A solution of 1.66 g of benzyl 4-oxopiperidine-1-carboxylate and 1 g of methyl 2-amino-5-({[(1,1-dimethylethoxy)carbonyl]amino}methyl)benzoate obtained in stage 5.1 in 20 ml of DCM is added dropwise at AT to a suspension of 2.04 g of NaBH(OAc)3 in a mixture of 20 ml of DCM and 0.41 ml of AcOH. The mixture is stirred at AT for 15 h 00 and then a further 2.04 g of NaBH(OAc)3 are added. After stirring for 6 h 00, 1.66 g of benzyl 4-oxopiperidine-1-carboxylate are added and the mixture is stirred at AT for 48 h 00. A saturated NaHCO3 solution is added and extraction is carried out with DCM. The organic phase is washed with a saturated NaHCO3 solution and twice with a saturated NH4Cl solution. It is dried over Na2SO4 and filtered, and the filtrate is evaporated under reduced pressure. The residue is chromatographed on silica gel, elution being carried out with an AcOEt/heptane mixture, (5/95, v/v) as far as (40/60, v/v), to give 1.6 g of the expected product.
