Reagents and purity

Modem plants generally produce carbon disulfide of about 99.99% purity. High product quaUty is ensured by closely controlled continuous fractional distillation. Reagent and U.S. Federal specifications, and typical commercial-grade quaUty are Hsted in Table 5. 

Many methods exist for the purification of reagents and solvents. A number of these methods are routinely used in synthetic as well as analytical chemistry and biochemistry. These techniques, outlined below, will be discussed in greater detail in the respective sections in this Chapter. It is important to note that more than one method of purification may need to be implemented in order to obtain compounds of highest purity. 

It was assumed throughout that the compound which separated out from the solution was chemically pure, but this is not always the case. The purity of the precipitate depends inter alia upon the substances present in solution both before and after the addition of the reagent, and also upon the exact experimental conditions of precipitation. In order to understand the influence of these and other factors, it will be necessary to give a short account of the properties of colloids. 

The enantioselectivity was highly dependent upon the purity of the organolithium reagent, and the presence of small amounts of alkoxide led to considerably lower enantioselectivities. 

Posner and coworkers have published a series of papers in which they described a successful application of the Michael reaction between a variety of carbanionic reagents and chiral cycloalkenone sulphoxides 557 to the synthesis of chiral organic compounds (for reviews see References 257, 649, 650). In several cases products of very high optical purity can be obtained. Subsequent removal of the sulphinyl group, serving as a chiral adjuvant, leads to optically active 3-substituted cycloalkenones 558 (equation 356 Table 27). 

For the synthesis of coumarins, the Pechmann reaction [145] is one of the most popular synthetic routes. As the reaction is conventionally carried out at high temperature, two microwave-assisted versions have been recently described. Besson and co-workers described the cyclocondensation of different m-amino phenols 226 with /1-ketoesters 227 on graphite/montmorillonite KIO support (Scheme 83). The use of graphite was crucial in the development of the reaction conditions. In fact, microwave irradiation of the reagents using different conditions gave poor results in terms of yields and purity. The optimized conditions, using a monomode microwave system, employed... 

The convergent approach overcomes these problems to an extent, but still gives more or less imperfect structures depending on the purity of the reagents and the efficiency of the reactions used. 

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