Diethyl azodicarboxylate (14.63 g, 84 mmol) was added dropwise to a solution of (R)-7-benzyl-1-(5-hydroxyhexyl)-3-methylxanthine (as prepared in Example 29) (20 g, 56 mmol), 4-nitrobenzoic acid (14 g, 84 mmol) and triphenylphosphine (22 g, 84 mmol) in tetrahydrofuran (200 ml). After stirring for 30 minutes, the reaction mixture was concentrated under reduced pressure. The residue was purified by flash chromatography on silica gel eluting with 30% ethyl acetate-hexane to provide (S)-7-benzyl-1-(5-(4-nitrobenzoyloxy)hexyl)-3-methylxanthine (25 g).
