Unsaturated 5 dehydroamino acid amides

The mechanism of the aminolysis and the electronic effects of substituents at C-2 or C-4 on the kinetics of amide bond formation have been studied. In some cases, ring opening with amines occurs with partial isomerization of the exocyclic double bond. However, with more hindered compounds, such as unsaturated oxazolones derived from ketones, ring opening is stereospecific.Ring opening using diamines has also been described. Selected examples of dehydroamino acid amides prepared by aminolysis of unsaturated 5(4//)-oxazolones are shown in Table 7.40 (Fig. 7.51). 

On the other hand, if a 2-phenyl-5(4//)-oxazolone 547 is the starting material, A -(benzoyl)dehydroamino acid amides 548 or unsaturated imidazolones 549 are... 

TABLE 7.40 N-ACYL DEHYDROAMINO ACID AMIDES FROM AMINOLYSIS OF UNSATURATED 5(4//)-OXAZOLONES, 237... 

The low ee-values obtained with simple unsaturated acids as compared to the enamides of dehydroamino acid derivatives show that the oxygen atoms of the amide is a key to complex formation with the metal center. Knowles also proposed a quadrant model that has been adapted for many reactions [5, 22]. The mechanism of the reaction has been investigated, and it is known that the addition of the substrate to the metal is regioselective and that competing catalytic cycles can occur [5, 10, 22, 25, 27, 30-46]. 

One ligand system which was developed during the late 1990s, and has proven quite versatile for the reduction of a wide variety of unsaturations, including a-and /i-dehydroamino acids, arylenamides and MOM-protected /1-hydroxy en-amides, is the rigid BICP (26) [113-118],... 

Complexation of a/S-unsaturated acids and amides has been extensively studied/ In such cases, both diastereomers of a species involving coordination of both olefin and carboxylate group are observed, usually in comparable proportions. The extent of complexation is enhanced and the rate of exchange of free and bound substrate reduced when base is added, or a preformed tetraalkylammonium carboxylate is employed. This suggests that structures (62) and (63) best describe the complex. The spectra observed are almost identical to those derived from E-dehydroamino acid derivatives, implying that the latter have structure (64) in accord with labeling studies. ... 

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