Oxygen Heterocydes

The (n,n ) excited state of a ketone has electrophilic character, similar to that associated with alkoxy radicals, and it is not surprising that these excited states readily attack carbon-carbon multiple bonds. The overall reaction that normally ensues is a cycloaddition, giving a tour-membered oxygen heterocyde—an oxetane from an alkene addend (4.62), or an oxete from an alkyne addend (4.63). Some oxetanes are of interest in their own right, but many are useful intermediates in the synthesis of other compounds. 

Typical Reactions oj Oxygen Heterocydes with Hydrogen Sulfide over NaX ... 

Other Oxygen Heterocydes, induding Polyether lonophores... 

Diverse Nitrogen and Oxygen Heterocydes via On-Resin Cydization... 

I 3 Synthesis of Medium-size Oxygen Heterocydes by Ring-dosing Athene Metathesis... 

The oxygen- or nitrogen-substituted allene adducts as described in the previous section are of major interest because of the possible subsequent cyclization leading to very valuable oxygen- or nitrogen-containing heterocydes (Scheme 8.25). The syn-... 

The properties of the related heterocydes containing oxygen and sulfur (e.g. dibenzofuran, dibenzothiophene) can be similarly interpreted. 

Heterocyde a cyclic organic molecule containing at least one noncarbon atom in the ring, typically, oxygen, nitrogen, or sulfur. 

Pyrazolyl- [85], dihydropyridyl- [86], and pyrrolizinylcarbene complexes [87] have also been subjected to the benzannulation to give the respective oxygenated benzo-N-heterocycles. Finally, a,/ ,y,dienyl carbene complexes containing a heterocyde at the internal double bond have been utilized to prepare 2,3-dihydro-l,2-benzisoxazoles and indazoles by intramolecular benzannulation [60h]. 

Oxygen only Nitrogen only AH other types of heterocyde... 

Thiophene is a usefiil template for four-carbon homologation via reduction [9], as well as a bioisostere of the benzene ring and other heterocydes in medicinal chemistry. Thiophene is a n-electron-excessive heterocycle. It favors electrophilic substitution, which, similar to metalation, takes place preferably at the a-positions due to the electronegativity of the sulfiir atom [10]. In comparison to the oxygen atom in fiiran, the sulfiir atom in thiophene has lower electronegativity, so its lone pair electtons are more effectively incorporated into the aromatic system. The aromaticity of thiophene is in between that of benzene and fiiran. As a consequence, the difference in reactivity of a-halothiophenes and 3-halothiophenes is not as pronounced as that of the corresponding halofiirans. 

Reid, S. T., The Photochemistry of Heterocydes, 11, I The Photochemistry of Oxygen- and Sulfur-Containing Heterocydes, 33, I Photochemistry of Nitrogen-Containing Heterocydes, 30, 239. 

Cyclization of 1,4-dicarb6ny compbuiidis with nitrogen, sulfur, or oxygen nucleophiles gives the five-membered aromatic heterocydes pyrrole, thiophene, andfuraii. 

To test the effect of a heteroatom, Starokon et al. [175] studied three five-membered heterocydes with a similar molecular structure, containing the oxygen (2,5-dihy-drofuran), nitrogen (3-pyrrole) and sulfur (butadiene sulfone). Only the oxygen-containing cyde was carboxidized selectively, while the others showed a strong tendency towards side reactions resulting in a set of unidentified products. 

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