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thiazazincolidine complexes 18, 24 and polymer-supported sulfonamidoalcohols 19. 25 Recently, Mukaiyama and co-workers reported the highly enantioselective reduction of ketones and imines with sodium borohydride in the presence of 1 mol % of chiral β-oxoaldiminatocobalt(II) complexes 20. 26 3. Asymmetric Reduction of Functionalized Ketones 3.1. Hydroxy Ketones and Diketones The stoichiometric, asymmetric reduction of α- and β-hydroxy ketones with ent-3 or 4 proceeded in excellent enantiomeric excess 6 VOL. 35, NO. 1 • 2002 Diol Reduction Conditions Yield (%) % de % ee 11 (B-OMe) (0.1 equiv), BMS (1.0 equiv), rt 91 >95 >98 13a (0.1 equiv), BMS (1.4 equiv), rt 58 94 4 (2 equiv), -78 °C to rt 93 >98 >98 (ee) via an intramolecular β-hydrogen shift (eq 1). 8b Recently, we reported an efficient and very highly enantioselective method for the preparation of optically active terminal 1,2-diols via the oxazaborolidine-catalyzed borane reduction of α-silyloxy or tetrahydropyranyloxy ketones (eq 2). 20a,27 A comparison of the enantioselectivities of structurally diverse oxazaborolidines 10−15 in the reduction of 2-tert-butyldimethylsilyloxyacetophenone showed that 11a provided the best result: 1-phenyl-1,2-ethanediol was obtained with near 100% ee. Very high ee’s were also observed in the same reduction of aromatic analogs using 11a as the catalyst. eq 1 eq 2 eq 3 eq 4 The first successful use of the more stable Nethyl-N-isopropylaniline-borane complex (BACH-EI TM ), instead of BH3 or BMS as the borane carrier (Method C), made the reduction more practical for large-scale applications. 20a Chiral ligand 16 was employed as part of an effective catalyst for the reduction of α-protected hydroxy ketones. 22b The CBS oxazaborolidinecatalyzed reduction of 1,2-diketones, in particular benzil derivatives and heterocyclic analogs, provided optically active (S,S)hydrobenzoins with both high enantiomeric and diastereomeric excesses (ee and de) (eq 3). 28 Similarly, the oxazaborolidine-catalyzed
orane reductions 29,30 of 1,4-diphenyl-1,4butanedione and its reduction with 4 31 produced the corresponding 1,4-diol with high ee’s and de’s (eq 4). 3.2. α-Halo and α-Sulfonyloxy Ketones The asymmetric reductions of α-bromoand α-chloroacetophenone, among the α-halo ketones, have been the most studied (eq 5). Reductions with 4, 9 5, 32 and most of the oxazaborolidine- and other-ligandcatalyzed reductions19,22,23,25b,33,34 provided 2halo-1-phenylethanols with high ee’s. The chiral borohydrides 1 and 2 afforded 77% and 92% ee’s, respectively. 6,7 Reductions of trihalomethyl ketones with 11c/CB or with 4 afforded the corresponding trihalomethyl Styrene Oxide Reduction Conditions Yield (%) % ee 1 (1.1 equiv), -78 °C 82 77, S 2 (1.1 equiv), -78 °C 99 92, S 4 (1.2 equiv), -25 °C 90 96, R 5 (neat), rt 91 96, R 11a (0.02 equiv), BH3:THF (0.5 equiv), rt 97 96, S 13a (0.1 equiv), BMS (0.5 equiv), -20 °C >90 92, S 15b (0.1 equiv), BH3:THF (0.8 equiv), -20 °C >90 96, S 16 (0.1 equiv), BMS (1.2 equiv), rt 84 84, S 17 (0.1 equiv), BMS (1.0 equiv), 110 °C 91 94, S 19a (0.15 equiv), NaBH4–TMSCl, 65 °C 98 96, S Trihalomethyl Carbinols X R Reduction Conditions Yield (%) % ee F aryl 11c (0.1 equiv), CB (1.5 equiv), PhMe, -78 °C or -20 °C >90 94-100, R F aryl 4 (neat, 1.1 equiv), 25 °C 48-93 78-90, S F alkyl 4 (neat, 1.1 equiv), 25 °C 70-87 87-96, S Cl aryl 11c (0.1 equiv), CB (2.0 equiv), PhMe–THF, -70 °C 64-74 >98, R Cl alkyl 11c (0.1 equiv), CB (1.5 equiv), PhMe, -78 °C to 23 °C 94-97 92-98, R CB = catecholborane carbinols in 78–100% ee’s (eq 6). 35,36a 4 was also effectively employed for preparing 1,1,1-trifluorooxirane in high ee by reduction of 1-bromo-3,3,3-trifluoropropanone (eq 7). 36b Optically active halohydrins or styrene oxide derivatives, obtained by reduction of α-halo ketones, have been widely used as key intermediates in the synthesis of many chiral drugs containing the β-amino alcohol moiety (Figure 5). Examples of such drugs include: (R)-denopamine (21), 37 (R)-isoproterenol (22), 38 (R)-salmeterol (23), 39 and (R,R)-formoterol (24). 40 However, the use of α-halo ketones as starting materials in commercial applications has severe drawbacks: α-halo ketones cause skin and eye irritation and are unstable to light. These disadvantages were successfully overcome by using the more stable and nonirritating α-sulfonyloxy ketones. The CBS oxazaborolidine-catalyzed reduction of α-sulfonyloxy ketones using BACH-EI TM as the hydride source provided the corresponding 1,2-diol monosulfonates and terminal epoxides in very high ee’s (eq 8). 20e The reduction with 4 also afforded high enantioselectivities. 41 Using this methodology, chiral drugs such as (R)-21, (R)-nifenalol (25), and (R)-pronethalol (26) were prepared highly enantioselectively by direct amination of the corresponding optically active 1,2-diol monosulfonates. 42 Moreover, enantiopure (1S,2R)-indene oxide (27)—which is a key starting material for the synthesis of indinavir (28), a highly effective HIV protease inhibitor—has been efficiently prepared by the same reduction from 2-(p-toluenesulfonyloxy)-1-indanone (eq 9). 20f VOL. 35, NO. 1 • 2002 eq 5 eq 6 eq 7 7
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Page 5 and 6: Boron-Based Reducing Agents for the
Page 7: Figure 3. Stoichiometric Boron-Base
Page 11 and 12: α-Hydroxy Acetals R Reduction Cond
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Page 15 and 16: Other reductions using 14, 64c,d 16
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Page 19 and 20: The synthesis of biaryl compounds v
Page 21 and 22: 52,396-8 [87199-18-6] 3-Hydroxyphen
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Page 41 and 42: from 2- and 3-carene turned out to
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Page 45: Abstr. 1968, 68, 29743j. (11) Refer
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