Ring closures, electrocyclic

Whether they go in the direction of ring opening or ring closure, electrocyclic reactions are subject to the same rules as all other pericyclic reactions—you saw the same principle at work in Chapter 35 where we applied the Woodward-Hoffmann rules both to cycloadditions and to reverse cycloadditions. With most of the pericyclic reactions you have seen so far, we have given you the choice of using either HOMO-LUMO reasoning or the Woodward-Hoffmann rules. With electrocyclic reactions, you really have to use the Woodward-Hoffmann rules because (at least for the ring closures) there is only one molecular orbital involved. 

Ring contraction and intramolecular cyclization constitute a convenient route to ring-fused systems that would be difficult to synthesize in other ways. H- 1,2-Diazepines (538) undergo electrocyclic ring closure to the fused pyrazole system (539) (71CC1022). Azepines also undergo similar valence bond isomerizations. 

This ring closure is the final step of the reaction of hydrazines with 1,3-difunctional compounds (Section 4.04.3.1.2(ii)), and numerous examples in the literature of pyrazoles have been described. In some cases the N—C ring closure occurs by a concerted mechanism, classified by Huisgen (80AG(E)947) as 1,5-electrocyclizations. 

The important synthesis of pyrazoles and pyrazolines from aldazines and ketazines belongs to this subsection. Formic acid has often been used to carry out the cyclization (66AHQ6)347) and N-formyl-A -pyrazolines are obtained. The proposed mechanism (70BSF4119) involves the electrocyclic ring closure of the intermediate (587) to the pyrazoline (588 R = H) which subsequently partially isomerizes to the more stable trans isomer (589 R = H) (Section 4.04.2.2.2(vi)). Both isomers are formylated in the final step (R = CHO). 

Methylvinyldiazirine (199) rearranges at room temperature in the course of some days. Formation of the linear isomer is followed by electrocyclic ring closure to give 3-methyl-pyrazole. The linear diazo compound could be trapped by its reaction with acids to form esters, while the starting diazirine (199) is inert towards acids (B-71MI50801). 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

All-ci5-cyclononatetraene undergoes a spontaneous electrocyclic ring closure at 25°C to afford a single product. Suggest a structure for this product. Also, describe an alternative symmetry-allowed electrocyclic reaction that would lead to an isomeric bicyclononatriene. Explain why the product of this alternative reaction pathway is not formed. 

Trifluoromethyl-substitutedazimines are surprisingly stable compounds. They are accessible by 1,3-dipole metathesis from tnfluoromethyl-substituted azomethine imines and certain nitroso compounds [187, 188] On photolysis, an electrocyclic ring closure first gives the triaziridines, which are stable at room temperature. On heating above 80-100 C, a valence tautomenzation takes place and azimines are formed [189] (equation 43). 

The reaction of 4,4-bis(tnfluoromethyl)-I,3-diaza-1,3-butadienes with certain a,P-unsaturated ketones yields pyrimidine derivatives A two-step mechanism, metathesis-electrocyclic ring closure and metathesis-intramolecular ene reaction, is a plausible explanation for the experimental results (pathway 4, equa-bon 25) [259]... 

In summary, these results were interpreted to support rate-determining electrocyclization for the ring closure, starting from the all-trans iminium 11, via the cis conformation 13 of the neutral form, followed by fast proton transfer and elimination of aniline (Scheme 8.4.6). 

Another total synthesis used the rich chemistry of iminophosphoranes (95AHC159). The /3-(3-indolyl)vinyl iminophosphorane 354 underwent an aza-Wittig/electrocyclic ring closure reaction to give the carboline 355 which was hydrolyzed with lithium hydroxide (Scheme 106). A selective reduction, deprotection, decarboxylation and diazotation followed by ring closure gave Fascaplysine (353) (94TL8851). 

An electrocyclic ring closure then leads to a cyclohexadienone complex 7, which upon migration of a proton, yields the chromium tricarbonyl-hydroquinone complex 3. 

Upon treatment of a divinyl ketone 1 with a protic acid or a Lewis acid, an electrocyclic ring closure can take place to yield a cyclopentenone 3. This reaction is called the Nazarov cyclization Protonation at the carbonyl oxygen of the divinyl ketone 1 leads to formation of a hydroxypentadienyl cation 2, which can undergo a thermally allowed, conrotatory electrocyclic ring closure reaction to give a cyclopentenyl cation 4. Through subsequent loss of a proton a mixture of isomeric cyclopentenones 5 and 6 is obtained ... 

However, if the azepine is C-monosubstituted, e.g. 14, or unsymmetrically substituted, then two isomeric 2-azabicycloheptadienes, e. g. 15 and 16, may result corresponding to electrocyclic ring closure involving C2-C5 or C4-C7 of the azepine ring. In practice, the ratio of the two isomers formed (which may be separated by vapor phase chromatography) varies with the position of the substituent.236 In contrast, irradiation of methyl 2,5-di-tm-butyl-l//-azepine-l-carboxylatein methanol yields only methyl 3,5-di-tert-bulyl-2-azabicycIo[3.2.0]hepta-3,6-diene-2-carboxylate (81 %).70... 

Diazocines are isolated, as byproducts, in another photochemical reaction which starts from fluorinated pyridazines. On irradiation of 6 a Dewar diazabenzene derivative is formed, via an electrocyclic ring closure, which looses a fluorinated nitrile to give the azacyclobutadiene system 7. This reactive intermediate then leads vide supra) to the 1,5-diazocine 8.50... 

Dotz reaction, since both an alkyne and CO are inserted. However, the additional double bond present in the starting complex participates in the subsequent electrocyclic ring closure, giving rise to eight-membered carbocycles. 

For the indene derivatives M two different reaction pathways have been discussed so far, starting from the ( )-metallatriene D. A strongly coordinating solvent may induce an electrocyclic ring closure yielding the metallacyclohexa-diene K, and the indene product is obtained after tautomerisation and reductive... 

Wan s group showed that the observed photodehydration of hydroxybenzyl alcohols can be extended to several other chromophores as well, giving rise to many new types of quinone methides. For example, he has shown that a variety of biphenyl quinone methides can be photogenerated from the appropriate biaryl hydroxybenzyl alcohols.32,33 Isomeric biaryls 27-29 each have the benzylic moiety on the ring that does not contain the phenol, yet all were found to efficiently give rise to the corresponding quinone methides (30-32) (Eqs. [1.4—1.6]). Quinone methides 31 and 32 were detected via LFP and showed absorption maxima of 570 and 525 nm, respectively (in 100% water, Table 1.2). Quinone methide 30 was too short lived to be detected by LFP, but was implicated by formation of product 33 that would arise from electrocyclic ring closure of 30 (Eq. 1.4). 

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