1,3-dipolar cycloaddition reactions solved problem

Electrocyclic, sigmatropic, and cycloaddition reactions are subsequently described in Chapters 2, 3, and 4, respectively. Chapter 5 is devoted to a study of cheletropic and 1,3-dipolar cycloaddition reactions as examples of concerted reactions. Many group transfer reactions and elimination reactions, including pyrolytic reactions, are included in Chapter 6. There are solved problems in each chapter that are designed for students to develop proficiency that can be acquired only by practice. These problems, about 450, provide sufficient breadth to be adequately comprehensive. Solutions to all these problems are provided in each chapter. Finally, in Chapter 7, we have compiled unworked problems whose... 

The structure of the reaction product of 2-aminopyridine and diethyl malonate, described by Chichibabin as 2,4-dioxo-3,4-dihydro-2//-pyrido-[l,2-<7]pyrimidine,96 was first questioned by Snyder and Robison253 on the basis of the high melting point and poor solubility of the compound. They suggested the tautomeric 2-hydroxy-4-oxo-4H-pyrido[l,2-a]pyrimidine structure. The problem was solved by Katritzky and Waring273 who compared the UV spectrum of the product with that of fixed tautomers and found that the product may best be described as anhydro- 2-hydroxy-4-oxo-4/f-pyrido[l,2- ]pyrimidinium)hydroxide (63). Because of the chemical behavior of these compounds, however, the contribution of other mesomeric forms to the structure has also been considered.122 Thus, PPP-SCF quantum chemical calculations suggest that 1,4-dipolar cycloadditions to the C-3 and C-9a atoms are to be expected.352 This type of reaction does in fact occur (see Section III,C,10). Katritzky and Waring273 estimated the ratio of the mesomeric betaine (63 R = H) and the 2-hydroxy-4-oxo tautomers to be about 20 1. 

Although this strategy had solved the problem of pyrrolidine ring formation, the unexpected incorporation of the oxygen functions in both products presented a new one. Strenuous measures to eliminate air from the reaction mixture failed to avoid this complication, suggesting that the reaction intermediates (which may be either zwitterionic or radical in nature) were remarkably efficient scavengers of O2. Thus, faced with additional steps required to manipulate these groups in a productive way and the already poor yield of the dipolar cycloaddition, this approach was abandoned. 

Typically, cycloaddition of azides with nonactivated alkynes leads to a regioiso-meric mixture of 1,2,3-triazoles. Dramatic improvement in the methods was achieved with a copper catalyst discovered in 2002 by two independent groups. " Since then, the number of studies concerning 1,3-dipolar cycloadditions of azides and terminal alkynes has increased dramatically. The introduction of the copper catalyst solved many problems, including long reaction times, high reaction temperatures, and poor... 

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