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Please login to access the full content or check if you have access via3.4.1.2 Metal-Free Catalytic Conjugate Reduction of Enals
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Mase, N., Science of Synthesis: Water in Organic Synthesis, (2012) 1, 156.
Chemoselective conjugate reduction of enals 5 or 9 has been a quite challenging topic in organic synthesis. Conjugate reduction of enals 5 or 9 to the corresponding saturated aldehydes has been achieved either through unusual reducing agents or indirectly through hydrosilylation; however, recently, metal-free catalysts, generally called organocatalysts, have been introduced for the reduction of C=C bonds using a Hantzsch compound 7 or 11 as a hydride source under aqueous conditions. Nornicotine (6), a native component of tobacco, is found to catalyze the chemoselective conjugate reduction of β-monosubstituted enals 5 under very mild aqueous conditions via iminium catalysis (Scheme 4).[10] Aqueous asymmetric conjugate reductions of β,β-disubstituted enals 9 are achieved using a resin-supported N-terminal prolyl peptide 10 having a β-turn motif and a polyleucine tether. Polyleucine accelerates the reaction with high enantioselectivities by providing a hydrophobic microenvironment around the prolyl residue. Not only aromatic aldehydes but also aliphatic aldehydes show good reactivities with excellent enantioselectivities (Scheme 5).[11,12]
Meeeee 8 Meeee-Meee Meeeeeeee Meeeeeeee Meeeeeeee ee β-Meeeeeeeeeeeeee Meeee[88]
| M8 | Meeeeee Meeeeeeeee | Meee (e) | Meeee (%) | Mee |
|---|---|---|---|---|
| 8-MeMMM8M8 | 88% MMMM | 8 | 88 | [88] |
| 8-M8MM8M8 | 88% MMMM eee 88 eee% 8 | 8 | 88 | [88] |
| 8-MeM8MM8M8 | ee MMMM | 8.8 | 88 | [88] |
| 8-MMM8M8 | 8% MMMM | 8 | 88 | [88] |
Meeeee 8 Meeee-Meee Meeeeeeee Meeeeeeee Meeeeeeee ee β,β-Meeeeeeeeeeee Meeee[88,88]
| M8 | Meee (e) | ee (%) | Meeee (%) | Mee |
|---|---|---|---|---|
| Me | 88 | 88 | 88 | [88,88] |
| 8-eeeeeeee | 88 | 88 | 88 | [88,88] |
| 8-MeMM8M8 | 88 | 88 | 88 | [88,88] |
| 8-MeM8M8 | 88 | 88 | 88 | [88,88] |
| (MM8)8MM=MMe8 | 88 | 88 | 88 | [88,88] |
Meeeeeeeeeee Meeeeeeee
8-Meeeeeeeeeeee 8 (M8 = Meee); Meeeeee Meeeeeeee:[88]
Me e eeeeeeee eeeeee eeeeeeeeee eee eeee 8 (M8 = eeee; 88 ee), 8,8-eeeeeeeeeeeeeee-8,8-eeeeeeeeeeee eeee (8.8 eeeee) eee MeMMM8 (8.8 eeeee) eee eeeee MMMM (8–8.8 eM) eee eeeeeeeee eeeeee (888 eM, eM 8.8; 8.8–8 eM). Meeeeeeeeee (8; 88 eee%) eee eeeee eee eee eeeeee eee eeeeee eee eeeeee ee ee eeeeeee eeeeee (888 eee) ee 88 °M eee 8.8–8 e. Meeee (8 eM) eee eeeee eee eee eeeeeee eee eeeeeeeee eeee MM8Me8 (8 × 8 eM). Mee eeeeeee eeeee eee eeeeeeeeeeee eee eee 8-eeeeeeeeeeee 8 (M8 = eeee) eee eeeeeeee ee eeeeeeeeeee MMM (MMMe8/eeeeeee ee eeeeeee/MeMMe); eeeee: 88–88%.
8-Meeeeeeeeee Meeeeeee 88; Meeeeee Meeeeeeee:[88,88]
Me e eeee ee eee eeee 8 (8.88 eeee) ee MMM (8.88 eM) eee eeeeeeeee M8M (8.88 eM) eee eeeee eee eeeeeee eeeeeeee MMM eeee 88 (888 ee, 8.88 eeee ee eeeeeeee eeeeee eeeee) eee Meeeeeee eeeee 88 (8.88 eeee). Mee eeeeeee eee eeeeeee ee ee eee 88–88 e. Meee, eee eeeeeeee 88 eee eeeeeeeee ee eeeeeeeeee eee eeeeee eeee MMM. Mee eeeee eeeeeee eee eeeeeeee ee eeeeeeee eee eeeeeee eeeee eeeeeee eeeeeeee. Meeeeeeeeeee ee eeeeeeeeeee MMM (eeeeee/MeMMe 8:8) eeeeeeee eee eeeeeeeeeeee eeeeeee 88; eeeee: 88–88%.
References
| [10] | Meeeee, M. M.; Meeeeeeee, M. M.; Meeee, M. M., Meee. Meeeee. (Meeeeeeee), (8888), 8888. |
| [11] | Meeeeee, M.; Meeeeee, M.; Meeeeeee, M.; Meee, M., Meeeeeeeeee: Meeeeeeee, (8888) 88, 888. |
| [12] | Meeeeee, M.; Meeeeee, M.; Meeeeeee, M.; Meee, M., Mee. Meee., (8888) 88, 8888. |
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