Cyclization probability

Iodine (or IC1, NBS, IpyrgBF promotes similar cyclizations, probably via an electrophilic mechanism (Equation (181)).147 147a... 

In some cases80 where the nitro group is ortho to an ethyl group, there is a competing reaction which leads, via cyclization probably involving the nitronic acid form of the nitro group, to the corresponding anthranil 71. 

Nitrobenzoylamino)-2,2-dimethylpropanamide (143 R = Me) reacts in methanol-DMSO solution with sodium methoxide to yield 5,5-dimethyl-2-(4-nitrophenyl)imidazol-4(5//)-one (144 R = Me). The 4-methoxyphenyl derivative and the parent phenyl derivative react similarly, as do compounds in which variation of the 2-substitutent (R = Pr , Ph, 4-O2NC6H4) was made. The mechanism of the cyclization probably involves initial formation of the anion of the alkanamide (145), which adds to the carbonyl group of the benzamido moiety to yield the tetrahedral oxyanion (146) proton transfer and dehydration then yield the heterocycle (144). The kinetics of hydrolysis in water at 70 °C and pH 2-11 of A-glycidylmorpholine (147) have been reported. ... 

Various mechanisms have been proposed to account for cyclization of 1,3-dienes. Cyclization probably occurs by attack of the propagating carhocation on trans 1,4-douhle bonds,... 

In HeLa cells and presumably in normal mammalian nuclei, the initial cleavage at the 5 end of the in-tron leaves a 3 phospho group (Fig. 28-18B, step a), which is cyclized, probably in an ATP-dependent process, to the 2, 3 -cyclic phosphate (step b). This is ligated to the other piece of RNA by a direct displacement on the cyclic phospho group (step c).602... 

Three l-hydroxy-3-phenylpyrroles (46) were formed (72JOC1561) in 26-44% yields, instead of the anticipated 6//-oxazines, when the corresponding E-oximes (44) were treated successively with one equivalent of sodium hydride in dimethylformamide (DMF) followed by triphenylvinylphosphonium bromide Z-benzil oxime gave no pyrrole. The cyclization probably proceeds via cyclization to the N-oxide 45 and tautomerism to the 1-hydroxypyrrole. 

Unexpectedly, the intramolecular allylation of a,/ -unsaturated aldehydes, under identical reaction conditions, gave bicyclic product 165 as a single diastereoisomer (Scheme 13.56). This cyclization probably proceeded through a [1,2]-migration of the silyl group in the /hsilyl cationic species 163, followed by a subsequent intra-... 

When phenylacetylenes (3 mol), tellurium dioxide (1 mol), and a lithium halide (14-18 mol) were refluxed in acetic acid, 3-halobenzotellurophenes were obtained in yields ranging from 21 to 92%2. A tellurium(IV) acetate halide, formed from tellurium dioxide, lithium halide, and acetic acid, probably adds to the carbon-carbon triple bond of the phenylacetylene. The intermediary 2-phenyl-2-haloethenyl tellurium acetate halide cyclizes probably by loss of acetic acid. The 3-halobenzotellurophene Te, 7e-acetate halide is then reduced to 3-halobenzotellurophene by an excess of the phenylacetylene. Diphenylacety-lene and diphenyldiacetylene did not react under these conditions. 

What must happen here is that the two compounds must exchange protons (or switch enolates if you prefer) before the aldol reaction occurs. Cyclization probably occurs next through C-N bond formation and, finally, dehydration is forced to give the Z-alkene. 

Brominations mth S Br and PBr. The cyclization of 4,4 -dichlorodiphenylamine (57) with S2Br2 to l,9-dibromo-3,7-dichloro-phenothiazine (59) was already mentioned in Section V,A,l,a. In this reaction cyclization probably takes place before bromination. 

The only task we should do is to estimate vR. and vL. Let us confine ourselves to sufficiently slow reactions as compared with the diffusion process so that the mixing is so perfect as to excuse the approximation of quasi-static changes. Then, vR is simply given by the product of the cyclization probability 2P of a j-chain and the total number of chances, j, for j-ring formation... 

