AbstractThe synthesis of biopterin labeled at the angular 8a position with13C is described. Cyano‐13C‐acetic acid was converted in three steps to benzyl 2‐aminocyanoacetate. Condensation of the amino cyano ester with the monoxime of 5‐deoxyl‐L‐arabinosone by a modification of the literature method afforded 2‐amino‐3‐benzyloxycarbonyl‐5‐(L‐erythro‐1′, 2′‐dihydroxypropyl) pyrazine‐1‐oxide‐2‐13C. This intermediate was condensed with guanidine to give biopterin‐8‐oxide‐8a‐13