Epoxides advantages/disadvantages

The use of a polyamino acid such as polyleucine as a catalyst for the asymmetric epoxidation of a,/l-unsaturated ketones is clearly established. The advantages and disadvantages of this methodology may be summarised as follows ... 

In conclusion, each of the three synthetic approaches to 256 has its advantages and disadvantages. Base-catalyzed isomerization of epoxides is certainly the simplest method however, it is not applicable in all cases. The use of a specific Base, not prone to side reactions, would be the best approach to the synthesis. 

The third method makes use of the one-flask procedure, which is advantageous from the preparative point of view. However, opening of certain stereoisomeric epoxides (263) with selenophenol suffers from low regioselectivity, resulting in a low yield of the final product. The other disadvantage is the basic reaction-medium occasioned by the method used for the generation of selenophenol, namely reduction of diphenyl diselenide with sodium borohydride in solution in anhydrous alcohol (see Ref. 356) some epoxides are sensitive to basic media. However, David (see Ref. 356) did not observe side reactions in his syntheses of 256. 

It is generally true that nearly all conmiercial epoxide resins have extenders incorporated in their formulation, the final selection being determined by the properties to be altered, the application and the other components of the system. Some of the properties affected by the addition of extenders are listed in Table 4.4. (For some specialised applications, certain of the listed disadvantages may in fact be advantages.)... 

In general, epoxidation modifies the physical properties of liquid polydienes. Bleaching, increased specific weight, and increased viscosity are the main effects. If the first is an advantage, the last represents a disadvantage. It was found that this increase in viscosity is less prenounced in polydienes with an increased content in 1,2-struclures. 

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