To a suspension of hexane washed 60% sodium hydride in mineral oil (91 mg, 2.3 mmol) in DMF (2.5 mL) under nitrogen atmosphere to 0° C. is added a solution of diethyl 1H-indole-2,6-dicarboxylate (619 mg, 2.4 mmol) in DMF (4 mL). The mixture is stirred for 20 min at 0° C. and a solution of tert-butyl (6R)-6-methyl-1,2,3-oxathiazinane-3-carboxylate 2,2-dioxide (518 mg, 2.1 mmol) in DMF (2.5 mL) is added. The reaction mixture is stirred for 30 min at 0° C. and is warmed to room temperature and stirred for 72 h. Water and NH4Cl solution are added and the mixture is stirred for 15 min. The aqueous mixture is extracted with EtOAc (50 mL) and the organic layer is washed with water, brine, dried (MgSO4) and concentrated to afford the title crude compound (1.1 g) which is used in the next step without purification.
