1,3-Dienes electrocyclic ring closure

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... 

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... 

In an electrocyclic ring-closure reaction a new o-bond is formed between the end atoms of a conjugated system of double bonds. Consider the cyclisation of buta-1,3-diene ... 

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... 

The initial photochemical reaction is intramolecular ortho cycloaddition leading to bicyclo[4.2.0]octa-2,4-diene derivatives. These normally are not detected because they undergo rapid thermal disrotatory ring opening to all-cis cyclooctatrienes which have large extinction coefficients and absorb a second photon to undergo electrocyclic ring closure to the photostable bicyclo[4.2.0] octa-2,7-dienes. 

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). 

A -(4-Tolyl)-A -(triphenylphosphoranylidene)carboximidamide 11 reacts with aliphatic and aromatic aldehydes in refluxing xylene to give quinazolines 12 and 3,4-dihydroquinazolines 13, the ratio of which depends on the aldehyde employed. The reaction proceeds under non-acidic conditions and probably involves an aza-Wittig reaction followed by a 67t-electrocyclic ring closure of the 1,3-diazabuta-l,3-diene intermediate. Subsequent [l,5]-sigmatropic hydrogen shift results in the formation of the 3,4-dihydroquinazoline which may aromatize under the reaction conditions to yield the quinazoline 12. °... 

Irradiation of s-c/s-2,3-dimethylbutadiene yields both electrocyclic ring closure and double-bond isomerization products (Scheme 28). While the ring closure in. s-c/.s-butadiene, isoprene, 2-isopropylbutadiene, and 1,3-penta-diene is considerably less efficient than s-cis-s-trans isomerization, in s-cis-2,3-dimethylbutadiene it is about 50 times faster (Squillacote and Semple, 1990). This effect is counterintuitive, since the two methyl groups appear to hinder the rotation about the central C—C bond in butadiene. 

Wirz, J., Persy, G., Rommel, E., Murata, I., Nakasuji, K., Photoisomerization Pathways of 8,16 Methano[2.2]metacyclophane 1,9 diene. A Model Case for Adiabatic Electrocyclic Ring Closure in the Excited Singlet State, Helv. Chim. Acta 1984, 67, 305 317. 

This looks as though each of the C—C bonds is independently the result of both HOMO/LUMO interactions, with an endo selectivity as well. In the presence of dienes, these species behave as allyl cations (see p. 259) and undergo clean [4 + 2] cycloadditions, as in the reaction of the oxyallyl 6.372 giving the tricyclic ketone 6.373, which is similar to the diene 6.369. Normally, oxyallyls are in equilibrium by disrotatory electrocyclic ring closures with cyclopropanones and with allene oxides, but the presence of the five-membered ring in these particular examples makes these pathways counter-thermodynamic. 

Electrocyclic ring closure of l-oxa-3,5-diaza-l,3,5-hexatrienes forms 2//-l,3,5-oxadiazines (see Section 6.18.10.1.1), while 4//-l,3,5-oxadiazines arise by (4 - - 2) t cycloaddition of electron-deficient l-oxa-3-aza-1,3-dienes with carbonitriles (see Section 6.18.10.2.2.i) and by cycloaddition of hexa-halogenopropanones (1 equiv.) with dialkylcyanamides (2 equivs.) (see Section 6.18.10.3.l.i). 

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 . 

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). 

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