The cyclization rate vR of j-chains is in proportion to the cyclization probability 3 multiplied by the total number (j)j of chances of cyclization of j-chains ... 

In the Flory model, no ring formation is allowed, and g>-— 0, so that from Eq. (104) it follows that yc —thus the critical dilution disappears. In real gelations, such disappearance of yc never occurs because of the presence of the finite cyclization probability. The critical dilution is a general theorem in real gelations. 

Prior to the comparison with simulation experiments, it is necessary to calculate the cyclization probability, 2P, which is the only unknown parameter with Eq. (114). Note that Eq. (114) was derived based on the lid expansion, so that one can apply Gaussian statistics to 2P. According to the definition, 2P has the form ... 

Ring polymers are synthesized by linking two reactive ends of linear polymers in dilute solution. The cyclization probability can be defined as the... 

Others have reported that there is a definite effect of aryl substituents and that the derivative 39 undergoes cyclization in the presence of dissolved oxygen to yield the hydroxyflavone 40. The cyclization involves the formation of a biradical 41 that cyclizes in the presence of oxygen to yield the hydroxyflavone 42. The cyclization of such chalcones has been known for many years and studied in some detail . Research showed that the derivatives 43 undergo efficient cyclization to 44 (Scheme 4) on irradiation at wavelengths >365 nm in dioxan or ethyl acetate solution . The reaction is solvent-dependent and poorer yields are obtained in benzene or chloroform solution . Further studies demonstrated, for the conversions shown in Scheme 5, that the cyclizations probably arose from a 7171 transition . ... 

Pheny(acetylenes (3 mol) react with Te02 (1 mol) and excess of lithium halide in refluxing HOAc to produce 3-halobenzotellurophenes via the addition of a Te(IV) acetate halide to the triple bond, cyclization (probably by loss of HOAc) and reduction of the Te(IV) cyclic product to 3-halobenzotellurophene by an excess of the pheny(acetylene. Owing to isolation facilities, the product is converted into the crystalline dichlorides that is reduced to the benzotellurophenes. ... 

Cyclization probability depends upon the radial distribution function... 

One anticipates that cyclization probability will decrease as... 

Quinolizidine derivatives formed by hydrogenation of the dicyano ketone 4 to catalyst loading affects the stereochemistry of cyclization, probably by isomerization. 

The pronounced sensitivity of alkynes to cyclization in the presence of external nucleophiles is illustrated in studies of formaldiminium ion (99) in which an alkyne and alkene compete as intramolecular ir-nucleophiles. While cyclization of (99) in water afforded 4-hydroxypiperidine (98) in 73% yield, cyclization in the presence of 10 equiv. of Nal gave vinyl iodide (100) in 76% yield (Scheme 34). It has been suggested that the mechanism of these nucleophile-promoted iminium ion-alkyne cyclizations probably involves rate-determining attack of the nucleophile on a ir-complex or bridged cation produced from reversible interaction of the iminium ion and alkyne groups. ... 

The occurrence of a fast cationic aza-Cope reairangement has been proven in the case of vinylsilane cyclizations (equation 109). Enantiomerically pure ethoxycarbamate (148) cyclizes under the influence of BF3-Et20 to a I I mixture of racemic tetrahydropyridines (149) and (150). Thus, the equilibration of iminium ions (151) and (152), attended with racemization (equation 110), occurs much faster than ir-cy-clization. Because allylsilanes are more nucleophilic than vinylsilanes, ir-cyclization probably takes place from (152). The fast rate of cyclization of allylsilanes is apparent from the cyclization via (140), where cationic aza-Cope rearrangement cannot compete (equation 105). 

Cyclization probably occurs in the course of double acylation of the tautomeric enamine 38 via the intermediate 39. The azadienes 37 are easily accessible [90]. 

Cyclic ethers. Alkenyl esters undergo deoxygenative cyclization, probably via an... 

---