Cascade reactions basic principles

The other type of radical chemistry of importance in the carbohydrate field is one-electron reductions. A handful of these reactions (such as the metallic Zn reduction of acetobromoglucose to triacetylglucal) have been used in synthesis for decades, but, starting with the Barton-McCombie deoxygenation of sugars in the mid-1970s there has been an explosion of interest, as increasingly sophisticated cascades of elementary radical steps have been devised. Such reactions are driven by the homolysis of weak bonds such as Sn-H or N-O under conditions of photolysis or mild thermolysis. Nature uses a similar basic principle in Type II ribonucleotide reductases, where the weak bond in question is the cobalt-carbon a bond in the corrin cofactor. ... 

In this context the emergence of more complex molecular structures starting from simpler ones is the central problem. In this respect basic principles of induced cascade-type reactions such as that given in Scheme 2 are of special interest ... 

Recently, multistep enzyme-catalyzed reactions have attracted the attention of chemists and biotechnologists, as they can be combined in a modular manner and often lead to high-value compounds. All naturally occurring metabolic pathways are basically cascade reactions. Based on natural principles, synthetic chemists search for universal multistep processes applicable to a vast number of chemical compounds. Multistep enzyme-catalyzed reactions involving nonphysiological substrates and selective enzymes are of particular interest because they may lead to tailor-made complex molecules with desired properties. Moreover, one of the most important advantages of multistep enzyme-catalyzed reaction sequences... 

One approach to provide resists with high sensitivity and contrast involves the principle of chemical amplification (CA) (6-8), The amplification effect is achieved by employing a photo generated acid as a catalyst to carry out a cascade of chemical reactions in the resist film. Catalytic chain lengths of >1000 have been reported in the literature (9,10), However, the high catalytic chain lengths also enhance the resist s sensitivity to airborne basic contaminants and basic moieties on the substrate (11,12), In a positive tone resist such deactivation of the photoacid by airborne or substrate bound basic contaminants results in T-tops or foot formation respectively. 

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