Water → Pyridine-2-carbaldehyde thiosemicarbazone

The corresponding pyridine-2-formaldehyde (23 or 28a-k, 10 mmol) was dissolved in ethanol-water (2:1 v/v, 150 mL). To the solution was added thiosemicarbazide (11 mmol). The solution was stirred for 30 min at ambient temperature. After addition of water (50 mL), the reaction mixture was stirred vigorously for 2 h at room temperature. The yellow precipitate was collected by filtration, washed with ethanol-water (1:4 v/v) and dried in vacuum to afford the corresponding pyridine-2-carboxaldehyde thiosemicarbazone (24, 29a-k). (2-Thiosemicarbazonomethylpyridin-3-yl)carbamic acid 2-bis(2-trimethyl silyl ethyl) phosphonooxy-5-chlorobenzyl ester (24). Following the above procedure, 23 (6.9 g, 12 mmol) gave 24 (4.9 g, 63%) as a yellow solid: