Trienes rearrangements

Following on from the original discovery that cw-bicyclo[6.1.0]nona-2,4,6-triene rearranges on heating at 75 °C to a mixture of cis- and tra j-8,9-dihydroindenes, the thermal decompositions of a considerable number of derivatives were investigated (Table 2). 

However, highly substituted arenes are attainable from iododienes and alkynes with a high degree of regioselectivity (Scheme Apparently, the primarily formed trienes rearrange to the aromatic system. 

Vkiyl aHenes (44) are rearranged with heat or metal catalysis and photosensitized isomerization to produce the vitamin D triene (156—160). 

Cross-conjugated dienones are quite inert to nucleophilic reactions at C-3, and the susceptibility of these systems to dienone-phenol rearrangement precludes the use of strong acid conditions. In spite of previous statements, A " -3-ketones do not form ketals, thioketals or enamines, and therefore no convenient protecting groups are available for this chromophore. Enol ethers are not formed by the orthoformate procedure, but preparation of A -trienol ethers from A -3-ketones has been claimed. Another route to A -trien-3-ol ethers involves conjugate addition of alcohol, enol etherification and then alcohol removal from la-alkoxy compounds. 

Reaction of 1-ethoxycyclohexene (34) with dichlorocarbene gives 1-ethoxy-7,7-dichloronorcarane (35) in 87 % yield. Rearrangement of dichlorocyclo-propane (35) in hot quinoline results in loss of both chlorine atoms to give l-ethoxycyclohepta-l,3,5-triene (37) in 37% yield. Hydrolysis of enol ether (37) with a very small quantity of hydrochloric acid in methanol produces cyclohepta-3,5-dienone (38) in 91 % yield. ... 

The method of choice for preparing tropone (45) is to treat the initial mixture of monoadducts (43a) and (43b) with methanolic 1 TV hydrochloric acid to complete ketal hydrolysis and then carry out the pyridine rearrangement to give 3-bromo-4-methoxy-A-homo-estra-2,4,5(10)-trien-17-one (44) as described above for monoadduct 17-ketone (43b). 

Hecogenin p-toluenesulfonylhydrazone, 402 Hofmann-Loffler reaction, 257 Homoallylic rearrangements, 379 A-homo-5a-cholestan-3-one, 356, 358, 362 A-homo-5a-cholestan-4-one, 359, 360, 368 A-homo-choIest-4a-en-3-one, 366 A-homo-estra-1(10), 2,4a-triene-4,17-dione, 367,370... 

The monomer (laurolactam) could he produced from 1,5,9-cyclododeca-triene, a trimer of hutadiene (Chapter 9). The trimer is epoxidized with peracetic acid or acetaldehyde peracetate and then hydrogenated. The saturated epoxide is rearranged to the ketone with Mgl2 at 100°C. is then changed to the oxime and rearranged to laurolactam. 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

The reaction of oxepin with dimethyl 5-oxo-2,3-diphenylcyclopenta-l,3-diene-l,4-dicarboxy-late takes a different course. Two products 7 and 8 can be isolated, 7 is the [4 + 2] adduct of the cyclopentadienone across the central C-C double bond of the oxepin, the other, 8, is thought to be a [4+6] cycloadduct across the triene system of the oxepin.237 In boiling benzene, the [4 + 2] adduct 7 undergoes no cycloreversion, but rearranges to the tricyclo[5.3.02,4]deca-5,8-dien-10-one system.237 The [4+6] adduct, however, is stable under thermal conditions. 

The parent thionine system 1 up to now has not been prepared probably because the C-S bond in valence isomeric forms is too weak giving rise to facile rearrangement or decomposition. The obvious synthetic route, photochemical transformation of cyclooctatetraenccpisulfide 2 (9-thiabicyclo[6.1.0]nona-2,4,6-triene), does not lead to 1, but intriguingly to another valence isomer, the sulfur-bridged homotropylidene system 3.20... 

