Cyclization reactions comparison

The products from cyclization reactions of 5-substituted isoindolinediimirtes are 2,9,16,23-tetrasubstituted phthalocyanines, e.g. 2.110121,4lf Again, a mixture of four structural isomers is obtained (see p 737). It should be emphasized that reaction conditions employing isoindoline-diimines are mild in comparison to the use of phthalonitriles. 

Although cyclizations from the direct anodic oxidation of acyclic 1,3-dicarbonyl compounds have not been reported, the analogous mediated reactions have been studied [24]. Snider and McCarthy compared oxidative cyclization reactions using a stoichiometric amount of Mn(OAc)3 with oxidations using a catalytic amount of Mn(OAc)3 that was recycled at an anode surface (Scheme 11). In the best case, the anodic oxidation procedure led to a 59% yield of the desired bridged bicyclic product with the use of only 0.2 equivalents (10% of the theoretical amount needed) of Mn(OAc)3- Evidence that the reaction was initiated by the presence of the mediator was obtained by examining the electrolysis reaction without the added Mn(OAc)3. In this case, none of the cyclized product was obtained. For comparison, the oxidation using... 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

The identity of the heteroatom present in a target molecule dictates the identity of the nucleophilic atom (Z) to be used in an electrophilic heteroatom cyclization reaction of the type shown in Scheme 1. However, successful application of this strategy to the synthesis of specific target molecules also requires selection of appropriate combinations of nucleophilic functionality (Z-R) and activating electrophile. Therefore, major subdivisions within this chapter are based on the identity of the heteroatom, although comparisons between results observed for the different heteroatoms will be made. 

A comparison of the behavior of the R = H, 5-Me-, and 6-Me-substituted succinates (77) in the competitive cyclization reactions (Table I) revealed that in ethanolic sodium ethoxide at 25°C the 6-methyl group fully inhibited the ring closure onto the ring nitrogen. It also revealed that in phosphoryl chloride-polyphosphoric acid at 120 C the formation of 4-oxo-4//-pyrido-[1,2-a]pyrimidine (78 R = 6-Me) was only hindered, whereas at 250 C under the conditions of thermal ring closure, only a slight, if any, steric hindrance occurred. This suggests that the formation of the 6-substituted pyrido[l,2-a]-... 

Table 6 Comparison of Reactivity for the Cyclization Reaction of QHC1...

In comparison to the cyclization reactions shown above, intermolecular Meerwein arylations are often more difficult to conduct. Since the aryl radical addition to the alkene is no longer favored by the close proximity of the reacting centers, the probability for a direct recombination of the aryl radical with scavengers Y is significantly increased (Scheme 17). To maintain the desired reaction course from 44 to 45 including steps (1) and (2) [89, 90], Meerwein arylations have for a long time mostly been conducted with activated alkenes, such as acrylates (R = COOR ), vinylketones (R = COR ), styrenes (R = Ph), or conjugated dienes [91,92]. These types of alkenes are known for fast addition of aryl radicals. 

Page and Jencks (16) estimated that the entropy changes in solution may correspond to an effective concentration of about 6 x 10 M, and high EM values [10 -10 M] have been measured for simple cyclization reactions (17). However, with rare exceptions, the simple approximation of reactants with the help of synthetic supramolecular assemblies results in rate increases that are tiny by comparison with those achieved by enzymes, as evidenced by typical EM values of less than 10 M. 

The following contribution is intended to continue our earlier review [6] dealing with the then known cesium assisted ractions only some very important earlier cesium assisted reactions are described here for the sake of completeness After a short description of the properties of some cesium compounds used in this respect in organic synthesis, those reactions are discussed that have been published more recently and which proceed under the intermediate formation of organic cesium salts at oxygen, nitrogen or sulphur functions. Only those cyclization reactions are considered which allow a direct comparison of the yields obtained with cesium compounds or cesium metal. The description starts with reactions leading to the formation of C—C bonds and proceeds to the syntheses of ethers, lactones, amines and sulfides. The discussion ends with a summary of the attempts to explain the cesium effect . 

