First Total Synthesis and Structural Elucidation of Microcystbiopterin C, a Pterin Glycoside From a <i>Microcystis</i> Bloom

ABSTRACT The first total synthesis and unambiguous structural elucidation of microcystbiopterin C, a pterin glycoside isolated from a Microcystis bloom, has been achieved. Two candidate biopterin glycosides ( 5a and 5b ) corresponding to the proposed structures of microcystbiopterin C were synthesized by glycosylation of a suitably protected biopterin derivative 10 with 2,4‐di‐ O ‐benzoyl‐6‐deoxy‐3‐ O ‐methyl‐α‐L‐mannopyranosyl bromide ( 9 ) and the corresponding α‐D‐mannopyranosyl bromide ( 22 ), respectively, followed by stepwise deprotection. Detailed comparison of the NMR data and the specific rotation of compounds 5a and 5b with those of the natural product established that microcystbiopterin C possesses the structure of 2′‐ O ‐(6‐deoxy‐3‐ O ‐methyl‐α‐D‐mannopyranosyl)biopterin ( 5b ).