Octa-2,4,6-triene

As an example of a six-electron system, trans-trans-octa-2,4,6- triene illustrates the opposite sense of the stereochemical nature of the thermal and photochemical reactions ... 

Among the methods at hand to synthesize cyclopropane derivatives, carbene addition to alkenes plays a prominent role 63). As a source of vinylcarbenes, cyclopropenes might be useful in this kind of approach. In 1963, Stechl was the first to observe a transition metal catalyzed cyclopropene-vinylcarbene rearrangement64). When treating 1,3,3-trimethylcyclopropene with copper salts, dimerization occurred to give 2,3,6,7-tetramethyl-octa-2,4,6-triene (9), the product from a formal recombination of the corresponding vinylcarbene (Eq. 8). 

CPE = cyclopentene CHX = cyclohexene 1,4-CHX = cyclohexa-1,4-diene 2,4,6-Cg = octa-2,4,6-triene. The small molecules formed, such as ethene or propene, are not listed. 

The Ci5-phosphonium salt 49 is a key intermediate not only in the industrial synthesis of vitamin A, but also in that of p,p-carotene (3). As the structural formula suggests, 3 can be produced by a double Wittig olefination of 49 with the symmetrical Cio-unit 2,7-dimethyl-octa-2,4,6 -triene-1,8-dialdehyde (58) (Scheme 16). 

SCHEME 2.8 Electrocyclization of (2Z,4Z,6E) or (2 ,4ZAQ-octa-2,4,6-triene under thermal... 

On the other hand, irradiation of (2 4 5 )-octa-2,4,6-triene (hexatriene system) promotes an electron to W4, which then becomes HOMO, and bond formation is possible only through conrotation (Scheme 2.9). 

Q 8. Electrocyclization of (2 ,4Z,6 )-octa-2,4,6-triene under photochemical conditions, gives ... 

Sol 8. (a) (2 ,4Z,5 )-Octa-2,4,6-triene (I) is a (4n + 2) rr-electron system, so under photochemical conditions it undergoes conrotatory electrocyclization to give frani-5,6-dimethylcyclohexa-1,3-diene (II). 

With trienes, the extrusion process dominates and the ring opens in a con-rotatOTy fashion. This indicates that in this reaction, one of the components is acting in an antarafacial manner. For example, cis- and [Pg.241]

Treatment of ll,ll-dichloro-l,6-methano[10]annulene with Bu"Li in an ether solvent yields C22H hydrocarbons of labile and complex nature/ Trapping experiments support the intermediacy of compounds ( )6) and (607) formed by the sequence of rearrangements (605)- (606)- (607). Reaction of cyclo-octa-2,4,6-trien-l-one with the anion of methyl 4-(dimethylphosphinyl)but-2-enoate gave (608) and (609) the predominant isomer (609) resulted from base-catalysed isomerization of (608) under the conditions of reaction. Low-temperature oxygenation of the enolate anion derived from the mixture of (608) and (609), followed by reduction with triethyl phosphite, gave a 1 1 mixture of 8-methoxycarbonylbicyclo[5,3,l]undeca-l,3,5,9-tetraen-8-exo-ol and -1,3,5,8-tetraen-lO-exo-ol. Pyrolysis of the p-nitro-benzoate esters of these alcohols effected their conversion into methyl 1,5-methano-[10]annulenecarboxylate (610). 

Scheme 11.6 Decomposition of complex 17 to yield 2,7-dimethyl-octa-2,4,6-triene and din-uclear complex 18.

III.3.1,6-Dioxa-3a-aza-6aA -pnjctapentalene (10-Pn-3 5-aza-2,8-dioxa-1- pnictal)icyclo[3.3.0]-octa-2,4,6-trienes)... 

Table 7. Relative (with respect to 20) Energies of the Isomers of 5-Aza-2,8-dioxapnictabicyclo[3.3.0]octa-2,4,6-trienes (Pn = P, As) ...

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