Cyclization classification

The participation of the lone-pair orbital in the cyclization process allows its classification as a so-called pseudopericyclic reaction (76JA4325 97JA4509), which is a subset of a general type of pericyclic reactions... 

In some ways this classification is arbitrary since, for example, the formation of 30 could be catalogued as [3 + 2] according to the overall synthesis, or as [5 + 0] if cyclization of the intermediate 29 is considered. We have often tried to classify a synthesis according to the... 

Three prominent types of reactions fall in this classification cyclizations by condensation, metal-mediated cyclizations and nitrenoid insertion reactions. 

Thus, it is called exo , when the cyclization occurs on the inside of the unsaturated carbon-carbon bond, and it is called endo , when the cyclization occurs on the outside of the unsaturated carbon-carbon bond. Moreover, it is tet (tetrahedral 109.5°), when the carbon-carbon bond at the reaction site is sp3 hybridization it is trig (trigonal, 120°), when the unsaturated carbon-carbon bond at the reaction site is sp2 hybridization and it is dig (digonal, 180°), when the unsaturated carbon-carbon bond at the reaction site is sp hybridization. For example, there are two types of cyclization manner in 5-hexen-l-yl radical, exo-trig and endo-trig, based on the above classification. Since a 5-membered cyclopentylmethyl radical is formed through exo-trig cyclization, it is finally... 

Consequently, the main class of photochromic compounds that shows change in chirality by photochromic transformation is that based on electrocyclization. Fulgides [11-13], diarylethenes [14-16], and spiropyrans [17,18] are included in this class. It may be somewhat strange to put spiropyrans into this category because the colored merocyanine forms. ure known to take zwitterionic structures, which do not cyclize through the electrocyclization mechanism. However, in order to simplify the classification, we consider spiropyrans to cyclize from the neutral dienone structures as the merocyanine forms. 

In summary, chiral solvents have only induced limited enantioselectivity into different types of photochemical reactions as pinacolization, cyclization, and isomerization reactions. These studies are nevertheless very important, because they are among the early examples of chiral induction by an asymmetric environ ment. Based on our classification of chiral solvents as chiral inductors that only act as passive reaction matrices, effective asymmetric induction by this means seems to be intrinsically difficult. From the observed enantioselectivities it can be postulated that defined interactions with the prochiral substrate during the conversion to the product are a prerequisite for effective template induced enantioselectivity. 

The authors feel, that the classification of the synthetic principles applied here is somewhat arbitrary. Multifunctional polycondensations which are conducted in a two-step manner (generation of single-strand intermediates, followed by cyclization), could be classified with the same justification as stepwise processes. On the other hand some of the stepwise syntheses of ladder structures constitute condensations of multifunctional monomers (e.g. the use of butadiynes as starting compounds, see Sect. 4.1.). 

In order to ensue a clear presentation of the results the authors decided to segregate both synthetic principles All synthetic strategies developed from the multifunctional condensations of Stille and Marvel were assigned to this general type of reaction. At the same time the first multistep sequences (polymer-analogous cyclization of poly(methyl vinyl ketone) and polyacrylonitrile) are used as point of reference for the classification of the other type of synthesis (stepwise procedures). 

This classification is illustrated in Scheme 260. Formation of the 1,2- (or 2,3-) bond via the activation of an amide or urea group is a versatile method to synthesize imidazoles. Many reagents, such as PPh3-CCl4, PCI5, or acids, etc., have been used for the cyclization process. 

This classification is illustrated in Scheme 300. The annelation of o-phenylenediamines with carboxylic acid derivatives is the most common method for the syntheses of benzimidazoles. As discussed in CHEC(1984) and CHEC-II(1996), either Br0nsted or Lewis acids can be used to promote the cyclization (e.g.. Scheme 301 also see Section 4.02.9.1(i)) <1998TA2245>. Carboxylic acids and their derivatives, such as acid chlorides, imidates, and phosgene iminium chloride, have been used in these reactions <2005JME8289, 2006BML4994>. 

This classification is illustrated in Scheme 330. Imidazole synthesis under this classification consists mainly of two approaches 1,3-dipolar cycloaddition and one-pot addition-cyclization involving isocyano derivatives. 

Scheme 359 illustrates this classification. Bredereck s formamide cyclization continues to be one of the choices for the synthesis of 4,5-disubstituted or 4(5)-monosubstituted imidazoles <2005JME6632, 2005H(65)2783, 1997S347>. Typically, the reaction proceeds at high temperature with formamides and a-haloketones as the starting materials. For instance, monosubstituted imidazole 1389 is prepared from compound 1388 in 25% yield. 4,5-Disubstituted imidazole 1391 with a hindered side-chain is obtained from 1390 in 48% yield (Scheme 360) <2000JOC8402>. 

As already mentioned [(3), (10), (13), 79, 82, 83] many other types of electron impact induced reactions exist which do not belong to the classification used in this chapter. Among these are reactions which proceed via cyclic reactive intermediates or cyclization processes accompanied by hydrogen transfer at one step of the reaction sequence. This last-mentioned decomposition shall be discussed briefly. Compounds of structure 108 yield intense fragments at m/e 133,110116. 2H labelling of the carboxyl function does not shift the signal mje 133, and it was argued by the authors that the reaction possibly proceeds via the phthalide, 109 (21). 

For purposes of classification the 4-aminopyrazoles are considered to be 4-imino-2-pyrazolines and analogs of 2-pyrazolin-4-ones. These compounds are listed in Table XL. Such compounds can be prepared by direct cyclization using ethyl diazoacetate and ethyl cyanoacetate.92 This is the same as eq. 243, except that the malonic ester is replaced by ethyl cyanoacetate. Purines can be hydrolyzed to 4-imino-2-pyrazolines by using strong acid.1210 1846 By far the most frequently used preparation is reduction of appropriately substituted pyrazoles, such as 4-nitro,368,812,819,1015,1019,1049 4-nitroso1165 or 4-aryl-azo.671 974,995 The hydrolysis of the carbethoxy 4-imino-2-pyrazolines derived from ethyl cyanoacetate and ethyl diazoacetate forms 4-imino-2-pyrazolin-3-carboxylic acid which is readily decarboxylated to the parent compound.92... 

Most syntheses which come under this classification are cyclizations of TV-substituted amidines or ureas. The amidines can be presynthesized or involved as unisolated intermediates formed... 

Establishments primarily engaged in manufacturing synthetic rubber by polymerization or copolymerization. An elastomer for the purpose of this classification is a rubber-like material capable of vulcanization, such as copolymers of butadiene and styrene, or butadiene and acrylonitrile, polybutadienes, chloroprene rubbers, and isobutylene-isoprene copolymers. Butadiene copolymers containing less than 50 percent butadiene are classified in Industry 2821. Natural chlorinated rubbers and cyclized rubbers are considered as semifinished products and are classified in Industry 3069. 

For most reactants, identification and the naming of the functions which participate in the ring closure is easy (as in the example in Section III.I), but several others presented difficulties. Brief comments on how the problems have been resolved may help the reader to appreciate the basis of classification. Several of the difficulties are collected in the (mythical ) compound (1.3) which has four functions, each of which must have an unambiguous name which determines in which chapter the cyclization is to appear. 

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