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.
Key words
enantioseparation - 1-(α-aminobenzyl)-2-naphthols - 1,3-diaryl-2,3-dihydro-1H -naphth[1,2-e ][1,3]oxazine - tartaric acid
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 CH2 Cl2 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 CH2 Cl2 . 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 CH2 Cl2 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 D
20 +169 (c 1.0, CHCl3 ) for S -(+)-1 }. 1 H 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, CHarom ) 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 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 D
20 +56.0 (
c
4.4, benzene)}.
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