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To a 0.1 M solution of tetrabenazine compound 17 in ethanol at 0° C. was added NaBH4 (2.85 eq). The reaction mixture was allowed to stir for 60 min. at room temperature. The excess solvent was carefully removed under reduced pressure, and the residue was taken up in dichloromethane and washed with three portions of saturated aqueous K2CO3. The aqueous washings were back extracted with two portions of dichloromethane. The combined organic extracts were dried (MgSO4), filtered, and concentrated under reduced pressure to provide a yellow foam. Purification of the crude product dihydrotetrabenazine compound 24 was achieved by chromatography on SiO2 (2.5-5% MeOH—CH2Cl2, elution was observed at 285 nm). The product 24 was a colorless foam 69%: 1H NMR (CD2Cl2) δ 0.99 (s, 6H), 1.02-1.06 (m, 1H), 1.16 (s, 9H), 1.48 (dd, J=20.2, 11.4 Hz, 1H), 1.63-1.82 (m, 4H), 2.06 (apparent t, J=11.4 Hz, 1H), 2.47 ((ddd, J=3.8, 10.6, 10.6 Hz, 1H), 2.60 (ddd, J=12.0, 3.4, 2.3 Hz, 1H), 2.68 (apparent br d, J=15.4 Hz, 1H), 2.96-3.04 (m, 1H), 3.05-3.14 (m, 2H), 3.17 (apparent br d, J=11.4 Hz, 1H), 3.31 (ddd, J=9.3, 9.3, 4.3 Hz, 1H), 3.85 (s, 6H), 3.87-3.92 (m, 2H), 6.66 (s, 1H), 6.75 (s, 1H), 7.43-7.56 (m, 6H), 7.76-7.86 (m, 4H); 13C NMR (CD2Cl2) δ 19.10, 26.83, 27.67, 27.77, 29.28, 32.73, 39.98, 40.64, 42.21, 44.66, 49.89, 51.75, 55.77, 55.94, 61.02, 61.24, 62.71, 73.88, 108.46, 111.79, 126.62, 127.76, 129.70, 134.10, 135.68, 147.44, 147.69. A small amount of the ring position-2 epimer of dihydrotetrabenazine compound 24 was isolated in about 12% yield and was characterized. The epimeric product, 2-epi-24, was a pale yellow oil: 1H NMR (CD2Cl2) δ 0.92 (s, 6H), 0.96-1.02 (m, 2H), 1.08 (s, 9H), 1.42 (dd, J=14.5, 4.7 Hz, 1H), 1.61-1.71 (m, 3H), 1.86-1.95 (m, 1H), 2.35 (apparent dt, J=13.7, 2.9 Hz, 1H), 2.43 (apparent t, J=11.6 Hz, 1H), 2.51 (ddd, J=11.4, 11.4, 3.9 Hz, 1H), 2.59-2.67 (m, 2H), 2.88-2.95 (m, 1H), 2.98-3.11 (m, 1H), 3.45 (br d, J=11.4 Hz, 1H), 3.76-3.88 (m, 8H), 3.94-4.01 (m, 1H), 6.61 (s, 1H), 6.67 (s, 1H), 7.40-7.54 (m, 6H), 7.68-7.81 (m, 4H); 13C NMR (CD2Cl2) δ 19.05, 26.72, 27.51, 29.31, 29.78, 32.81, 36.51, 39.36, 41.99, 44.53, 52.34, 55.77, 55.86, 55.96, 57.71, 61.16, 69.62, 108.45, 111.80, 127.19, 127.71, 129.64, 130.43, 134.12, 135.65, 147.30, 147.53. The minor epimer, 2-epi-24, was converted by a series of steps analogous to Method steps 19 (protection of the hydroxy methine group as a THP ether to provide 2-epi-26), 21 (removal of the t-butyldiphenylsilyl group to provide 2-epi-28), and 23 (reaction of the primary hydroxy group with DAST to provide 2-epi-30); and then removal of the THP ether protecting group in a step analogous to that described in Example 3 to provide the alpha-fluoroalkyl dihydrotetrabenazine 2-epi-32 (See Table 15), a compound identical in structure to compound 32 in all respects save the configuration at ring position-2 which is “S” rather than “R”. The intermediate 2-epi-28 was characterized by low resolution mass spectroscopy: LRMS-(ESI+) calcd for (C24H37NO5+H) [M+H]+ 448.31. found 448.26. Alpha-fluoroalkyl dihydrotetrabenazine 2-epi-32 was characterized by high resolution mass spectroscopy: HRMS-(ESI+) calcd for (C21H32FNO3+H) [M+H]+ 366.24445. found 366.24333.
