Heterocydes 3-membered

Table 34 Operating Conditions for GLC Separation of Five-membered Heterocydes with More Than One Heteroatom ...

Figure 4.14 Schemauc represemaiion of (a) 4-membered, (b) 6-membered and (c) 8-membered (LiN) heterocydes showing pendant groups on N lying both above and below the plane of the ring, (d) the laddered structure formed by lateral bonding of iwo l.iiN uniK.

Dipolar cydoadditions are one of the most useful synthetic methods to make stereochemically defined five-membered heterocydes. Although a variety of dia-stereoselective 1,3-dipolar cydoadditions have been well developed, enantioselec-tive versions are still limited [29]. Nitrones are important 1,3-dipoles that have been the target of catalyzed enantioselective reactions [66]. Three different approaches to catalyzed enantioselective reactions have been taken (1) activation of electron-defident alkenes by a chiral Lewis acid [23-26, 32-34, 67], (2) activation of nitrones in the reaction with ketene acetals [30, 31], and (3) coordination of both nitrones and allylic alcohols on a chiral catalyst [20]. Among these approaches, the dipole/HOMO-controlled reactions of electron-deficient alkenes are especially promising because a variety of combinations between chiral Lewis acids and electron-deficient alkenes have been well investigated in the study of catalyzed enantioselective Diels-Alder reactions. Enantioselectivities in catalyzed nitrone cydoadditions sometimes exceed 90% ee, but the efficiency of catalytic loading remains insufficient. 

An important area of progress in zirconacyclopentadiene chemistry has been the heteroatom transfer developed by Fagan and Nugent, leading to five-membered heterocydes (Eq. 2.19) [26],... 

Sulfenyl chlorides and halogens react with 1,2-alkadienylphosphonic acids to afford phosphorus-containing heterocydes [72], However, the electrophilic addition of dialkyl 4-methyl-2,3,5-hexatrien-2-yl phosphonates with sulfenyl or selenyl chloride afforded 2-thienyl methylphosphonates or the seleno analogues [73, 74]. The conjugate addition of sulfenyl or selenyl chloride with the 2,4-diene moiety in the starting allene leads to the formation of the five-membered skeleton (Scheme 10.69). 

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

SYNTHESIS BY TRANSFORMATION OF OTHER HETEROCYCLES 3.12.4.1 Ring Contraction of Six-membered Heterocydes... 

Etliyleneimine (aziridine, azacydopropane) is the smallest cyclic imine consisting of a three-membered iV-heterocydic ring (n... 

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). 

Preparation of Five-Membered Carbocydes and Heterocydes via Intramolecular Carbene Insertion... 

Cuevas, G. Juaristi, E. Manifestation of stereo-electronic effects on the calculated C—H bond lengths and one bond 1JC-h NMR coupling constants in cyclohexane, six-membered heterocydes, and cydohexanone derivatives. 

The additive-modified catalyst mixture developed for intramolecular olefin coupling is sufficiently active to catalyze the analogous intermolecular reaction. A variety of five-membered N-heterocydes can be catalytically alkylated at the C-2 position (Table 2) [8], Additional functional groups on the heterocyde[15] (Table 2, Entry 1) and on the olefin (Table 3) are well tolerated. Products corresponding to linear addition are usually obtained exclusively, even when the olefin is rapidly isomerized under the reaction conditions. 

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. 

Table 25 Structures Five-membered N-Heterocydic Complexes of Cobalt (II, III)...

In Table 2, the ring strains, bond lengths, and bond angles for some 3-and 4-membered heterocydics are compared with these of corresponding cycloalkanes [63]. 

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)... 

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

The aromatic five-membered heterocyde imidazoie is important in many biological processes. One of its nitrogen atoms is pyridine-like in that it contributes one w electron to the aromatic sextet, and the other nitrogen is pyrrole-like in that it contributes two IT electrons. Draw an orbital picture of imidazole, and account for its aromaticity. Which atom is pyridine-like and which is pyrrole-like ... 

The carbenes derived from six- [20-22] and seven-membered heterocydes [22-24, 70] of type 14 can be obtained both with an aliphatic or an aromatic backbone. The characteristic NMR resonances for the C2 carbon atoms in six-membered carbene heterocydes ( 236-248 ppm) [20, 21] have been recorded in the same range as the Ccarbene resonances for saturated imidazolidin-2-ylidenes of type 1. Dimerization to enetetramines occurs for some carbenes derived from a six-membered heterocycle [21] and this behavior is independent from the nature of the backbone (sartirated or unsaturated), but depends strongly on the steric bulk of the /V,/V -substiments. 

No enetetramine formation has been observed with NHCs derived from sevenmembered heterocydes. The C2 resonance in these derivatives is shifted further downfield (5 250-260 ppm) in the C NMR spectra. The most striking difference in comparison to all NHCs discussed previously is the twisted, nonplanar conformation of the seven-membered heterocycle both in the azolium precursor and in the carbene ligand. The twisted conformation appears to be essential to accommodate the strain within the seven-membered heterocycle. 

DHA gave a blue color with pyrrole, 3,4-dimethylpyrrole, and 2,2 -bipyrrole (69MI1 70MI2). On the other hand, pyrrole having substituents at the 2- or 2.5-positions, or other five-membered heterocydes such as furan, thiophene, or pyrrolidene, failed to give such a blue color when they were substituted for pyrrole. Although other attempts to isolate the reaction product failed, the use of the 6-benzoyl derivative of DHA instead of DHA did allow isolation. Based on IR spectroscopy and elemental analysis, it was concluded that the blue compound possesses an indophe-nine structure. 

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