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Scope of Enamide Substrates

Although it has some limitations, the Rh-DIPAMP system developed by Knowles has been well studied and this allows for a fair degree of certainty that it can be used with untried substrates. For this reason, it has been used to prepare a range of unnatural amino acid derivatives at scale [7, 11]. A model has been reported that allows others to predict whether Knowles catalyst can be used in a productive manner with their desired enamide. The prediction of the model is based on dividing the enamide substrate into four quadrants (Fig. 2) [7]. [Pg.260]

If we start at the top left-hand corner and work round anti-clockwise, the scope and limitations of the system can be explained. This quadrant has to be hydrogen. Any substitution results in a drop in the reaction rates and a significant lowering of enantioselectivity. Thus, / -branched a-amino acid derivatives cannot be accessed with Knowles catalyst. This limitation has been overcome by the use of other catalyst systems such as Burk s DuPHOS [12]. The need for hydrogen at this position [Pg.260]

The final quadrant, the top right-hand, has to contain an electron withdrawing group. This group activates the carbon-carbon unsaturation. A wide variety of groups are tolerated including a carboxylic acid, ester, amide, nitrile, ketone, and aldehydes [7, 9]. This shows that the catalyst system can be used to prepare a wide range of amino acid derivatives without recourse to a number of transformations on the product unnatural amino acid. [Pg.261]

Many of the ligands that have been advocated for analogous reductions provide high enantioselectivity and some can provide good turnover numbers and frequencies [4, 5], Knowles catalyst often results in an ee of about 94-95% if the reaction mixture is monitored. However, crystallization of the N-acylamino acid product often results in enantioenrichment [10, 11]. In addition, hydrolysis of the amide to provide the amino acid itself also provides an opportunity for enantioenrichment (Fig. 3) [10]. [Pg.261]

The enantioselectivities did not change upon decrease of the catalyst loading (S/ C) from 5 000 to 20000 in the reduction of the 2-naphthyl enamide (2a), but the [Pg.261]


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Enamide

Substrate Scope

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