Metformin hydrochloride (3.97 g, 24 mmol) was added to a
solution of NaOH (0.960 g, 24 mmol) in ethanol (60 ml) and the
resulting suspension was stirred for 2 h. Then, it was filtered. In
the second step, dipicH2 (2.00 g, 12 mmol) was added to the filtered
solution. Then, the reaction mixture was stirred, heated to
50–60 C while adding 65 ml ethanol to obtain a homogenous solution.
The solid L compound 3.20 g (63% yield), was subsequently
precipitated by slow evaporation of solvent at room temperature,
m.p.: 190 C.