Trienes electrocyclic ring closure

The first electrocyclic ring closure involves eight electrons, so it is conrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the tetraene, which are both in, become trans. The second electrocyclic ring closure involves six electrons, so it is disrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the triene, which are both out, become cis. This is the arrangement observed in the natural product. 

Recent theoretical and spectroscopic studies indicate that in aliphatic dienes and trienes, excitation to the spectroscopic l1 state usually results in facile twisting about the termini in the stereochemical sense dictated by orbital symmetry selection rules for the appropriate electrocyclic ring closure, motions which are often accompanied by some degree of planarization of the carbon framework. In general, relatively minor distortions... 

For example, the electrocyclic ring closure of cycloocta-1,3,5-trienes 1 to bicyclo[4.2.0]octa-2,4-dienes 2 has been widely used for the preparation of cyclobutanes (see Houbcn-Weyl, Vol. 4/4,... 

Computational studies of the electrocyclic ring closures of cycloheptatetraene to bicyclo[3.2.0]hepta-l,3,6-triene and bicyclo[3.2.0]hepta-2,3,6-triene have been reported,292 and the reaction of P-chloroiminophosphine with 1 -(dialkylaminojalkynes has been shown to yield 1,2-azaphosphetines via the (E)-1 -aza-2-phosphabuta-1,3-dienes.293... 

Photochemical electrocyclic ring-closure in a 4-electron system works well for many acvclic dienes (2.17) and related cvclic systems (2.18). The situation with conjugated trienes is more complex, and they can act as 6-e ectron systems (2.19) leading to cyclohexal, J-dienes, or as 4-electron systems (2.20) giving cyclobutenes. In addition they can undergo other photochemical reactions such as geometrical isomerization about the central double bond (which must be cis if a 6-electron electrocyclic nng-closure is to take place). 

Aryl-substituted butenynes do appear to undergo a photochemical electrocyclic ring closure, which is followed by a hydrogen shift in the initially formed cyclohexa-1,2,4-triene to produce a compound with a new fused aromatic ring (equation 13), but the process has been shown to occur by way of non-concerted radical or ionic pathways. ... 

The photochemistry of w-vinyl-orf/to-quinodimethanes is typical of trienes in which at least one of the two C—C bonds is frozen in the s-cA conformation competing electrocyclic ring closure to regenerate the precursor, formation of benzobicyclo[3.1.0]hex-2-enes and [1,5]-H shifts to arylallenes. The only triene photoproduct which is not generally... 

Electrocyclic ring closures occur in the gas phase and in solution, are relatively insensitive to solvent polarity and do not require catalysis. Typically, they are also stereospecific bond formation between terminal carbons of triene systems occurs in a disrotatory manner, and bond formation between termini of conjugated diene and tetraene systems occurs in a con-rotatory manner Kinetic data on electrocyclic ring closures are summarized in Table . 

Electrocyclic ring closures of acyclic trienes and tetraenes have substantial negative entropies of activation, as anticipated for reactions in which rotational freedom is lost on assembling the cyclic transition states. Conversions of the more rigid cyclic trienes and tetraenes to bicyclic hydrocarbons usually have less negative entropies of activation. 

Azepines 3 and (alkylsulfanyl)-dihydroazepines 2 were synthesized by the cyclization of 2-aza-l, 3,5-trienes 1. The proposed mechanism involves deprotonation with base followed by electrocyclic ring closure and either elimination of the alkylsulfanyl group to form the azepine, or reprotonation with water to form the dihydroazepine (14TL2495). 

Hydride shift ensues to generate ruthenium haxa-l,3.5-triene III, which undergoes 671-electrocyclic ring closure and reductive elimination to furnish cyclopentadiene IV. Ultimately, the most stable regioisomer 211 is yielded via a [1,5]-hydrogen shift. 

Theoretical studies have been carried out on the conrotatory and disrotatory reaction pathways of hexa-l,3,5-triene to cyclohexadiene, and the effect of solvent and salt effects on the rates of the electrocyclic ring closure of (IZ, 3Z, 5 )-l,2,6-triphenylhexa-l,3,5-triene has been determined. An ab initio study of the electro-cyclization of (Z)-hexa-l,3,5-triene and its hetero-substituted analogues has been undertaken.The involvement of a lone pair on the nitrogen or oxygen atom appears to facilitate the interaction between the terminal atoms that bond to each other to close the ring. It has been reported that the intramoiecular aza-Diels-Alder reaction of an a./S-unsaturated hydrazones to a quinone, as in (197), is followed by an unprecedented rearrangement in which the aminoaryi moiety formally undergoes a 1,2-shift to yield benzo- or pyrido-[fc]acridine-6,11-dienes (198). 

In Summary Conjugated dienes and hexatrienes are capable of (reversible) electrocyclic ring closures to cyclobutenes and 1,3-cyclohexadienes, respectively. The diene-cyclobutene system prefers thermal conrotatory and photochemical disrotatory modes. The triene-cyclohexadiene system reacts in the opposite way, proceeding through thermal disrotatory and photochemical comotatory rearrangements. The stereochemistry of such electrocychc reactions is governed by the Woodward-Hoffmann rules. 

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... 

The thermal ring closure reaction of a 1,3,5-triene to a 1,3-cyclohexadiene occurs by a concerted disrotatory electrocyclic mechanism. An example of the latter is the oxepin-benzene oxide equilibrium (7) which favors the oxepin tautomer at higher temperatures (Section 5.17.1.2). Oxepin (7) was found to rearrange to phenol during attempted distillation at normal pressure (67AG(E)385>. This aromatization reaction may be considered as a spontaneous rearrangement of the oxirane ring to the dienone isomer followed by enolization (equation 7). 

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