Biomimetic reduction

An intriguing use of a quaternary ammonium salt in a two-phase reaction is to be found with the regeneration of 1 -benzyl-1,4-dihydronicotinamide by sodium dithionite in a biomimetic reduction of thiones to thiols [12], The use of sodium dithionite in the presence of sodium carbonate for the 1,4-reduction of the pyri-dinium salts to 1,4-dihydropyridines is well established but, as both the dithionite and the pyridinium salts are soluble in water and the dihydropyridine and the thione are insoluble in the aqueous phase and totally soluble in the organic phase, it is difficult to identify the role of the quaternary ammonium salt in the reduction cycle. It is clear, however, that in the presence of benzyltriethylammonium chloride, the pyridine system is involved in as many as ten reduction cycles during the complete conversion of the thione into the thiol. In the absence of the catalyst, the thione is recovered quantitatively from the reaction mixture. As yet, the procedure does not appear to have any synthetic utility. 

Scheme 6.25 Product range of the biomimetic reduction of a.P-unsaturated nitroalkenes catalyzed by 9 in the presence of 19.

Scheme 6.26 Proposed model of the biomimetic reduction of conjugated nitroalkenes in the presence of thiourea catalyst 9 and 19.

Biomimetic reduction enamides by a model of NADH was also carried out in the presence of magnesium salts to afford the corresponding saturated compounds221 (Scheme 144). 

Biomimetic Reductions Amino Acid Dehydrogenases as the Role Model for the Brpnsted Acid Catalyzed Transfer Hydrogenation... 

Since CIgSiH is known to be activated by DMF and other Lewis bases to effect hydrosilylation of imines (Scheme 4.2) [8], it is hardly surprising that chiral formamides, derived from natural amino adds, emerged as prime candidates for the development of an asymmetric variant of this reaction [8]. It was assumed that, if successful, this approach could become an attractive altemative to the existing enzymatic methods for amine production [9] and to complement another organo catalytic protocol, based on the biomimetic reduction with Hantzsch ester, which is being developed in parallel [5]. 

Volume E 21 D.2.3.6. Enzyme-Catalyzed and Biomimetic Reductions of Carbonyl Groups 4143... 

S )-Enantiomcrs of fluorophcnylalanincs and 4-(trifluoromethyl)phenylalanine were successfully prepared from the corresponding 2-oxo acids by the transfer of an amino group from (S)-aspartic acid catalyzed by a specific transaminase of microbial origin20. The biomimetic reduction of other imines with NAD coenzymes has also been described21 28. 

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