Unsaturated isomers

Anhydrous ammonia adds tofluorooleftns to produce nitriles Tins phenomenon IS used to characterize chemically the terminal difluoromethylene olefin that IS claimed to be m equilibrium with the internal isomer [4] (equation 2) Thus isomerization to the terminally unsaturated isomer prior to attack by ammonia yields the cyanoenamine... 

The reaction mechanism has been discussed by several earlier authors more recently, Elderfleld and King proposed that the onol form (79) of the ketone underwent acylation, whereas Balaban and Nenitzescu assumed that the )3,y-unsaturated isomer (80) of the ketone was acylated. Support for the latter view is given by the high concentration of the j8,y-unsaturated isomer (80) in equilibrium with the a,j8-isomer (78). In the case of mesityl... 

Nonconjugated jS.y-unsaturated ketones, such as 3-cvclohe enone. are in an acid-catalyzed equilibrium with their conjugated a-,/3-unsaturated isomers. Propose a mechanism for this isomerization. 

This is the only convenient way to make these compounds, since elimination by any other route gives the thermodynamically more stable a,P-unsaturated isomers. This is an illustration of the utility of the Wittig method for the specific location of a double bond. Another illustration is the conversion of cyclohexanones to alkenes containing exocyclic double bonds, for example, ... 

The related unsaturated compounds, N-nitroso-1,2,3,6-tetra-hydropyridine and N-nitroso-1,2,3,4-tetrahydropyridine, have been tested for carcinogenicity and both were potent esophageal carcinogens, as is nitrosopiperidine. However, N-nitroso-1,2,3,6-tetrahydropyridine also produced liver tumors, whereas N-nitro-so-1,2,3,4-tetrahydropyridine also gave tumors of the forestomach and oropharynx. The difference in tumor spectrum between the two unsaturated isomers may be related to differences in metabolism. N-Nitroso-1,2,3,6-tetrahydropyridine isomerized to N-nitro-so-1,2,3,4-tetrahydropyridine in vivo, but the reverse reaction was not observed (49). 

Protonation of the enolate provides a method for converting a,(3-unsaturated ketones and esters to the less stable (3,y-unsaturated isomers. 

This review will first concentrate on the unimolecular gas-phase chemistry of diene and polyene ions, mainly cationic but also anionic species, including some of their alicyclic and triply unsaturated isomers, where appropriate. Well-established methodology, such as electron ionization (El) and chemical ionization (Cl), combined with MS/MS techniques in particular cases will be discussed, but also some special techniques which offer further potential to distinguish isomers will be mentioned. On this basis, selected examples on the bimolecular gas-phase ion chemistry of dienes and polyenes will be presented in order to illustrate the great potential of this field for further fundamental and applied research. A special section of this chapter will be devoted to shed some light on the present knowledge concerning the gas-phase derivatization of dienes and polyenes. A further section compiles some selected aspects of mass spectrometry of terpenoids and carotenoids. 

W2S+W2S] Electrocylizations have been effected for all the fully unsaturated isomers as shown overleaf and for some dihydro derivatives, diazepinones and fused systems (76H(4)1509, 80JCS(P1)1230, 80JCS(Pl)17l8, 76JCS(Pl)362). In some cases such reactions are followed by spontaneous extrusion of the two-atom bridge thus 5H- 1,2-benzodiazepines (88) are... 

In this case, however, double-bond migration is an advantage, because nonconjugated p,y-unsaturated esters are usually much more difficult to prepare than their a,p-unsaturated isomers. 

Explain why /3,y-unsaturated aldehydes and ketones usually are relatively difficult to synthesize and are found to rearrange readily to the a,/3-unsaturated isomers, particularly in the presence of basic reagents ... 

Photochemical isomerization of a,/3-unsaturated ketones to 0,7-un-saturated ketones via the enol appears to be a general reaction. An equilibrium mixture of 4,5,5-trimethyl-3-hexen-2-one (Formula 166) and its d,7-unsaturated isomer (Formula 167) is converted by irradiation in a quartz vessel to the pure /3,7-unsaturated ketone (Formula 167) (66). 

A /3-hydroxyketone (542) forms with 543 the corresponding l,3-dioxa-2-sila-4-cyanocyclohexane (544) and the unsaturated isomer 545. With methanol, de-cyano-silylation is obtained. The /3-diketone 546 forms 544, too in that case it is better cleaved by AgF/THF or only methanol. 

Thus, according to Scheme 28, (-)-shikimic acid 169 was converted to cyclohexadiene derivative 170 via esterification of carboxyl group, protection of the m-disposed hydroxyls, and elimination of the remaining carbinol moiety. Catalytic dihydroxylation of 170 gave unsaturated esters 171 and 172 as a separable 1 1 mixture. The 5,5a-unsaturated isomer 171 was finally elaborated into the desired (-)-MK7607 (174) via simple protection-reduction-deprotection sequence. 

Murai et al. [28] found that the reaction of a,/J-unsaturated imines with CO results in a [4+1] cycloaddition to give unsaturated y-lactams (Eq. 12). For the reaction of imines which contain a /1-hydrogen, the initially produced /J,y-un-saturated y-lactams are isomerized to the stabler a,/J-unsaturated isomers. This success can be attributed to the facile coordination of the sp2 nitrogen of the substrates to a ruthenium center that assembles the substrates to the ruthenium complex. 

The carbon-carbon double bond is not isomerized to a detectable extent during saponification of the ester or distillation of the acid thus the 2-methyl-2-dodecenoic acid will have the same isomeric composition as the sample of ester from which it was obtained, and it consists entirely of a,/3-unsaturated isomers. 

Tetrahydro compounds still contain one ring double bond and thus can exist in several tautomeric forms (cf. Section 2.2.5.2). Little systematic work is available regarding the position of such equilibria, but l-methyl-l,4,5,6-tetrahydro-pyridine is more stable than the 3,4-unsaturated isomer by 16 kj mol1. 

Deconjugation of oL, -enones. The photochemical deconjugation of a,P-unsat-urated acids and esters via a dienol is often a useful preparative route to the P,y-unsaturated isomers. The corresponding reaction with a,p-enones is erratic it can result in reversion to the original enone or the trans-isomer. Irradiation in the presence of a mild base such as pyridine (or imidazole) can promote conversion of a,P-enones to p,7-enones. A stronger base such as triethylamine catalyzes reconjugation of the intermediate dienol. Base-catalyzed deconjugation is more efficient in polar solvents (DMF, CH,CN, CH,OH) than in ether or hydrocarbon solvents. Thus under proper conditions several a,p-enones previously considered inert to irradiation can be isomerized to the p,y-isomers. 

Draw a condensed formula for an unsaturated isomer with the molecular formula C4H8, and with a branch. Name the compound. 

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