The existence of a metalated epoxide was first proposed by Cope and Tiffany, to explain the rearrangement of cyclooctatetraene oxide (8) to cydoocta-l,3,5-trien-7-one (11) on treatment with lithium diethylamide. They suggested that lithiated epoxide 9 rearranged to enolate 10, which gave ketone 11 on protic workup (Scheme 5.4) [4],... 

Tin compounds, as reducing agents 954, 955 Transition metal compounds as oxidizing agents 982—985 as reducing agent 949, 950 Trichloromethanesulphenates, rearrangement of 718, 721 Trienes 748 synthesis of 956... 

Indeed, cw-l,2-divinylcyclopropanes give this rearrangement so rapidly that they generally cannot be isolated at room temperature,though exceptions are known. When heated, 1,5-diynes are converted to 3,4-dimethylenecyclobu-tenes. A rate-determining Cope rearrangement is followed by a very rapid electro-cyclic (18-27) reaction. The interconversion of 1,3,5-trienes and cyclohexadienes... 

When the lactone silyl ketene acetal 18-1 is heated to 135° C a mixture of four stereoisomers is obtained. Although the maj or one is the expected [3,3] -sigmatropic rearrangement product, lesser amounts of other possible C(4a) and C(5) epimers are also formed. When the reaction mixture is heated to 100° C, partial conversion to the same mixture of stereoisomers is observed, but most of the product at this temperature is an acyclic triene ester. Suggest a structure for the triene ester and show how it can be formed. Discuss the significance of the observation of the triene ester for the lack of complete stereospecificity in the rearrangement. 

Intramolecular cyclopropanation of 4-aryl-1 -diazo-2-butanones 240 allows construction of the bicyclo[5.3.0]decane framework 12). In a reaction sequence analogous to that described above for the intermolecular ketocarbenoid reaction, bicyclo-[5.3.0]deca-l,3,5-trien-8-ones 241 are formed. They rearrange to the conjugated isomers 242 at the high temperatures needed if the reaction is catalyzed by copper 2311 or CuCl 232), but can be isolated in excellent yield from the Rh2(OAc)4-promoted reaction which occurs at lower temperature 233... 

The similarity of the MS spectra of isoterpinolene (11), terpinolene (12), a-terpinene (13) and the alio-ocimene (7) is striking. Whereas the hydrogen rearrangements suggested to explain this similarity might be speculative, they offer a reasonable explanation for the almost identical MS of the open and closed diene structures with that of the triene (7) spectrum. 

The two-step process, depicted by path b, involves initial addition of the carbene carbon to an adjacent it bond to form bicyclo[4.1,0]hepta-2,4,6-triene (2a). This process has precedent in the analogous rearrangement of vinylcar-bene to cyclopropene (Scheme 6),lc18 and is supported by Gaspar s work on 1-cyclohexenylcarbene.17 In the second step of the mechanism in Scheme 5, subsequent six-electron electrocyclic ring opening of 2a yields the cyclic allene 3a. 

Scheme 6.114, a carbene-carbene rearrangement transforms diphenylcarbene to o-phenylphenylcarbene, which is the progenitor of 565. Two phenylbicydo[4.1.0]-hepta-2,4,6-trienes and l-phenyl-l,2,4,6-cycloheptatetraene (562) have to be assumed as further intermediates. The participation of 562 is supported by the structure of the products 563 and 564, which should result from the addition of 562 to diphenylcarbene and the dimerization of 562, respectively. By thermolysis of the sodium salt of 2-phenyltropone tosylhydrazone, 562 was generated directly. At 100 °C in diglyme as solvent, 564 was identified as the only product and at 340°C/4Torr in the gas... 

With this mechanistic scheme, the chemoselectivity of the addition and the formation of rearranged chlorides (but not acetates) have been chosen as criteria to differentiate the ion pair mechanism from the purely ionic one and, on the basis of both criteria, the authors suggest the involvement of a tight ion pair for the addition of ArSCl in AcOH to diene 62 and of solvent separated ion pairs to triene 108. The effects related to the presence of added electrolytes, which favor the formation of rearranged acetates, have been considered in this work127 as evidence that even a larger separation of ions, which should lead to more electrophilic species, is possible. 

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