1.3- Diene isomerization

Baldry has studied the influence of aryl substituents on the photoreactions shown by arylbutadienes (149). The author concludes that the formation of the products (150), (151), and (152) correlates with the ground-state a or cr substituent constant rather than with the excited-state constant The mechanism for the formation of the products (150—155) was also discussed. Product (153) 

Irradiation of 7,8-didehydrocholesterol (167) at 310—312 nm gave the previtamin D3 (168, 20.4%), lumisterol3 (169, 5.9%), and vitamin D3 (170, 69.7%). The position of the photoequilibrium between the cis- and the trans-isomer of vitamin D is dependent upon the energy of the triplet sensitizer employed. Vycor-filtered irradiation of the diene (167) in pentane afforded the triene (168) in a photoequilibrium of diene. triene 30 70. When the diene (167) was irradiated at 270—330 nm the triene was absent and the product was identified as the tricyclononene (169). Normally the diene (167) was prepared in situ by the decarbonylation of the ketone (170). This afforded a ready method for the preparation of large quantities of the diene and also permitted the successful 

Direct irradiation at 254 nm of the acetal (188) leads to two reaction modes, viz, valence-bond isomerization to yield (189, 3%), and bond fission to yield the 

Irradiation of the sultone (194) in methanol affords the ring-opened products (195) and (196). Subsequent irradiation of these compounds brings about elimination of the S03Me group and formation of the fluoranthene (197). This compound is thought to be the result of cyclization within the anion (198). 

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Conjugated dienes sucb as butadiene and its open-chain analogues can act as 17 ligands the complexes are u.sunlly prepared from melal carbonyl complexes by direct replacement of 2CO by the diene. Isomerization or rearrangement of the diene may occur a.s indicated schematically below ... 

Tricarbonylchromium complexes are useful for 1,4-addition of hydrogen to 1,3-dienes to afford monoenes selectively (40,42,43,44). With 1,4-dienes, isomerization into conjugation precedes hydrogenation. Isolated double... 

Since sulfoxides and sulfones are versatile synthetic intermediates, and since in both the thiolene oxide and dioxides the reverse dethionylation114 ( — SO), and cheletropic extrusion of sulfur dioxide296, respectively, readily take place thermally, these cycloadditions are expected to find a useful place in organic synthesis. It should be kept in mind, however, that the retrograde SO-diene reaction and interconversion of the thiolene oxides compete effectively against SO extrusion on heating, and that diene isomerization accompanies the forward reaction (SO + diene). 

In contrast LP-DE gives disappointing results for intramolecular imino Diels-Alder reactions, even in the presence of CSA. This is due to the fact that weak acids become strong acids in highly polar media such as 5.0m LP-DE and the protonation of diene, with concomitant diene isomerization, competes with cycloaddition [42]. This observation was supported by using trifluoroacetic acid (TEA). The imine 33 (Scheme 6.21) in LP-DE at room temperature in the presence of TEA gave a 1 1 mixture of cycloadduct 34 and the isomerized diene 35 within the unreacted imine 33. No Diels-Alder cycloadduct 36 was detected. 

The question as to whether these pathways involve an intermediate or are concerted has major implications for the stereochemistry of the product expected. This subject will be discussed more fully in the next section. The diradical obviously does not exist long enough to react with another molecule of olefin since no tetrahydropyrans have ever been detected from these reactions. It should also be pointed out that if the formation of the intermediate is reversible, olefin (or diene) isomerization may, in some cases, involve this pathway rather than triplet-triplet transfer. 

Isomerization of monoepoxides of 1,3-dienes.1 In the presence of this catalyst, monoepoxides of a 1,3-diene isomerize in benzene at 105° to a,p-unsaturated aldehydes or ketones with high (E)-selectivity. 

CoH(BH4)(PCy3)2] in benzene solution hydrogenates terminal alkenes and styrene faster than internal alkenes or dienes. Isomerization of terminal alkenes caused a rapid deterioration of the reaction rate.69 An ort/io-metallated triphenyl phosphite complex, for which the structure (20) was proposed, gave a turnover number of more than 300 in the hydrogenation of 1-butene. Isomerization was not observed in this case.70... 

The behavior of stilbene radical cations in the semiconductor catalysis is in keeping with the result of photoisomerization of other olefins like 6-methylstyrene sensitized by electron acceptors like chloranil in polar solvents (48). The semiconductor photocata-lyzed isomerization of strained cyclobutanes to strained dienes (isomerization of quadricyclene to norbomadiene and similar reactions of complex cage compounds (49)) is related to the olefin isomerization discussed above. 

Non-conjugated dienes isomerize during complexation to afford tricarbonyliron-coordinated conjugated dienes. This isomerization has been applied to a wide range of substituted cyclohexa-1,4-dienes available by Birch reduction from aromatic... 

Dendrobium Alkaloids.—8-Epidendrobine (14) has been synthesized employing a route involving a diene isomerization in a Diels-Alder reaction (Scheme 3). To account for the unexpected stereochemical result of the latter addition [(12)->(13) and epimeride] it is proposed that ester (15) assumes a very hindered conformation in the transition state, involving no secondary orbital overlap between diene and ester groups. However isomerization of the trisubstituted double bond to give (16) leads... 

