A New Approach to the Enantioseparation of Betti Bases

 

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Abstract

An improved method for enantioseparation of racemic 1-(α-aminobenzyl)-2-naphthols has been developed by the reaction in situ of Betti base product mixtures with l-(+)-tartaric acid taken in a 1:1 ratio. The products of this reaction are (-)-1-(α-aminobenzyl)-2-naphthol tartrate, the acetal of benzaldehyde and tartaric acid as well as (+)-1,3-diaryl-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazine, which can be easily separated by crystallization.

References and Notes

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Separation Procedure of 1 Using l-(+)-Tartaric Acid. To 1.48 g (4.4 mmol) of 1 dissolved in 15 mL of dry CH₂Cl₂ at 35-40 °C was added a solution of 0.66 g (4.4 mmol) l-(+)-tartaric acid, 7 mL of abs. MeOH and 3 mL of CH₂Cl₂. The mixture was refluxed for 2 h, then kept at r.t. during 3 d, the crystalline sediments being periodically filtered off. The crystals so obtained were washed with a boiling mixture of CH₂Cl₂ and benzene (1:1), filtered and dried in vacuo. Yield: 0.75 g (85.4%) of (S)-(+)-1-(α-aminobenzyl)-2-naphthol tartrate (2). The mother liquor was evaporated in vacuo under dried argon, leaving a brown residue which was extracted by hot EtOAc (2 × 7 mL). The insoluble residue was isolated and recrystallized from a (4:1) mixture of benzene-diisopropyl ether. Yield: 0.45 g (61%) of (R)-(-)-1,3-diphenyl-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazine [(R)-(-)-1], mp 162-163 °C; [α]_D ²⁰ -170 (c 1.0, CHCl₃) {lit. [11] mp 163-164 °C; [α]_D ²⁰ +169 (c 1.0, CHCl₃) for S-(+)-1}. ¹H NMR (CDCl₃) δ (tautomer A) = 6.48 (s, CH-N), 8.64 (s, CH =N) ppm; δ (tautomer C) = 5.68 (s, CHN), 5.73 (s, CHO) ppm; δ (tautomer C) = 5.86 (s, CHN), 5.99 (s, CHO) ppm; δ (tautomers A + B + C) = 7.23-7.92 (m, CH_arom) ppm. Tautomeric mixture (%): A (27.5), B (63.2), C (9.3). From the EtOAc extract a solvent was removed in vacuo under dry argon. Yield: 0.25 g (48%) of crystalline residue 2,3-O-benzylidene tartaric acid (3), mp 127-128 °C. IR (KBr): ν = 1728 (C=O) cm^-1 (lit. [12] mp 128.5-129.5 °C). From 2 following a published procedure [8] (S)-(+)-1-(α-amino-benzyl)-2-naphthol was obtained; mp 136-137; [α]_D ²⁰ +56.0 (c 4.4, benzene) {lit. [8] mp 136-137 °C; [α]_D ²⁰ +56.0 ( c 4.4, benzene)}.