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Aldol reactions, another class of carboncarbon bond-forming techniques, conventionally require a tradeoff between inefficiency and selectivity. New biomimetic catalytic methods are improving the environmental profile of aldol chemistry. Proline, for example, can be used to give excellent regio- and stereoselectivity in many cases (45). Aldol reac- tions are common in nature; engineered aldolase enzymes can convert a variety of substrates to chiral products with excellent selectivity (46). Alternative synthetic methods Cascade reactions can lead to reductions in the amounts of solvents, reagents, and unwanted bypro- ducts. In the absence of biocatalysts, they typically proceed by cationic, anionic, radical, or pericyclic mechanisms. Intramolecular radical cascades are useful in streamlining synthesis of polycyclic natural products. With careful planning, cyclizations, 1,2- group migrations, hydrogen shifts, fragmentations, and substitutions can be ‘‘programmed’’ in a useful order (47) (Figure 7). Since the late 1980s, microwave-assisted organic synthesis has developed rapidly; specialized commer- cial equipment is now available for measuring and controlling power input, temperature, and pressure, allowing for improved reproducibility. Often the major benefit of microwave energy is reaction accel- eration due to increased temperature, but microwave techniques can also facilitate green chemistry meth- ods such as solvent-free reactions, use of high- temperature water as a reaction medium, and use of recyclable solid-supported materials. Microwave heating potentially allows shorter residence time at high temperatures and more rapid cooling, enabling preservation of thermally unstable reaction products. Rapid heating of organic material under closed conditions is a well-known safety hazard, but with properly designed reactors the ability to rapidly remove the source of heat is an advantage over conventional heating systems (49,50). There is no evidence that shortened reaction times and higher yields are related to anything but thermal effects, but the enhancements may be difficult to achieve by classical techniques. Energy savings compared to conventional heating depends on efficiency in con- verting electric energy to microwave energy, as well as the characteristics of the reactor, the amount of reaction mixture, and the capability of the reaction components to absorb microwave energy. The use of microwave heating on an industrial scale is relatively unexplored (51). Sonochemistry, the use of ultrasonic waves to promote chemical reactions, is similar to microwave- assisted synthesis in that rate and yield enhancements are usually the primary benefits. In many cases, the chemical pathway is not affected; the benefits can be explained by mechanical effects analogous to high- speed stirring. However, unlike microwaves, ultra- sound can indirectly induce homolytic cleavage of chemical bonds. Transition metal complexes may be activated, and ‘‘sonochemical switching’’ can occur: the reaction mechanism changes depending on the application of ultrasound. Green chemistry benefits have been demonstrated, including use of milder reaction conditions, less aggressive reagents, and access to selectivity that conventionally requires material-intensive routes (52). Photochemistry provides methods for achieving chemical transformations without added reagents. Photon absorption can occur independently of temperature and solvent, providing flexibility in environmentally friendly synthetic design (53). Sol- vent-free ‘‘solid-to-solid’’ photochemical reactions have been developed as a technique for synthesizing molecules with adjacent quaternary stereogenic cen- ters (54) (Figure 8). Microreactors (in which reaction components are manipulated in channels as small as 10 mm in diameter) can result in reduced reaction time and increased yields, because of improved mixing. Some autocataly- tic, UV-light-intensive, or highly exothermic (‘‘run- away’’) reactions are more safely performed in Green chemistry: the emergence of a transformative framework 13 Figure 7. One-pot cascade of radical cyclization and fragmentation reactions, giving 70% product yield (48).PDF Image | Green chemistry: the emergence of a transformative framework
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