1,3,5,7-Octatetraene, electrocyclization

The nucleophilic attack with 2-700 and 2-701 can proceed in a syn or anti manner to provide either 2-702 or 2-703, or both [357]. If2-703 is formed, it follows a charge-driven conrotatory opening of the cyclobutene ring with generation of the coiled 1,3,5,7-octatetraene 2-704. This intermediate is capable of a rapid helical equilibration [357] and a regioselective 8jt electrocyclic ring closure to give 2-705 [358]. 

The predicted conrotatory cyclization of octatetraenes was confirmed for the case of the methyl-substituted compounds, which above 16 °C readily formed the cyclooctatrienes shown in equations 13 and 14)14. We conclude this section with an electrocyclic reaction involving ten TT-electrons, that is, the formation of azulene (17) when the fulvene 16 is heated (equation 15)15,16. 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

A number of electrocyclic reactions have been carried out with systems of other sizes, e.g., conversion of the 1,3,5,7-octatetraene 97 to the cyclooctatriene 98.383 The stereochem-... 

With two more electrons in the conjugated system, making eight in all, the octatetraenes 4.51 and 4.54 show conrotatory closure giving the cyclo-octatrienes 4.52 and 4.55, However, the reaction can only just be stopped at this stage, for the products undergo a second electrocyclic reaction giving the bicyclic dienes 4.53 and 4.56 as a result of the allowed disrotatory reaction of the all-m hexatriene units. 

Fig, 4. Transition structure for the electrocyclic ring closure of 1,3,5,7-octatetraene to 1,3,5-cyclooc-tatriene... 

Thomas IV, B. E. Evanseck, 1. D. Houk, K. N. Electrocyclic reactions of 1-substituted 1,3,5,7-octatetraenes. An ab initio molecular orbital study of torqu-oselectivity in eight-electron electrocyclizations, J. Am. Chem. Soc. 1993, 115,... 

There are abundant examples in the literature which reveal that the thermal electrocyclization of 8e systems to form eight-membered rings proceeds with lower activation energies than for the lower vinyl-og, (Z)-hexatrienes. The helical geometry of the transition state leads to less steric congestion about Ae reacting termini of the octatetraene and this likely accounts for the facility of the thermal 8e conrotatory process. Minor structural differences can induce cycloreversion of the process. ... 

Alkynes undergo cycloaddition on irradiation with benzene or naphthalene derivatives or with other aromatic compounds. With a benzene derivative the product is usually a cyclo-octatetraene which results from thermal electrocyclic ring-opening of the bicyclo-octatriene formed initially by 1,2-addition of the alkyne to the benzene ring (equation 61) . The intermediate can be trapped using a dienophile such as tetracyanoethylene (equation 62) ". The first step of the photoaddition process involves excitation of the alkyne , and orbital symmetry considerations suggest that concerted 1,2-addition is allowed if the alkyne is excited but not if the benzene is excited ... 

Sol 12. (b) The product, 7,8-dimethyl cycloocta-l,3,5-triene (1) with tmns stereochemistry is obtained upon disrotatory ring closure of octatetraene system, which occurs only under photochemical conditions. Similarly, a bicyclic product (11) with cis stereochemistry at fused carbons is obtained when a triene system undergoes disrotatory electrocyclization under thermal conditions. 

Sol 14. (a) The c/i-orientation of hydrogens in the product indicates that cyclization involves disrotatory mode. Under thermal conditions, cyclo-octatetraene preferentially undergoes a hrr-electron disrotatory electrocyclization forming only the cw-isomer. It should be noted that cyclooctatetraene does not undergo the thermally allowed 4TT-electron conrotatory electrocyclization as the frani-fused bicyclic structure is highly strained. 

The thermal (8 r,67r)-electrocyclization of 1,3,5,7-octatetraene with one double bond embedded in an aromatic moiety has been described (Scheme 31). ... 

Scheme 6.39 Assembly of a 1,3,5,7-octatetraene by a Stille coupling followed by two consecutive electrocyclizations [158].

A theoretical study of the sequential addition of a chiral alkyl-substituted cyclopen-tenyl lithium and then an achiral cyclopentenyl lithium to a squarate ester has shown that the second carbonyl addition initiates a domino process whereby the dialkox-ide intermediates undergo electrocyclic four-membered ring cleavage to yield endo-and exo-octatetraene rotamers which are interconvertible these then form isomeric cyclooctatriene products by electrocyclization. ... 

The higher analogs in the series of electrocyclic reactions, the electrocyclic ring closures of 1,3,5-hexatriene and 1,3,5,7-octatetraene, have been studied using HF, MP, and DFT methods. Figure 2 shows the calculated transition structure for the disrotatory ring closure to 1,3-cyclohexadiene, and selected results for this reaction are summarized in Table 2. 

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