A comparison of the yields of the 16-membered ring compound 56i in cyclization reactions using different metal carbonates is shown in Table 6 In the series of the alkali metal carbonates the best yields are obtained with CS2CO3. Tl ions, which possess the highest polarizability in the periodic system of the elements [51a], led to yields which are comparable to those obtained with Rb2C03 and K2CO3, whereas by using the alkaline earth metal carbonates no reaction could be observed [51]. 

The particular influence of cesium ions on the course of cyclization reactions which is proven by many comparison experiments with different metal cations has been called the cesium effect . As a rule, equimolar amounts of cesium compounds are used, which means that one cannot normally speak of a cesium catalysis. Nevertheless, a catalytic effect with respect to the cesium cation often cannot be excluded. 

By comparison, the reaction of L-tryptophan with peroxyacetic acid21 or photosensitized oxygenation22, involving ring closure and introduction of hydroxy and hydroperoxy groups at the 3a-position, proceeded with poor stereoselectivity. The related cyclization reactions of tryptophan and tryptamine derivatives mediated by electrophiles are described in Section 7.2.6.2.1. 

There is continued interest in micellar and surface effects, and unusual reaction media such as polymers, de Mayo and his co-workers have reported further examples of the way in which photoreactions can be modified if one reactant is adsorbed on to silica gel. Irradiation of nonan-5-one as a urea clathrate leads to reduced fragmentation and relatively increased and more selective cyclization in comparison with behaviour in methanol (Casal and... 

C—H bond 280,281) comparison, only trace amounts of cyclopentane resulted from the CuSO -catalyzed decomposition of l-diazo-2-octanone or l-diazo-4,4-dimethyl-2-pentanone It is obvious that the use of Rh fOAc) considerably extends the scope of transition-metal catalyzed intramolecular C/H insertion, as it allows for the first time, efficient cyclization of ketocarbenoids derived from freely rotating, acyclic diazoketones. This cyclization reaction can also be highly diastereo-selective, as the exclusive formation of a /r< i5-2,3-disubstituted cyclopentane carboxylate from 307 shows The stereoselection has been rationalized by... 

The tertiary-amine catalysis will favour the intramolecular cyclization reactions which will lower the crosslink density of the network in comparison with the situation if only intermolecular reactions took place. 

Radical Cyclizations Section C of Table 11.3 shows some reactions involving cyclization of unsaturated radicals. This type of reaction is an important application of free radical chemistry in synthesis, and is discussed more thoroughly in Section 10.3.3 of Part B. Rates of cyclization reactions have also proven useful in mechanistic studies, where they can serve as reference points for comparison with other reaction rates. 

Nearly all of the cyclization reactions that we have discussed have been intramolecular Sn2 reactions where one end of the molecule acted as the nucleophile displacing the leaving group on the other end. We kept to this sort of reaction in order to make valid comparisons between different ring sizes. But you can imagine making saturated heterocycles in plenty of other ways—intramolecular substitution at a carbonyl group, for example, such as happens in this lactonization reaction, or intramolecular addition of an oxyanion on to an alkyne. 

Under the conditions of a kinetically controlled polycondensation reaction, cyclization reactions compete with propagation steps. The extent of cyclization depends on the flexibility of the polymer chain and on the concentration of the active species. Blends of PES with its homologous macrocyclic oligomers show greatly lowered melt viscosities in comparison to the corresponding original PES. 

These cyclization reactions are suppressed if a phosphorus-modified B-pentasil zeolite is used, as shown by comparison with the conventional silica-based catalyst. Only the silica carrier is replaced by a zeolitic carrier. The amount of cyclic compounds is reduced from 6.7 mol % to 1.4 mol %, and the yield of the desired product adipodinitrile is about 10 % higher [20]. This is evidently due to the restricted transition state shape selectivity of the pentasil zeolite the steric constrains of the zeolite framework prevent the formation of the transition state of the cyclic compounds. The reactions on the outer surface are responsible for the 1,4 mol-% yield of cyclic compounds. 

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