1,3-Dimethylisoguanine.

Methylated purine bases are frequently isolated from marine sponges and ascidians, and their presence is often associated with pronounced biological activity. Rapid dereplication of these compounds in polar extracts of sponges is complicated by dif®culties in their characterization. One such example is 1,3-dimethylisoguanine, (I), which has been described by three different groups but with differences in the C NMR and MS data published in each report (Chehade et al., 1997; Mitchell et al., 1997; Jeong et al., 2003). In addition to the form shown as (I), other tautomers, namely (II) and (III), are also possible.