Modification of Stereo and Enantioselectivity

The most important aspect of the synthesis of organic molecules that contain one or more stereogenic elements is usually that of stereochemical control. Indeed, such control is an essential aspect of the design of a good synthesis and, in addition, affects selection of the most appropriate method - including the choice of a particular route [83]. Several reports have described how microwave irradiation can be used to modify stereoselectivity in some reactions. The possibility of stereochemical modification by simply changing the mode of activation is a very attractive prospect. Slight modification of the stereoselectivity in the Rodionov reaction has been reported by Romanova [80]. 

This effect was explained by Cossio, who postulated that under the action of microwave irradiation the route involving direct reaction between the acyl chloride 

Synergism between microwave irradiation and enzyme catalysis has been ascribed to reduction of the Michaelis constants by use of microwaves, without any associated change in the form of the rate equation [93]. 

A similar reduction in stereoselectivity in ethyl alcohol was observed by Touko-niitty [99] in the hydrogenation of ethyl pyruvate to ethyl lactate, while similar stereoselectivity was observed in toluene. The authors consider that this result is a con- 

In conclusion, it has been extensively shown that microwave irradiation is a selective mode of activation. Radiation is selectively absorbed by polar molecules in the presence of apolar molecules, a property that leads to selective thermal gradients. 

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