Four-membered heterocydes

Formation of Four-membered Heterocydes from Three- and Four-membered Heterocydes... 

Most reactions of this type were found to involve acyclic 1,4-dipolar intermediates which cydize to four-membered heterocydes or are intercepted by isocyanate or C=X components, such as C=N, C=S, and CR2, to form a six membered ring. This group of reactions is illustrated in Figure 1. Depending on the nature of the isocyanate and the double-bond system, any of the products shown in Figure 1 can be obtained. Variations in the component ratio or judicious choice of reagents are noted to have pronounced control of product type. Additional reaction details, as well as a description of the multiple transformations involving adjacent substituents, have been summarized (28). 

Four-membered heterocydes are easily formed via [2+2] cycloaddition reac tions [63] These cycloaddition reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [77d] Via this route, p lactones are formed on addition of ketene derivatives to hexafluoroacetone [777, 77S] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, l,4-benzoxazin-2-ones, l,2,4-tnazin-5-ones, and l,2,4-tnazm-3,5-diones accelerates [2+2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluorinated oxetanes are formed thermally and photochemically [779, 720] The reaction of 577-1,2-azaphospholes with fluonnated ketones leads to [2+2] cycloadducts [727] (equation 27)... 

In this large group of heterocycles, ring strain is of little or no importance. Ring-opening reactions are, therefore, rarer than in three- and four-membered heterocydes. The crudal consideration is rather whether a compound can be regarded as a heteroarene or whether it has to be classified as a heterocycloalkene (see p. 2). Various aromatidty criteria apply to heteroarenes, and, as a consequence, different opinions have been expressed on this matter [1]. As will be shown by means of examples of the various systems, the nature and number of heteroatoms are the critical factors. The parent compound of the five-membered heterocydes with one oxygen atom is furan. 

Mesoionic miinchnones are usually prepared by cyclodehydratation of amino acid derivatives and are in equilibrium with the corresponding amido-substituted ketenes. These compounds are well known as substrates for 1,3-dipolar and [2 + 2] cycloaddition reactions giving rise, respectively, to five- and four-membered heterocydes. Amdtsen and coworkers recently developed a new and elegant way to... 

Carbenes Derived from Four-Membered Heterocydes... 

Four-Membered Sulfur Heterocydes TABLE 7. THIETANE SULFILIMINES AND 5-AMINO SALTS... 

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