The bicyclic ester (I) was reduced to alcohol (II) using LiBH4. Subsequent Swern oxidation of (II) afforded aldehyde (III). Wittig reaction of aldehyde (III) with methoxymethylene triphenylphosphorane gave rise to the enol ether (IV). Diels-Alder reaction of (IV) with tetrazine (V), with concomitant elimination of nitrogen, furnished the pyridiazine (VI). Finally, acid hydrolysis of the carbamate group of (VI) provided the title compound.