Cyclizations mechanism

Jug and co-workers investigated the mechanism of cycloaddition reactions of indolizines to give substituted cycl[3,2,2]azines <1998JPO201>. Intermediates in this reaction are not isolated, giving evidence for a concerted [8+2] cycloaddition, which was consistent with results of previous theoretical calculations <1984CHEC(4)443>. Calculations were performed for a number of substituted ethenes <1998JPO201>. For methyl acrylate, acrylonitrile, and ethene, the concerted [8+2] mechanism seems favored. However, from both ab initio and semi-empirical calculations of transition states they concluded that reaction with nitroethene proceeded via a two-step intermolecular electrophilic addition/cyclization route, and dimethylaminoethene via an unprecedented two-step nucleophilic addition/cyclization mechanism (Equation 1). 

While little biosynthetic information is available, it has been suggested [38] that 25 and 26 may be formed from AA (24) and EPA (14) via a cyclization mechanism (Scheme 3) similar to that which forms trans-cyclopropyl-containing diol 28 upon treatment of linoleic acid with performic acid [40]. An alternative biogenetic mechanism (Scheme 4), based upon that proposed for the structurally related red algal metabolites constanolactone A and B [41], would involve the formation and opening of an allylic epoxide intermediate created as a result of a 15-/ -LPO acting on either AA or EPA. Related compounds have been isolated from the coral Plexaura homomalla and the mollusc Aplysia kurodai (see below). 

In addition to the stepwise mechanism, Dautzenberg and Platteeuw proposed another platinum-catalyzed cyclization mechanism (23). This might correspond simply to a disguised stepwise aromatization where the further reaction of unsaturated intermediates is very rapid compared with their desorption. Thus, hydrogen pressure would govern the probability of desorption versus further reaction. Since the cyclization of triene is irreversible, a very low steady-state surface triene concentration must be sufficient to ensure a measurable reaction rate. 

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

The experiments of Bragin et al. 26) support the existence of two cyclization mechanisms. They introduced alternative 2-methylpentane and... 

The photochemistry of a number of long chain aliphatic a-diketones has been reported.46 68 Included in the study were 2,3-pentanedione, 3,4-hexane-dione, 4,5-octanedione, 5,6-decanedione, 2,7-dimethyl-4,5-octanedione, and 1,2-cyclodecanedione. Without exception, irradiation of these compounds in cyclohexane or benzene yielded cyclobutanols, presumably by an internal abstraction-cyclization mechanism. The reaction is quite selective, producing... 

Methyl-o-benzyloxyphenylglyoxylate (32) cyclizes on irradiation to give an isomeric mixture of 2-phenyl-3-carboxymethyl-3-hydroxyl-2,3-dihydro-benzofurans (33), apparently by an internal hydrogen abstraction-cyclization mechanism.103 This abstraction must take place through a seven-membered... 

As a consequence of the conformational mobility of the thiepane 1-oxide ring (115) it was possible to form the necessary planar five-membered cyclic transition state for a thermal E elimination reaction (equation 19). The acyclic sulfenic acid intermediate was not isolated but rearranged to c/s-2-methylthiane 1-oxide by an intramolecular cyclization mechanism (75TL2235). 

The catalysis may not depend on the configuration of the oxime, since both syn and anti function with similar efficacy138. For this reason a mechanism involving the oxime anion acting as a general base catalyst for solvent attack has been favored. However, the observed large rate enhancement is more in accord with a cyclization mechanism, and, until an anti-syn interconversion prior to hydrolysis can be unequivocally eliminated, the direct attack of the oximate anion on phosphorus remains a strong possibility. 

Neither C5- nor C6-cyclization involve carbonium-ion intermediates over platinum metal. The rates of the -propylbenzene - indan reaction (where the new bond is formed between a primary carbon atom and the aromatic ring) and the n-butylbenzene- 1-methylindan reaction (which involves a secondary carbon atom) are quite similar (13). Furthermore, comparison of the C6-cyclization rates of -butylbenzene and n-pentylbenzene (forming naphthalene and methylnaphthalene, respectively) over platinum-on-silica catalyst shows that in this reaction a primary carbon has higher reactivity than a secondary carbon (Table IV) (29). Lester postulated that platinum acts as a weak Lewis acid for adsorbed cyclopentenes, creating electron-deficient species that can rearrange like carbonium ions (55). The relative cyclization rates discussed above strongly contradict Lester s cyclization mechanism for platinum metal. 

The cyclization mechanism is unknown but a one-electron transfer mechanism has been suggested (1997JOC9177). Anodic oxidation of 2-hydroxyiminohydrazones 345 gives respectable yields of 1,2,3-triazole 1-oxides 346 (1982ZC25). 

Reductive coupling of acrylates 521 and imines in the presence of iridium provides azetidin-2-ones bearing aromatic, alkenyl, and alkynyl side chains, with high diastereoselection (Equation 213) <2002OL2537>. The reaction has been proposed to occur through a reductive Mannich addition-cyclization mechanism. 

The cyclization mechanism is explained by the protonation of the double bond in the side chain, producing the stable terciary carbocation. This carbocation is intramolecularly trapped by the hydroxyl group located on... 

Inositol is formed from D-glucose 6-phosphate by direct cyclization of the chain. D-Glucose 6-phosphate was also shown to be a precursor of deoxyinositols in leaf tissues, as D-glucose-6-t 6-phosphate (72) was converted193 into quercitol-2-t (l-2,3,5/1,4) (73) and194 viburnitol-4-t (l-1,2,4/3,5) (74). Labeling in the precursor and the products corresponded to a direct cyclization-mechanism. 

Both direct and sensitized irradiations of cyclohexene 78Z to give trans-anti-transcis-trans-, and cis-anti-cis- 2 + 2]-cyclodimers 80-82 in different ratios [59]. This photodimerization is believed to proceed through the initial formation of the highly strained ( )-isomer 78E, which is followed by the thermal concerted and/or stepwise cyclodimerization with 78Z, although no direct evidence for the intervention of 78E has been obtained and the cyclization mechanism(s) involved are not very clear. In this photocyclodimerization, the enantiodifferentiation occurs not in the cyclodimerization but in the initial photoisomerization step hence we classify this formally bimolecular reaction as a unimolecular enantiodifferenti-ating photosensitization. 

Forasmuch as 1,7- and 1,5-divinylcyclohexasiloxanes, used in polyaddition, represent mixtures of cis- and tram-isomers of the approximate 52 48 ratio, synthesized copolymers are atactic. Reprecipitation of copolymers from toluene solution by methyl alcohol has given viscous or solid (with regard to the value of flexible junction) transparent products with T sPec=0.09-0.29, well soluble in different organic solvents. It is found that at short length of dimethylsiloxane unit (n < 4), copolymer yields are slightly decreased that may be explained by partial proceeding of hydride polyaddition by intramolecular cyclization mechanism (see Tables 3 and 4). 

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