Cycloaddition reactions dipolar properties

Gothelf presents in Chapter 6 a comprehensive review of metal-catalyzed 1,3-di-polar cycloaddition reactions, with the focus on the properties of different chiral Lewis-acid complexes. The general properties of a chiral aqua complex are presented in the next chapter by Kanamasa, who focuses on 1,3-dipolar cycloaddition reactions of nitrones, nitronates, and diazo compounds. The use of this complex as a highly efficient catalyst for carbo-Diels-Alder reactions and conjugate additions is also described. 

The alternating electronic properties of the Co, and Cp atoms in the vinylidene ligand enable dipolar molecules to enter into cycloaddition reactions. Intramolecular [2 + 2]-... 

From the foregoing survey of heterocyclic hydrazonoyl halides, it appears that the main emphasis has been restricted to both their preparation and use as intermediates for further synthesis. Large areas of their chemistry, particularly regarding their physical and biological properties, remain to be developed. A deeper understanding of some aspects of their 1,3-dipolar cycloaddition reactions, such as regiochemistry and site selectivity in terms of the frontier molecular orbital method, is also needed. 

Fused aziridines are interesting compounds owing to the fact that the strained three-membered ring can easily open and cause dipolar cycloaddition reactions as well as their photochromic properties. Therefore, most of this chapter covers the chemical and photochemical properties of bi- and tricyclic aziridines. Some properties of aziridinyl ketones are also reviewed, in particular, reactions leading to aziridinyl anils. 

Table 8 Global properties and global electrophilicity3 scale for common dipole models involved in 1,3-dipolar cycloaddition reactions...

The design of covalently linked donor-fullerene systems capable of undergoing photoinduced electron-transfer processes has been widely studied as a result of the remarkable photophysical [35] and electronic [36] properties of fullerenes. Porphyrins, phthalocyanines, tetrathiafulvalenes, carotenes, and ferrocene [37] have been covalently attached to the fullerene sphere, usually as pyrrolidine[ 60] fullerene derivatives by 1,3-dipolar cycloaddition reactions. 

The 1,3-dipolar character of the azido group has been previously exploited for [2 + 3]-cycloaddition reactions of glycosyl azides with compounds containing triple bonds. It is known that formation of 1,4-disubstituted 1,2,3-triazoles is favored over the 1,5-disubstituted ones. Motivated mainly by pharmacological considerations (in order to obtain compounds with cytostatic properties), syntheses of a great number of 1-A -glycosyl-1,2,3-triazole derivatives have been reported. " " "" ... 

A large number of peptidomimetics are currently entering clinical trials, typically protease inhibitors and anti-cancer agents, which emphasizes the importance of developing reactions which efficiently modify peptides or peptide-like structures to increase their drug-like properties. Azides are important dipoles in 1,3-dipolar cycloaddition reactions and react with dipolarophiles such as alkynes and nitriles, to afford [l,2,3]-triazoles and tetrazoles, respectively. 

Synthesis of new hydroxybisphosphonate derivatives of ciprofloxacin 49 has been performed by using Cu-catalyzed 1,3-dipolar cycloaddition reaction between the corresponding azide and N-alkynyl substituted quinolone [169] (Scheme 22). Derivatives of gati- and moxifloxacin have been obtained similarly. All of these modified compounds maintained antibacterial activity of the starting quinolones and, in addition to that, exhibit osteotropic properties. 

Perhaps the most characteristic property of the carbon-carbon double bond is its ability readily to undergo addition reactions with a wide range of reagent types. It will be useful to consider addition reactions in terms of several categories (a) electrophilic additions (b) nucleophilic additions (c) radical additions (d) carbene additions (e) Diels-Alder cycloadditions and (f) 1,3-dipolar additions. 

This chapter is divided into four major sections. The first (Section 2.1) will deal with the structure of both alkoxy and silyl nitronates. Specifically, this section will include physical, structural, and spectroscopic properties of nitronates. The next section (Section 2.2) describes the mechanistic aspects of the dipolar cycloaddition including both experimental and theoretical investigations. Also discussed in this section are the regio- and stereochemical features of the process. Finally, the remaining sections will cover the preparation, reaction, and subsequent functionalization of silyl nitronates (Section 2.3) and alkyl nitronates (Section 2.4), respectively. This will include discussion of facial selectivity in the case of chiral nitronates and the application of this process to combinatorial and natural product synthesis. 

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