A mixture of N,N′,N″-(boroxin-2,4,6-triyltris{[(1R)-3-methylbutane-1,1-diyl]imino[(2S)-1-oxo-3-phenylpropane-1,2-diyl]})tripyrazine-2-carboxamide (0.270 g, 0.246 mmol) and L-tartaric acid (149.5 mg, 0.33 mmol) were mixed in acetone (3 mL). The mixture was heated to form a solution. The solution was cooled uncontrolled until the internal temperature was about 25° C. Heptane (2.5 mL) was added. White solid precipitated out and the resultant slurry was agitated at ambient temperature for 1.5 h. The slurry was filtered to collect solid (R)-2-hydroxy-2-((R)-2-((R)-3-methyl-1-((S)-3-phenyl-2-(pyrazine-2-carboxamido)propanamido)butyl)-5-oxo-1,3,2-dioxaborolan-4-yl)acetic acid (0.388 g) which also contained a dimeric species. MS (m/z) in CH3CN: [M+H] calculated for C23H28BN4O8, 499.2000. found, 499.2.
