The synthesis of five membered rings

Although some of the oxidative ring closures described above, e.g. reactions with lead tetraacetate (Section 4.03.4.1.2), may actually involve radical intermediates, little use has been made of this reaction type in the synthesis of five-membered rings with two or more heteroatoms. Radical intermediates involved in photochemical transformations are described in Section 4.03.9. Free radical substitutions are described in the various monograph chapters. 

Dipolar cycloaddUions. Interest in 1,3-dipolar cycloadditions increased dramatically during the past 20 years, largely because of the pioneering studies of Huisgen [7, 2] The versatility of this class of pericychc reactions in the synthesis of five-membered-ring heterocyclic compounds is comparable with that of the Diels-Alder reaction in the synthesis of six-membered-ring carbocyclic systems (equation 1)... 

C. The Synthesis of Five-membered Ring Sulfoxides and Sulfones. .. 461... 

The preparation of five-membered rings in solid-phase organic chemistry has been reported in several publications. Versatile syntheses of these heterocycles with different numbers and kinds of heteroatoms have been described. The synthesis of five-membered rings containing one nitrogen atom (Fig. 3.6) as pyrrolidines (231) [311-316] pyrroles (232) [317-320] pyrrolidinones (233) [321-323] pyr-rolinones (234) [324—326] 2,5-pyrrolidinediones (235) [327-329] 2,4-pyrrolidine-diones (236) [330-332] 2,5-pyrrolinediones (237) [333] or heterocycles with one oxygen or one sulfur atom like tetrahydrofurans (238) [334—336] 2,5-dihydrofurans (239) [337], furans (240) [338, 339], yS-lactones (241) [340-343], 2,5-dihydrofura-nones (242) [344] (Scheme 3.35) and thiophenes (243) [345, 346] can be accomplished on solid supports. 

The synthesis of five-membered rings containing three oxygen or sulfur atoms in the 1,2 and 4 positions can be placed into one of two broad groups ... 

Allenylsilanes serve as valuable three-carbon components in a general [3+2] annulation method for the synthesis of five-membered rings. A variety of general synthetic approaches to allenylsilanes have recently been developed and a number of specialized routes to various specific functionalized derivatives are available as well. The present procedure... 

Despite the numerous studies devoted to the synthesis of five-membered ring heterocycles using enamines as synthons, most publications on the use of enamines in the synthesis of heterocycles deal with six-membered heterocyclic ring systems, especially pyridine, pyrimidine, and pyran derivatives. 

The synthesis of five-membered ring systems can be achieved by a formal [4+1]- or a [2 + 2 + 1]-cycloaddition process essentially depending on the point of view with which one chooses to consider the reaction. Here we discuss the progress which has been made with iron complexes as catalysts for such transformations. 

It is well established that 1,3-dipolar cycloadditions are an important method for the synthesis of five-membered ring heterocycles. In particular, for the l,4-oxa/thia-2-azole system, nitrile oxides or nitrile sulfides are usually employed as the 1,3-dipoles and C=0 and/or C=S groups as the dipolarophiles. There are many examples of all possible dipole-dipolarophile combinations leading to azoline derivatives 176-179 and also to compounds of the general structures... 

The application of this approach to the synthesis of five-membered rings from 1,2-dihvdroxy compounds does not appear to have been... 

This TiCU-promoted allene cycloaddition has also been extended to the synthesis of five-membered ring heterocycles. In this case, increasing the steric shielding about the silicon atom seems to improve the cyclization process by suppressing the unproductive desilylative alkylation. For example, 1,3-di-methyl(r-butyldimethylsilyl)allene (39) reacts smoothly with cyclohexanecarbaldehyde to give in high yield the dihydtofuran predominantly as one stereoisomer (equation 31). Similar reaction with an N-acyl-iminium ion precursor (40) produces the pyrrolizidine system as a mixture of bicyclic isomers (equation... 

Compared with the synthesis of five-membered rings relatively little has been done on the synthesis of 8-lactones. Homoallylic alcohols can be converted into 8-lactones by rhodium-catalyzed hydroformyla-tion followed by oxidation (equation 48). The thallation and subsequent palladium-catalyzed carbonylation described earlier can also be used for the synthesis of six-membered rings (equation 49). ... 

Atom transfer carbonylation of 3-iodo alcohols provides a useful method for the synthesis of five-membered ring lactones (Scheme 4-49) [85]. The method is also applicable to six- and seven-membered ring lactones. 

Intermolecular cycloadditions have only been modestly successful for the synthesis of five-membered ring systems. 

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