. Dehalogenation of perophoramidine (1) by ammonium formate catalyzed transfer hydrogenation confirmed the type and number of halogen atoms present in 1.

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