1.3.5- Trienes, ring closur

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... 

It will be clear from the results so far presented that both C5 and C dehydrocyclization products can be formed, with aromatization proceeding (one would expect) by further dehydrogenation of the initially formed C6 ring-closure species. There is another pathway for the production of aromatics based upon cyclization of a linear triene (133), but this is of relatively small importance, and is only significant at all at quite high temperatures and low hydrogen partial pressures. 

The stereoselectivity of the thermal ring-closure of the dodecatrieneones 42 is determined by the nature of the remote group R. trans-Fused products 43 predominate over cw-products 44 and their ratio increases as R varies from MeO through Me to H (equation 29). If the reactions are catalysed by diethylaluminium chloride only fraws-compounds are formed. The homologues 45 behave similarly34. In contrast, the 7-azadeca-l,3,9-trienes 46 (X, Y = H2 or O) yield more of the cis- than the traws-fused compounds, regardless of the nature of X and Y (equation 30)35. 

With the C12,C13 disconnection producing an effective solution to the synthesis of epothilone A (4), it would seem likely that the metathesis approach could be extended readily to the preparation of epothilone B (5). However, installation of the desired C12 methyl group requires ring-closure of a diene precursor in which one of the olefins is disubstituted. Recently, such reactions have been shown to be problematic for Grubbs initiator 3 but more successful with Schrock s molybdenum initiator 1 [19]. Consistent with these reports, Danishefsky demonstrated that triene 38 would not undergo RCM with 3, whereas 1 was effective in promoting the transformation of 38 into a 1 1 mixture of 39a and 39b in good yield [14b] (Scheme 8). 

Irradiation of trans, cis, trans 1,6 dimethylhexa-1,3,5 triene (I) gives rise to trans-5, 6 dimethylcyclohexa 1,3 diene (II) while thermal ring closure gives rise to cis 5, 6 dimethyl cyclohexa 1,3 diene III. 

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... 

The photochemistry of ru-vinyl-ortho-qui nodi methanes is typical of trienes in which at least one of the two C—C bonds is frozen in the s-cis conformation competing electro-cyclic 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... 

Stepwise Cg dehydrocyclization (aromatization) involving the gradual loss of hydrogen atoms from an alkane followed by a triene - cyclo-hexadiene ring closure step 20, 21). This can be ... 

In an alternative mechanism a monodentate intermediate (1) is in rapid equilibrium with reactants and it undergoes at high [XY] rate-determining ring closure. Such a type of mechanism is believed to operate for Ni(trien) + interacting with XY." Reasons for the preferred mechanisms are given. The isomerization may take the form of a conformational change in a metalloprotein. 

Ni + displays normal (/ )) behavior. With Ni(trien)(H20) + ring closure of Ni(trien)(H20)XY + is rate determining at high [XY]. A first order reaction of Ni([12]aneN4) + controls the addition of XY. 

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). 

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,... 

Ring closure to cyclobutanes via valence isomerization of cycloocta-l,3,5-trienes is also possible when carbonyl functions are present in the ethane fragment. Thus, reaction of cyclohepta-2,4,6-trienone (tropone) with diazopropane gives 8,8-dimethylcycloocta-2,4,6-trienone (16, R = H), which rearranges quantitatively to 8,8-dimethylbicyclo[4.2.0]octa-2,4-dien-7-one (17, R = H).67... 

Photochemical electrocycltc ring-closure in a 4-electron system works well for many acyclic dienes (2.17) and related cyclic systems 12.18). The situation with conjugated trienes is more complex, and they can act as 6-electron systems (2.19) leading to cydohexa-1,3-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 Iwhich must be c/s if a 6-electron electrocydic ring-closure is to take place). 

An important controlling factor is the conformation of the triene about the two single bonds that link the three double bonds (2.21) an s-c/s-s-cis conformation is required for 6-electron ring-closure, but 4-electron closure can occur also in an s-cis-s-trans conformation. The different conformations interconvert rapidly in acyclic trienes, but because each has different absorption characteristics, the wavelength of irradiation can influence the course of reaction dramatically. 2,5-Dimethylhexa-1,3,5-triene undergoes efficient cis trans isomerization at 254 nm (where the major absorbing species is the s-trans-s-frans conformation), but it cyclizes to a cyclohexadiene (2.22) using 313 nm radiation. 

Z)-Perfluorohepta-1.3,6-triene (18) undergoes thermal ring closure to perfluoro(3-allylcyclo-butenc) (19) and perfluorobicyclo[3.1.1]hept-2-ene (20) in a product ratio that depends on the reaction temperature.41 At 250 C the bicyclie product 20 is formed quantitatively. In contrast, direct photolysis of tricne 18 gives bicyclie diene 19 very cleanly. The -isomer of 18 is more difficult to cyclize. 

Platinum catalyzes at least two types of C6- dehydrocyclization, one of which involves olefinic intermediates (13, 28, 29). In the case of paraffins, this latter reaction involves the ring-closure of hexatrienes (30, 31). In the C6-dehydrocyclization of n-butylbenzene and n-pentylbenzene, phenyl-butadiene and phenylpentadiene could correspond to these triene intermediates (13, 14). The second C6-dehydrocyclization mechanism is similar to C5-dehydrocyclization, and may not involve olefinic intermediates. 

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... 

Scheme 33 Thermal ring opening of an ortho adduct to cycloocta-l,3,5-triene and photochemical ring closure of this triene.

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