DEDICATED TO PROFESSOR HC BROWN ON HIS ... - Sigma-Aldrich

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3.3. α-Keto Acetals and α-Keto Thioketals Chiral hydride 2 efficiently reduced aliphatic and aromatic α-keto acetals to the corresponding α-hydroxy acetals in 87−99% ee (eq 10). 43 The CBS reduction provided excellent ee’s for aromatic analogs but moderate ee’s for aliphatic ones. 20b Similarly, the CBS reduction of both acyclic and cyclic α-keto thioketals was highly enantioselective (eq 11). 44 3.4. Keto Acids, Esters, and Amides Treatment of α- and β-keto acids with 4 and triethylamine led to the corresponding Figure 5. Applications of Optically Active Halohydrins to the Synthesis of Chiral Drugs. 8 VOL. 35, NO. 1 • 2002 Epoxides Reduction Conditions Yield (%) % ee 11b (0.1 equiv), BACH-EI (1.0 equiv), 25 °C 94-96 94-99, S 4 (1.1 equiv), -25 °C 72-92 92-97, R α- and β-hydroxy acids in excellent ee’s. 45 Similarly, 3 successfully reduced α-, β-, and γ-keto acids to the corresponding hydroxy acids with very high ee’s (eq 12). 46 In the case of α-keto esters, both 1 6 and 5 32 produced high ee’s (eq 13). However, these reagents failed to reduce β-keto esters. 9 In contrast, the reduction was successfully achieved by mixed borohydrides, “(R)-Mix A” [(R)-7] and “Mix B” (8), obtained from the reaction of NaBH4 with (R,R’)-N,N’dibenzoylcystine and (2R,3R)-tartaric acid, respectively. (R)-7 11 produced the desired aromatic β-hydroxy esters in 84−92% ee, while 8 12 provided the aliphatic analogs in 68−84% ee (eq 14). “Mix C” (9), modified with (S)-tert-leucine, was effectively used to prepare the optically active α-hydroxy amide 29 as a key intermediate in the synthesis of chiral diltiazem (30), a potent calcium channel blocker (eq 15). 47 3.5. α,β-Enones and Ynones eq 8 The oxazaborolidine-catalyzed reduction of α,β-enones, using 11 35a,48 or 12 49 as the catalyst, has been extensively studied. The reduction is effective for all cases having acyclic, endocyclic, or exocyclic double bonds. To minimize the hydroboration of the C=C bond by borane, these reductions were carried out at low temperature. Interestingly, the olefinic portion in acyclic enones generally behaves as the large group for the reduction. The stoichiometric reagents, 1, 4,
α-Hydroxy Acetals R Reduction Conditions Yield (%) % ee aryl 2 (1.1 equiv), -78 °C 80-87 92-96, S alkyl 2 (1.1 equiv), -78 °C 70-80 87-99, S aryl 11b (0.1 equiv), DEANB (1.0 equiv), 25 °C 91-97 83-99, S alkyl 11b (0.1 equiv), DEANB (1.0 equiv), 25 °C 65-98 42-71, S α-Hydroxy Esters R Reduction Conditions Yield (%) % ee aryl 1 (1.1 equiv), -78 °C 78-85 89-96, S alkyl 1 (1.1 equiv), -78 °C 75-87 81-98, S aryl 5 (2.0 equiv, neat), 0 or 25 °C 89-95 83-92, R alkyl 5 (2.0 equiv, neat), 0 or 25 °C 50-98 75-92, R VOL. 35, NO. 1 • 2002 eq 9 eq 10 eq 11 eq 12 eq 13 9
Page 1 and 2: DEDICATED TO PROFESSOR H. C. BROWN
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Page 5 and 6: Boron-Based Reducing Agents for the
Page 7 and 8: Figure 3. Stoichiometric Boron-Base
Page 9: orane reductions 29,30 of 1,4-diphe
Page 13 and 14: and 5 provided moderate ee’s in t
Page 15 and 16: Other reductions using 14, 64c,d 16
Page 17 and 18: Masuda, H.; Ito, K.; Hirao, A.; Nak
Page 19 and 20: The synthesis of biaryl compounds v
Page 21 and 22: 52,396-8 [87199-18-6] 3-Hydroxyphen
Page 23 and 24: Josiphos is a family of chiral phos
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Page 35 and 36: B-O bond. 15 The mechanism involves
Page 37 and 38: Scheme 4. Tartrate Ester Auxiliary
Page 39 and 40: eagent with aldehydes at low temper
Page 41 and 42: from 2- and 3-carene turned out to
Page 43 and 44: Scheme 22. Application of Crotylbor
Page 45: Abstr. 1968, 68, 29743j. (11) Refer
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