Hydrochloric Acid → Octahydropyrazino[2,1-c][1,4]thiazine 2,2-dioxide

To a suspension of hexahydropyrazino[2,1-c][1,4]thiazin-9(6H)-one 2,2-dioxide (D18; 165 mg, 0.81 mmol) in 1 ml of anhydrous THF were added successive aliquots of a 1M solution of borane THF complex in THF (total 20.4 ml, 20.4 mmol) and the reaction stirred at room temperature and 50° C., until disappearance of the starting material. Hydrochloric acid (5M, 2 ml) was added to the solution and stirring was continued at 50° C. for 12 h. The solvent was removed under reduced pressure and the residue loaded on a SCX cartridge, washed with methanol and eluted with 0.5M methanolic ammonia. The basic fractions were collected and the solvent removed leaving 172 mg of the target product, partially contaminated by the starting material.