Ring closure process

Simple examples of diazoalkylideneamine-l,2,3-triazole equilibria have been demonstrated for a series of l,2,3-triazolo[l,5-a]pyrimidines by variable-temperature NMR [74JCS(CC)671]. Tautomers A, B, and C interconvert rapidly at elevated temperatures the energy barrier for these ring-opening-ring closure processes was found to be AG = 76 kJ mol (for = H, Me R = CONH2) (Scheme 111). 

Another approachl2a e makes use of bilenimincs which can be photochemically cyclized to isobacteriochlorins. A plausible mechanism for this ring-closure process involves the formation of an enamine tautomer from the imine at the 1-position, which is then cyclized in a photochemical 1 STc-n-isomcrization process followed by elimination of the leaving group at the 19-... 

The driving force behind this remarkable reaction in which the sulphone participates intramolecularly in a 5-exo-tet ring closure process is that the geometry of the intermediate bromonium ion is nicely set up for the attack by the sulphone oxygen. Probably, the process is assisted by the ready cleavage of the sulphur to the t-butyl bond, with the concomitant formation of t-butyl bromide. 

Of note, Quintela and coworkers used a related aza-Wittig/l,5-electrocydic ring-closure process to obtain access to multiheterocydic structural elements such as py-... 

Stoichiometrically protonated cycloheptadienones appear to undergo electrocyclic ring-opening/ring-closure processes R. Noyori, Y. Ohnishi and M. Kato, J. Am, Chem. Soc., 94, 5105 (1972). 

The two-step Staudinger reaction has been investigated in depth and 13-azadienes 56 have been isolated in some cases. The evidence suggests that the enol ether group is crucial in the stabilization of the diene and also plays an important role in promoting the conrotatory ring closure process . 

A common feature of any cyclization reaction is that a new intramolecular C—C bond is produced that would not have been formed in the absence of the catalyst. Those reactions in which one ring closure step is sufficient to explain the formation of a given cyclic product will be called simple cyclization processes, although their mechanism is, as a rule, complex. We shall distinguish those cases in which any additional skeletal rearrangement step(s) is (are) required to explain the process. Some specific varieties of hydrocarbon ring closure processes are not included. A recent excellent review deals with the formation of a second ring in an alkyl-substituted aromatic compound (12). Dehydrocyclodimerization reactions have also to be omitted—all the more since it is doubtful whether a metallic function itself is able to catalyze this process (13). 

A very brief survey of ring closure processes over oxides (first, chromia) may reveal some common features with metal catalysis. This must have led in some cases to very similar ideas (see also Section II,B). 

It has been found that the cupric sulfate-mediated aminolysis of 3-bromo[ N]isoquinoline with ethanolic ammonia (130°C, 7 days reaction time) yields a mixture of 13 and 14, ratio 75 25 (74RTC198). It shows that also with the weaker nucleophile ammonia a part of 3-bromoisoquinoline can undergone a ring-opening, ring-closure process. 

Analogous 4,1,5-benzoxadiazocinones have been prepared by several complementary ring closure processes (67JOC2417). Treatment of the syn-oximes (318) of haloacety-... 

The transition metal catalyzed synthesis of five membered heterocycles, particularly of condensed ring systems, has attracted considerable attention. The ease of the formation of five membered rings has been utilised both in intramolecular ring closure processes, and in the combination of two (three) fragments through the formation of a carbon-carbon and a carbon-heteroatom bond. This chapter is dedicated to examples, where the construction of the five membered heterocycle is achieved in a transition metal catalysed step. 

Free radical reactions are used less frequently, to effect six- rather than five-membered ring closures, because carbohydrate-derived hept-6-enyl radicals are less available than are hex-5-enyl analogues, these being the species required for normal exo ring closure processes. Such reactions can, however, be used with good efficiency to produce cyclohexane ring systems bearing a carbon substituent (see Chap. 25). 

Discussion of preparative methods for 1,5-benzodiazepines is expanded compared to the corresponding sections in CHEC(1984) and CHEC-II(1996), reflecting the development of new synthetic methodology to access these heterocycles. Ring-closure processes are discussed in alphabetical order based on the bond disconnections noted below. 

Aniline nucleophiles readily participate in this type of ring-closure process under palladium catalysis (Pd(OAc>2, BINAP, and CS2CO3 in hot toluene) without resort to protecting groups when an aryl iodide is used <2005T61>. 

All the carbon atoms of the purine ring were supposedly provided by HCN molecules through a complex step-by-step condensation process. In particular, oligomers of HCN, such as the HCN-trimer aminomaleonitrile (AMN) and the HCN-tetramer diaminomaleonitrile (DAMN), were found to be intermediates in this transformation (Scheme 1) [43,44]. In accordance with the present-day biosynthesis of purines in the cell, two 4,5-di-substituted imidazole derivatives, 4-aminoimidazole-5-carbonitrile (AICN) and 4-aminoimidazole-5-carboxamide (AICA) were successively formed from AMN and DAMN by chemical or, most probably, photochemical reactions [45-47]. Finally, a ring-closure process of AICA and HCN yielded adenine 1. 

Substituted 2,3-dihydro-l-H-indoles 26 are accessible by the versatile application of a 5-exo ring closure process during the propagation cycle in the SRN1 reaction [67]. Following the initial ET and fragmentation of the radical anion of the substrate, the... 

The novelty of this work involves the versatile application of 5-exo ring closure processes during the propagation cycle of the SRN1 reaction the alkyl radical intermediates formed then reacted with the nucleophiles to afford the ring closure-substituted heterocycles. The factors governing the observed product distribution are discussed and one of the examples is illustrated below <02JOC8500>. 

One of the most important classes of Michael initiated ring closure processes in the construction of carbo- and heterocycles are stepwise cycloaddition reactions where a metal induces dipolar behavior in otherwise unreactive organic compounds to be reacted with activated olefins. In this area, Pd-assisted cycloaddition reactions which involve zwitterionic zr-allylPd complexes of type I (linear type), II, or III (Pd-Trimethylenemethane (TMM) type and analogs) as reactive dipole partners are popular methods that provide highly functionalized, saturated ring systems often with high stereocontrol and atom economy (Scheme 1). Discovered in the early 1980s, they have been extensively covered in the review literature [8-16]. 

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