The 22,23-epoxystigmastane side-chain (299) rearranges with boron trifluoride to form 22,25-epoxy-derivatives [tetrahydrofurans (300)] two 1,2-hydride shifts or a single 1,3-hydride shift are required to activate C-25 for attack by the oxygen atom. 3-Methoxyoestra-2,5(10)-dienes isomerize during g.l.c. to the 3-methoxy-... 

It is noteworthy that the Fe(CO)3 moiety has a pronounced affinity for 1,3-diene groups and irradiation of 1,4-dienes in presence of Fe(CO)5 and heating with Fe3(CO)12 are efficient methods of effecting 1,4- to 1,3-diene isomerization.6 Examples are illustrated in the annexed formulae. 

The same group also applied boron reagents to trap reactive dienes and used them in a subsequent IMDA reaction [20b]. o-Quinodimethanes are useful dienes since they allow rapid access to the tetralin skeleton. However, they are fairly unstable and only undergo cycloaddition with reactive dienophiles in the presence of less reactive dienophiles, the diene isomerizes or dimerizes preferentially. 

The use of highly polar media such as water clearly assists the Diels-Alder reaction, and similar rate enhancement has been observed in other reactions. Grieco et al. showed that 5.0 M lithium perchlorate in diethyl ether is an effective medium for these reactions, There is a problem, however, because weak acids in highly polar media such as this become strong acids, and protonation of dienes in intramolecular reactions can lead to diene isomerization that is competitive with cycloaddition. 25 Grieco and Kaufman found that for such... 

Only c/5-1,3-dienes can attain this geometry, and only cis dienes isomerize by this mechanism. If R and R differ in size, the most stable transition state is that in which the bulkier group is oriented equatorially. This accounts for the observation that isomerization of m-1,3-hexadiene gives predominantly cw,/rflAi -2,4-hexadiene. ... 

The reactions appear to be forbidden 2 + 2 retro- and cycloaddition. The latter reaction probably proceeds via a bicyclo[3.3.0]diyl. In the case of tricy-clo[3.3.0.0 ]octa-3,7-diene, isomerization to a semibullvalene occurs at only 0°C indicating either the [3.3.0]diyl is easily accessible or a forbidden 1,3-shift occurs easily. Relief of ring strain probably plays a role in these cases. However, a semibullvalene cannot be formed in the current case, so the biradical probably has no choice but to reclose to the tricyclic system which apparently is more stable than the cyclooctadiene. 

Saturated and monoene alcohols and acetates are quite stable in rubber septa. In general in traps in field applications, they neither hydrolyse nor oxidize (5, 25). However, several studies showed that conjugated dienes isomerize in rubber to ultimately form an equilibrium mixture (25-28). At equilibrium the content is 65-70% EE, 2-5% ZZ and the percentages of EZ and ZE are comparable to each other (27, 28). The exact values depend on the position of the double bonds. Because non-pheromone isomers may decrease or prevent trap catch, the useful life of a lure may be determined by this factor. Complete equilibration only requires about a month (27. 28) The reason for the rapid isomerization appears to be a combination of sunlight and catalysis by the sulfur used to cure the rubber (26-28). An alternate synthetic-elastomeric septum is available which reduces the rate of isomerization by about 8-fold (28). 

Evidence for such a scheme has been presented for hydrogenations conducted with simple Ru(II) salts, HCo(CO)4, [HCo(CN)s] , or [Pt(SnCl3)s] as catalysts 188-191). These homogeneous catalysts function best with terminal olefins and acetylenes. With dienes as substrates, the anionic catalysts selectively hydrogenate only one of the two double bonds. These complexes also catalyze olefin isomerization and, in the case of nonconjugated dienes, isomerization to a conjugated diene may precede hydrogenation. 

The dienes 49 and 50 isomerize slowly on standing or extended contact with silica gel and the actual isolated E Z ratio depends on the method of purification. Using flash-chromatography the diene isomerizes with 0-5% change in the values reported in Table 24.18. 

Attachment of the dienoic ester (222) onto 15-acetoxyverrucarol (225) was achieved via the sodium alkoxide. This esterification was accompanied by some diene isomerization which required that the desired Z, -isomer (226) be purified by HPLC. Esterification of the second acyl piece (224) onto the C-15 hydroxy group was sluggish although inclusion of ammonium iodide allowed realization of a 60% yield of diester. Desilylation then gave a pure sample of trichoverrin B (71). 

In a separate communication Roush and Spada (119) utilized their chiral ribbons in a synthesis of trichoverrol B (69) as shown in Scheme 26. Verrucarol (82) was selectively esterified at C-15 to yield the monoester (232). Formation of the sodium alkoxide and acylation with the mixed anhydride (230) successfully introduced the diene fragment with no observable diene isomerization. The authors hypothesized that the lack of a nucleophilic species capable of undergoing reversible Michael additions with the diene unit accounts for the absence of olefin isomerization. Simple treatment with fluoride then yielded the monoester trichoverrol B (69). 

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