Synthesis of N-(2-hydroxy-3-methoxybenzylidene) - 2, 3, 4, 6-tetra-O-acetyl -β-D galactopyranosylamine as a new chiral Schiff base for asymmetric [2+2] cycloadditions

The importance of carbohydrates for asymmetric synthesis is well recognized [1,2,3,4].The reason that carbohydrates contain too many chiral centers and functional groups has limited their research about stereodifferentiating selection processes in asymmetric synthesis for a long time. During the past decades few carbohydrates have received increasing attention as stereodifferentiating auxiliaries in stereoselective syntheses [5]. Carbohydrates have been applied for stereo- and regioselective chemical reactions for last two decades. They could be used for biological recognition on membrane, and they play a prominent role in numerous biological processes such as tumor-cell growth [6], bacterial and viral infection or inhibition of glycosidases [2,7,8,9]. Glycosylamines are valuable intermediates in the preparation of nucleosides and drugs [10,11,12]. It has been proposed that when the Schiff base is derived from an optically active amine and an achiral aldehyde, the degree of distereoselectivity in the [2+2] cycloaddition varies [13,14]. The asymmetric Staudinger reaction utilizing 2,3,4,6-tetra-O-acetyl-β-D-galactopyranosylamine and 2,3,4,6-tetra-O-pivaloyl-β-D-galactopyranosylamine as the chiral auxiliary in the synthesis of 2-azetidinones has been reported by us and others [15,16,17,18]. We now report compound 6 as a new chiral auxiliary Schiff base for β-lactam syntheses.

D-(+)-Glucose 1 was converted to 2, 3, 4, 6-tetra-O-acetyl-β-D-galactopyranosylbromide 2 by a reported method [18]. Treatment of 2 with NaN₃ in 9:1 aceton-water afforded galactosylazide 3 in 95% yield. Reduction of 3 by hydrogenation in the presence of Raney Ni gave 2, 3, 4, 6-tetra-O-acetyl-β-D-galactopyranosylamine as a white solid which was recrystallized from ethanol. o-Vanillin 5 (0.87 g, 5.71 mmol) was added to a solution of 2,3,4,6-tetra-O-acetyl-β-D-galactosylamine 4 (2.00 g, 5.76 mmol) in ethanol (35 ml). The mixture was refluxed for 5 h. The resulting yellow solid N-(2-hydroxy-3-methoxybenzylidene) - 2,3,4,6-tetra-O-acetyl -β-D galactopyranosylamine 6 was collected in 90% yield by filtration.

${\left[\mathsf{\alpha }\right]}_D^{22}$ = + 24.5 (c = 0.02, CHCl₃)

Melting point: 180-182 °C.

IR (KBr, cm^-1): 3357.8-3620.1 (OH); 1743.5 (C=O); 1633.6 (C=N) cm^-1.

¹H-NMR (250MHz, DMSO-d₆): δ= 12.44 (OH, br, 1H); 8.53 (NCH, s, 1H); 7.30-6.82 (Ar-H, m, 4H); 3.84 (OCH3, s, 3H); 1.91-2.10 (4CH₃CO, s, 12H),

¹³C-NMR (62.9 MHz, DMSO-d₆): δ= 170.82-165.03 (C=O); 124.90-115.21 (Ar); 21.10-20.97(OCH₃).

MS (m⁄z, %): 481 (2.00); 331 (2.10); 169 (21.60); 109 (18.10); 43 (100.00).