To a solution of 1-t-butoxycarbonyl-4-hydroxypiperidine (3.62 g), 2-methyl-4-nitrophenol (2.55 g) and triphenylphosphine (5.25 g) in dichloromethane (100 ml) was added dropwise diethyl azodicarboxylate (3.2 ml) in an ice bath and the mixture was stirred at room temperature for 18 hours. The reaction mixture was concentrated in vacuo. The residue was purified by chromatography on a silica gel column using dichloromethane as an eluant to give the crude desired compound (4.07 g) as a pale yellow oil.
