Ring compounds heterocyclic five-membered rings from

Ring synthesis of five-membered heterocycles from 1,4-dicarbonyl compounds... 

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Five-membered ring heterocycles can be the result of cycloaddition reactions of ADC compounds acting as 2n components with 1,3-dipoles, or as 47t components in cheletropic reactions. They can also result from nucleophilic attack on the ADC compound, followed by ring closure of the initial adduct. 

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). 

As with five-membered ring formation, the reactions of ADC compounds which lead to six-membered ring heterocycles can be classified according to how the ADC compound reacts in the initial step. Most common is the Diels-Alder reaction, with the ADC compound acting as dienophile. Six-membered rings also result from the reaction of monoenes with ADC compounds acting as the 4n component, and by cyclization or other transformation of an initial adduct. 

Scheme 3. New five-membered ring heterocyclic compounds predicted from the acetoin-ammonium sulfide model system.

When tozylhydrazone 368 is stirred in benzene at room temperature, within 2 h pyrazoline 369 is formed, while its subsequent refluxing in the same solvent converts the five-membered ring to cyclopropane (compound 370) [380] (Scheme 3.120). Heterocycle 370 can be obtained directly from hydrazone 368 by heating in benzene for 2 h. 

The cyclic hemiacetal that has a five-membered ring is called a furanose. This name is derived from that of tire five-membered, oxygen-containing heterocyclic compound fu-ran. The hemiacetal with a six-membered ring is known as a pyranose after the heterocycle pyran. 

Thiophene is an aromatic compound. Its structure can be assumed to be derived from benzene by replacement of two annular CH groups with sulfur. The sulfur atom in this five-membered ring acts as an electron-donating heteroatom by contributing two electrons to the aromatic sextet and thiophene is thus considered to be an electron-rich heterocycle. 

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