Forming four-membered

Enamines ° form four-membered rings with Michael-type aikenesand ketenes.In both cases, only enamines from aldehydes give stable four-... 

A useful method to synthesize ten and fourteen-membered ring imides 346 involved an initial condensation of macrocyclic -ketoestes 343 with alkyl or aryl isocyanates and carbodiimides, respectively, in the presence of a base [68]. After a nucleophilic attack of the enolate on the isocyanate C, the resultant amide N anion 344 induced a ring closure by addition to the keto group. Then, the intermediately formed four-membered ring 345 underwent a fragmentation... 

Photosensitized decomposition of oxime oxalyl amides proceeded via carbamoyl radicals which underwent A-exo cyclizations forming four-membered /3-lactams as main products. Irradiation of solution of oxime derivative 22 and 4-methoxyacetophenone (MAP) in toluene at 100 °C with a 400 W UV lamp led to azetidinone 23 in 3 1 ratio of diastereoisomers (equation 10) . ... 

Structures C and D have never been observed for gallium hydrazides, which exclusively form four-membered heterocycles. In contrast, all kinds of structures were observed with aluminum. Quantum-chemical calculations showed that for aluminum the five-membered heterocycle is the most... 

Not many examples of [3 + 1] combinations to form four-membered heterocycles are known, and it is apparently not a general method. 

In a similar manner, the reaction of 6.28 with CuCl in CH2CI2-CH3CN produces the spherical complex [ Cp Ta(CO)2(t " -P4) 6 CuCl g] (6.31). The tantalum complex 6.31 is comprised of 32 core atoms forming four-membered P4 rings and six-membered P4CU2 rings in an extended cubic arrangement with... 

The trithiocarbonato complex anions [M(CS3)212 (M = Zn or Cd) have been isolated as their tetraphenyl-phosphonium and -arsonium salts.881 The trithiocarbonate is bidentate and forms four-membered chelate rings. 

This increase in and 0B is experimentally observed and is shown in Table 10, which is restricted to molecules with bidentate ligands that form four-membered chelate rings with small normalized bites. Exact agreement is not expected with Figure 32, which was calculated for i (unidentate/bi-dentate) = 1.0. Nevertheless, in all cases the largest distortion from the parameters of the regular octahedron is the increase in 0B, whereas A slightly increases. 

Several complexes of bivalent cobalt and zinc with imidazoline-(l//,3i/)-2-thione (34 R = H) of the forms ML2X2 (X = halide ion) and ML4X2 (X = N03 or C104) have been isolated and characterized by physicochemical techniques.247 In contrast with the S monodentate behaviour concluded for the earlier-reported Ni11 complexes, the ligands are N—S bidentate in these metal salts to form four-membered chelate rings. Similar coordination behaviour has been observed for the... 

Dihydropyridines (42) behave in a similar way except that the initially formed four-membered ring (43) does not open under the influence of either heat or light in the cases examined.261 If the 3-sub-stituent of the dihydropyridine 42 is a carboxyl group, or a primary or a... 

The alkaline earths, Mg, Ca, Sr, and Ba, also form four-membered chelate systems with 1,8-naphthyridine and its 2,7-dimethyl derivative (151).101 When dissolved in polar media the complexes show 3-ion conductance. 

Several alkali halides have been studied at elevated temperatures394 and dimeric species forming four-membered rings have been found to predominate. There are indications of rather large amplitude vibrations in these dimers. It should be pointed out that monomeric species are also present in addition to the predominating dimers and several of these have been indentified by microwave spectroscopy44 ... 

An interesting aspect of fluorinated 4-pentenyl radicals that distinguishes them from their hydrocarbon counterparts is their ability to cyclize to form four-membered rings. As mentioned in Sect. 5.3.2, Piccardi and his coworkers reported in 1971 that C2F5I underwent free radical addition to 3,3,4,4-tetra-fluoro-l,5-hexadiene to form a four-membered ring product [173]. Subsequently they observed similar results in the addition of CC14 [323]. 

Cycloaddition reactions of thioketene and isocyanic acid forming four-membered ring products have been studied by means of the ab initio RHF/6-31G method. The two alternative reactions are concerted but nonsynchronous, taking place through twisted or planar four-membered cyclic transition states. The activation barriers were calculated to be... 

Fig. 5.5 A model of conformational autocatalysis for the oligopeptide of Fig.5.4. The lowest-energy a-helical form (two-helix bundle, left-hand side) interacts with the frozen metastable (3 form (four-member (3 barrel, center). After a long Monte Carlo run (symbolized by an arrow), which consists of a million of intermediate conformations of various type, both molecules form a strong complex of two (3 type metastable forms right). During this run the a conformation unfolds and then refolds to the (3 conformation. Thus, the (3 metastable form plays a role of a catalyst in the transformation of the a native structure into the (3 form (autocatalysis)...

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