A mixture of 1,1-dimethylethyl {1-[(4,9-dioxo-1,2,8,9-tetrahydro-4H,7H-imidazo[1,2,3-ij]-1,8-naphthyridin-2-yl)methyl]-4-piperidinyl}carbamate (2:1 mixture of R:S) (1.83 g, 4.55 mmol) and DDQ (1.6 g, 6.83 mmol) in 1,4-dioxane (100 ml) was stirred at 60° C. under argon overnight. More DDQ (1.6 g, 6.83 mmol) was added and the reaction was stirred at 60° C. for another 1 h. The reaction was cooled to room temperature, treated with 5% aqueous solution of potassium carbonate (600 ml) and extracted with 20% methanol/dichloromethane (3×500 ml). The combined organic extracts were dried over magnesium sulphate, filtered and evaporated to afford the crude as a brown oil. LCMS showed that there was still ˜8% of starting material left so the oil was combined with more starting material (220 mg) recovered from the aqueous in the previous step and was dissolved in 1,4-dioxane (100 ml), treated with 1 eq. of DDQ and heated at 60° C. for 1 h. LCMS showed that the reaction was not complete so 0.5 g of DDQ was added and the reaction was stirred at 60° C. for 0.5 h. A small work-up showed that the reaction was complete so the reaction was treated with 5% aqueous solution of potassium carbonate (500 ml) and extracted with 20% methanol/dichloromethane (2×500 ml). The combined organic extracts were dried over magnesium sulphate, filtered and evaporated to afford the product as a light brown foam (1 g, 50%).
