Trans-5,6-dimethyl- 1,3-cyclohexadienes

Der Cyolohexadien-(l,3)-RingschluB vollzieht sich photochemisch conrotatorisch. Aus all-trans-Oetatrien-(2,4,6) entsteht iiber die fraJw,cis, rare[Pg.252]

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... 

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

Cyclobutyl-cis-4-trans-5-dimethyl- 1,3-dioxolane, by reaction of erythro-3-methane-sulfonyloxy-2-butyl cyclo-butanecarboxylate with sodium borohydride, 51, 12 hydrolysis to cyclobutane-carboxaldehyde, 51, 13 3,5—CYCLOHEXADIENE—1, 2-DI CARBOXYLIC ACID, 50, 50 A1 a-Cyclohexaneacetaldehyde, 53, 104... 

A study on the homo- and copolymerization of a variety of dienes such as 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, E-l,3-pentadiene, E-l,3-hexadiene, E-l,3-heptadiene, E-l,3-octadiene, E,E-2,4-hexadiene, E-2-methyl-l,3-pentadiene, 1,3-cyclohexadiene mainly focused on mechanistic aspects [139]. It was shown that 1,4-disubstituted butadienes yield frans-1,4-polymers, whereas 2,3-disubstituted butadienes mainly resulted in cis- 1,4-polymers. Polymers obtained by the polymerization of 1,3-disubstituted butadienes showed a mixed trans-1,4/cis-1,4 structure (60/40). The microstructures of the investigated polymers are summarized in Table 26. 

Stereochemistry Electrocyclic reactions, like all pericyclic processes, exhibit great stereospecificity. The stereospecificity of such reactions is demonstrated by thermal closure of fransjds,trans-2,4,6-octatriene (8.12) to ds-5,6-dimethyl-1,3-cyclohexadiene (8.13) and of the isomeric frans,cis,czs-octatriene (8.14) to frans-5,6-dimethyl-1,3-cyclohexadiene (8.15). 

The uncatalyzed Diels-Alder reaction is well known to be highly stereospecific, preferentially occurring via syn addition to both the diene and dienophilic components. Stereochemical studies of the cation radical Diels-Alder reaction have confirmed an analogous stereospecificity in two distinctly different systems. The initial study was carried out using the cycloaddition of the three geometric isomers of 2,4-hexadiene as dienophilic components and 1,3-cyclohexadiene as the diene component [39]. Each of the three isomers of the acyclic diene was found to add stereo-specifically to cyclohexadiene. In a more recent study, the cis and trans isomers of 1,2-diaryloxyethenes were found to add stereospecifically to 1,3-cyclopentadiene (Scheme 17) and also to 2,3-dimethyl-l,3-butadiene [46]. 

The three systems are (1) 1,3-cyclohexadiene with all three geometric isomers of 2,4-hexadiene (2) 1,3-cyclopentadiene with cis- and trani-diaryloxyethenes and (3) 2,3-dimethyl-1,3-butadiene with cis- and Irani-1,2-diaryloxyethenes. Further, stereospecific cyclobutanation has been observed in two distinct systems (1) cyclobutadimerization of cis- and trani-anethole (in both dichloromethane and acetonitrile) and (2) 2,3-dimethyl-l,3-butadiene with cis- and trans-, 2-diaryloxyethanes. 

Fhe most striking feature of electrocyclic reactions is their stereochemistry. For example, (2E,4Z,6 )-2,4,6-octatriene yields only ds-5,6-dimethyl-1,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trans-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

Oxo-1,5-di-tert.-butyl - cyclohexadien - (1,4) - (3-spiro-l)-cis- 2,3 -dime -thyl-cyelopropan isomerisiert sich durch einen intramolekularen Energie-Transfer vom Dienon-System iiber einen Dreiring zum 6-Oxo-l,5-di-tert.-butyl-cyclohexadien-(l,4)-(3-spiro-l) - trans -2,3- dimethyl-cyclopropan 2 ... 

Bei Belichtung von III mit unterschiedlichen Resten sind zwei verschiedcne trans,cis,cis-Anordnungen moglieh 2-Methyl-5-isopropyl-cyclohexadien-(l,3) geht beispielsweise in 3,7-Dimethyl-cis,trans- und cis, cis-octatrien-(l,3,5) liber5. Bei Verbindungen vom Typ IV ist... 

Wahrscheinlich spielt die GroBe der Substituenten auch bei der Bildung des trans-verkniipften Addukta aus Cyclohexadien-(l,3) und Dimethyl-maleinsaure-anhydrid (XI, S. 384) eine wichtige Rolle. 

Over 300 diol metabolites are known.8 Several diols and some secondary synthons derived from them have recently become commercially available Eastman Fine Chemicals, Genencor (1 S-cis)-3-Chloro-3,5-cyclohexadiene-1,2-diol, (1 S-cis)-3-bromo-3,5-cyclohexadiene-1,2-diol, (1 S-cis)-3-iodo-3,5-cyclohexadiene-1,2-diol, (5S-cis)-5,6-dihydroxy-1,3-cyclohexadiene-1 -carbonitrile, cis-2R,3S-2,3-dihydroxy-2,3-dihydrobenzonitrile acetonide, (1R-cis)-1,2-dihydro-1,2-naphthalenediol, (1R-cis)-1,2,3,4-tetrahydro-1,2-naphthalenediol, (4S-trans)-4,5-dihydroxy-3-oxo-1 -cyclo-hexene-1-carboxylic acid, furo[3,4-d]-1,3-dioxol-4(3aH)-1 -dihydro-6-hydroxy-2,2-dimethyl-[3aR-(3aa,6aa)]. 

Evidence for disrotatory cyclization in a simple case was provided by Marvell and by Vogel who found that trans, cis, ra/i5 -2,4,6-octatriene gives at least 99.9% pure c/5 -5,6-dimethyl-1,3-cyclohexadiene with an activation energy of 29.4 kcal/ mol and an entropy of activation of — 7 e.u. (Scheme 7.40). 

A soln. of 1,2-dimethyl-1,4-cyclohexadiene in 3.6 10 water/acetone added to a yellow slurry of bis(acetonitrile)dichloropalladium(II), KHCO3, and CUCI2 in acetone at 20°, and worked up after 24 h di-p-chlorobis[(l,2,3-r )-5-hydroxy-l,2-dimethyl-2-cyclohexen-l-yl]dipalladium (Y 77%), in methanol containing Na-propionate pressurized in a Fischer-Porter tube with 3.8 atm. CO for 4.5 h methyl trans-S hydroxy-... 

Recent work by Rault-Berthelot et al. considered an alternative pathway to 115 (Scheme 34) [33]. Diels-Alder cyclization with trans,tranr-l,4-diphenyl-1,3-butadiene (116) and dimethyl acetylenedicarboxylate (117) resulted in cyclohexadiene 118 where subsequent oxidation with Pd/C afforded terphenyl 119. Direct cyclization of 119 using concentrated sulfuric acid does afford 115, but only in 12% yield. Instead, saponification of 119 to diacid 120 followed by cyclization improves the yield of 115 to 38%. 

Can these observations be generalized Let us look at the stereochemistry of the cis-1,3.5-hexatriene-cyclohexadiene interconversion. Surprisingly, the six-membered ring is formed thermally by the disrotatory mode, as can be shown by using derivatives. For example, heated trans,cis,trans-2,4,6-oclatriene gives cii-5,6-dimethyl-l,3-cyclohexadiene, and cw,cw,trfl 5 -2,4,6-octatriene converts into irfl i-5,6-dimethyl-l,3-cyclohexadiene, both disrotatory closures. 

---