Cyclization s. Ring closure

A prototype for the new cyclization reaction is the Ziegler-Hafner synthesis of azulene (J)4 by heating the fulvenoid decapentaene (/). Woodward28 was the first to argue that a thermal, electrocyclic [rr 10 s] ring-closure could be involved in this thermolytic process. Of course it seems likely that the nonisolable intermediate (2) may be formed by an intramolecular electrophilic attack of atom 10 to the electron-rich position 1 in compound (1). Indeed a distinction between the two different reaction modes is mechanistically irrelevant in this case. But only its interpretation as an electrocyclic process allows the generalization to [ir 6 s] cyclizations too. 

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

Radical cyclization. Ring closure of hept-6-en-1 -y 1 radical yields two products, methyl-cyclohexyl radical (85 percent) and cycloheptyl radical (15 percent).39 The overall rate constant is 3.5 X 104 s l. What are the rate constants for each pathway ... 

Poly(arylene ether ketone)s can also be modified by introducing the functional groups using similar approaches to polysulfones. For example, poly(arylene ether ketone)s were sulfonated.189 In addition, o-dibenzoylbenzene moieties in the poly(arylene ether)s can be transformed to heterocycles by cyclization with small molecules. These polymers can react with hydrazine monohydrate in the presence of a mild acid in chlorobenzene or with benzylamine in a basic medium.190 Another example of the use of the o-benzyl cyclization strategy is the intramolecular ring closure of poly(arylene ketone)s containing 2,2/-dibenzoylbiphenyl units to form poly(arylene ether phenanthrenes).191... 

The thermal cyclization of N-(2-alkenyIthiophenyl)-C-azidohydrazones 514 gave 515 [84JCR(S)364], The ring closure proceeds via uncommon 1,2,4-triaza-l, 3-diene intermediates. 

The key step in Hu s synthesis of 51 was cyclization of 50 by heating with copper(I) iodide and sodium hydride in DME, followed by a 10% aqueous ammonia work-up. Intermediate 50 was prepared via Michael addition of ethyl acetamidocyano acetate to the appropriate chalcone followed by acid-catalyzed ring closure [42,43]. 

In Case 1 both the ring closure steps (with rate constants ks and kR) are faster than dissociation of the 7r-complexes to reform the alkenol and N-X+ species. Here the [S]/R] product ratio is determined only by the difference in the activation energy leading to the 7is- and rcR-complexes because immediately after these are formed the cyclization occurs. Thus, the chirality is set by the approach of the alkene to the halonium ion. 

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