Five-membered rings, heterocycle synthesis

The concept of five-membered ring heterocyclic synthesis by transformation of the initial adduct of the ADC compound and substrate is not limited to cyclization of substitution products. l,3,4-Oxadiazol-2-ones (30, Scheme 3) result from heating the initial DEAZD-dichlorocarbene adduct.72 Treatment of the Diels Alder adducts 96 with zinc in acetic acid gives pyrroles in good yield (Eq. 17).151 The reaction has been extended to the synthesis of dipyrroles from the appropriate Diels-Alder adduct (96, R = pyrrol-2-yl). 

Figure 6. Five-membered ring heterocycle synthesis.

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)... 

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. 

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... 

Organotin-zinc derivatives can be used as reagents in organic synthesis. Thus, the stannyl zincate complex Me3Sn(Et)2ZnLi reacts with alkoxy-unsaturated ketones to form five-membered ring heterocycles. ... 

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... 

Saturated heterocycles and Flattened four-and five-membered rings Asymmetric synthesis ch41... 

As part of our current interest in SPOS, we examined the efficient and rapid synthesis of nitrogen- or sulfur-containing five-membered ring heterocycles and their fused counterparts by solid-phase strategies using Merrifield resin 1 and carbon disulfide (Figure 10.1). In this chapter, we review the synthesis of these five-membered ring heterocycles and their fused counterparts on solid support with carbon disulfide and Merrifield resin 1 as a solid support. 

Five-membered ring heterocyclic compoxmds containing nitrogen are one of the most common heterocycles. In fact, the synthesis of a great variety of that such as pyrrole, pyrazole, imidazole, triazole, tetrazole, lactams, and imides involving porous catalytic systems have been investigated. 

Phosphorus heterocyclic compounds, 1, 493-538 five-membered ring systems, 1, 513-523 nomenclature, 1, 496 six-membered ring systems, 1, 497-513 Photoaromatization oxirenes from, 7, 125-126 Photobleaching chromenes in, 3, 880 Photochemical reactions heterocyclic compound synthesis from, 5, 159 reviews, 1, 56 heterocyclic compounds reviews, 1, 71, 72... 

Many versatile approaches to the construction of fused heterocyclic systems (6 5 6) with ring junction heteroatoms have been reported. More general reactions which can be used for synthesis of derivatives of several tricyclic systems, and transformations which have potential for use in the preparation of a series of substituted compounds, are discussed in this section. Formation of the five-membered ring is presented first because it is a conceptually simple approach. It should be noted, however, that the addition of a fused six-membered ring to a bicyclic component offers much more versatility in the construction of a (6 5 6) system. Each subsection below starts with intramolecular cyclization of an isolated intermediate product. Reactions which follow are one-pot intermolecular cyclizations. 

A general approach to the synthesis of (6 5 6) fused heterocyclic ring systems involves intramolecular cyclization of two substituents attached to a five-membered ring of the fused (6 5) system. A typical example is cyclization of 211 to... 

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