N-Boc-L-(4-Pyridyl)alanine (I) is deprotected and esterified by means of a methanolic solution of SOCl2 to produce amino ester (II). Subsequent acylation of (II) with benzoyl chloride affords benzamide (III). Ester group reduction in (III) by means of NaBH4 yields alcohol (IV). The amide function of (IV) is then hydrolyzed under acidic conditions to furnish (S)-2-amino-3-(4-pyridyl)-1-propanol (V). Coupling of amino alcohol (V) with the known 6-(methylthio)purine (VI) in hot pyridine leads to adduct (VII). Intramolecular cyclization of the purine alcohol (VII) employing thionyl chloride produces the target tricyclic compound, which is finally isolated as the corresponding D-tartrate salt.
In a related method, the purine thioether (I) is oxidized to sulfone (II) by means of oxone under phase-transfer conditions. Coupling of sulfone (II) with the pyridyl amino alcohol (III) yields adduct (IV), which is finally cyclized to the target tricyclic compound upon treatment with thionyl chloride.