N[C@@H:2]([C:7]([OH:9])=[O:8])[C@H:3]([CH2:5][CH3:6])[CH3:4].N([O-])=[O:11].[Na+].C([O-])(O)=O.[Na+].[Na+].[Cl-]>OS(O)(=O)=O.O>[OH:11][C@H:2]([C@@H:3]([CH3:4])[CH2:5][CH3:6])[C:7]([OH:9])=[O:8]To a solution of (-)-D-allo-isoleucine [1.00 g, 7.634 mmol, [α]D20 =-37°(5% in 6M HCl)] in 26 ml of 1N aq. H2SO4 at 2° was slowly added (ca. 12 h) a solution of NaNO2 (1.67 g, 24.13 mmol) in 26 ml of demineralised water. After stirring overnight at room temperature, 0.62 g of powdered NaHCO3 were added to bring the pH above 2. The solution was saturated with NaCl and extracted 6 times with ethyl acetate, maintaining the pH between 2 and 3 (1N aq. H2SO4, pH meter). After drying (Na2SO4), the organic layer was concentrated and the crude product (0.91 g, 91%) was directly used in the next step without further purification.
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