Applications of -Cycloaddition Reactions

Cycloaddition of ethoxy ketene 1 with cis- and tranj-2-butene is a concerted process and gives stereospecific products 2 and 3, respectively [2], 

Similarly, ethylketene 4 and dimethylketene 5 react with cyclopentadiene to give 6 and 7, respectively. 

Ketene 8 reacts with ethoxy acetylene to give 9 [3] and phenyl methyl ketene 10 reacts with Z-alkene 11 to give 12 [4]. 

Ethoxyethylene reacts with diphenylketene 13 to give cycloadduct 14 in high yield [5], 

Ketenes also undergo [jt +jt ]-cycloadditions with ketones, in the presence of Lewis acids as catalysts, e.g., 8 reacts with ketone 15 to give 16 and with 17 to give 18 in high yields [6]. 

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Acyl azides which produce the nitrenes 3 and 4 upon photolysis are the most suitable for synthetic applications of cycloaddition reactions. Therefore, this subchapter will mainly deal with alkoxycarbonyl and aroyl nitrenes. 

Methods and Applications of Cycloaddition Reactions in Organic Syntheses, ed. N. Nishiwaki, John Wiley sons, Hoboken, New Jersey, 2014. 

This chapter gives an overview about applications of cycloaddition reactions to study nucleic acids. These cycloaddition reactions are typically grouped under the heading click chemistry despite varying other chemistries, and an enormous amount of different applications in the context of DNA and RNA have been found so far. Only selected recent implementations for cycloaddition reactions on nucleic acids can be presented in the limited scope of this chapter. Today s click reactions, as first defined by Sharpless et al. [1], are associated with biocompatibility, fast reaction kinetics and bio-orthogonality. 

A firm understanding of concerted cycloaddition reactions developed as a result of the formulation of the mechanism within the framework of molecular orbital theory. Consideration of the molecular orbitals of reactants and products revealed that in some cases a smooth transformation of the orbitals of the reactants to those of products is possible. In other cases, reactions that appear feasible if no consideration is given to the symmetry and spatial orientation of the orbitals are found to require high-energy transition states when the orbitals are considered in detail. (Review Section 11.3 of Part A for a discussion of the orbital symmetry analysis of cycloaddition reactions.) These considerations permit description of various types of cycloaddition reactions as allowed or forbidden and permit conclusions as to whether specific reactions are likely to be energetically feasible. In this chapter, the synthetic applications of cycloaddition reactions will be emphasized. The same orbital symmetry relationships that are informative as to the feasibility of a reaction are often predictive of the regiochemistry and stereochemistry of the process. This predictability is an important feature for synthetic purposes. Another attractive feature of cycloaddition reactions is the fact that two new bonds are formed in a single reaction. This can enhance the efficiency of a synthetic process. 

METHODS AND APPLICATIONS OF CYCLOADDITION REACTIONS IN ORGANIC SYNTHESES... 

Methods and applications of cycloaddition reactions in organic syntheses /... 

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