Double bonds isomers and

If we were to measure what are called heats of hydrogenation (A/Phydrog)tor the two double-bond isomers and find their difference, we could determine the relative stabilities of cis and Ivans isomers without having to measure an equilibrium position. In fact, the results bear out our expectation. For c/s-2-butene, AEP y rog -120 kj/mol (-28.6 kcal/mol) for the trans isomer, AFP]lvdrfM, = -116 kj/mol (-27.6 kcal/mol). 

Disproportionation reactions of acyclic mono-olefins can be classified into three types (I) reactant is a single olefin, (II) reactants are double-bond isomers, and (III) reactants are different olefins. Generalized equations and examples are ... 

The main regioisomer with a new stereogenic center a to the original stereocenter showed a diastereomeric ratio [(R, S )/(R, R )] of 81 19. These results confirm the stereoselective, suprafacial mechanism of the ene reaction and the importance of secondary orbital interactions between the allylic hydrogens and the reacting enophile (which serves as the basis for the explanation of the cis effect). The minor regioisomer is formed as a mixture of the two double-bond isomers, and with a similar diastereomeric ratio of 80 20. 

The synthesis of dihydroagarofuran is a further example of the construction of tricyclic compounds via radical six-ring formation80. ( —)-Carvone is converted to a bridgehead chloride 6 as a radical precursor treatment with tributyltin hydride gives a mixture of products consisting of a 4 1 mixture of the two vinylsilane double-bond isomers and the noncyclized acetylenic silane. Desilylation and stereoselective reduction leads to dihydroagarofuran. 

The term Vitamin A" is here collectively used to encompass (all E)-retinol and (all E)-retinal, also their 3-dehydro-derivatives and double bond isomers, and retinol esters. 

As follows from Fig. 4-1 and Fig. 4-2, the separation characteristics of graphitized thermal carbon black are unique. Graphitized carbon blacks are useful for analyzing cis-trans double bond isomers and positional isomers of multisubstituted aromatic (homo- and hetero-nuclear) compounds. Many examples of the separations are given in [36]. 

However, as pointed out earlier (Chapter 5), conjugated alkenes possess an enhanced stability over that of their isolated double bond isomers and thus, it can be argued, it is not surprising that benzene (CgHg) is more stable than what is expected when it is compared with cyclohexene (CgHm). A conjugated system would make a better choice for the comparison. 

Detailed experimental conditions have been reported for the direct synthesis of 2-alkenenitriles from acetonitrile and carbonyl compounds. Whereas the reaction is successful with aromatic aldehydes and diaryl ketones, dialkyl ketones yield mixtures of double-bond isomers, and aliphatic aldehydes do not condense satisfactorily. Similarly, 2-alkenenitriles are also the products of the reaction between the trimethylsilylacetonitrile anion and carbonyl compounds. ... 

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

Includes cis, trans, and terminal double-bond isomers. 

While the reductive procedure appears simpler, it also provides a higher proportion of the A -double bond isomer on dehydration than is obtained from the tertiary carbinol. This may be a consequence of the lower steric requirements of a A -olefin having only one alkyl group attached to C-20. In either case, all other double bonds must be protected before the ozonolysis and other free hydroxyls should be esterified prior to dehydration at C-20. 

It is interesting to see that the addition of methyl acrylate to the pyrrolidine enamine derivative of 2-methylcyelohexanone in benzene gave equal amounts of 2-methyl-2-carbomethoxyethyl and 2-methyl-6-earbomethoxy-ethylcyclohexanone even though the less substituted double-bond isomer predominates in the starting enamine (199,200,237). In contrast, the methylation of the same enamine mixture led only to 2,6-dimethyleyclo-... 

Intramolecular allylzinc aldehyde additions lead, in one synthetic step, to bicyelic a-methylene-y-lactones41. Interestingly, the (Z)-isomer gave only the r/5-fused bicyclic products, which arises from a. vyw-selectivc addition, while the ( >isomeric bromide furnished a cisjtrans mixture (yield 56%, cisjtrans 72 28)41 indicating that the double-bond torsion and the intramolecular carbonyl addition have similar rates. A similar sequence was the key step in a confertin synthesis28. 

Pi oblem The pheromone gossyplure (21) of the pink bollworm moth is a mixture of double bond isomers at bond (a). The c-is double bond (b) can be made from an acetylene so disconnection (24c) is suggested, Hou would you make both cis and trans (23) ... 

Owing to the concerted mechanism, chirality at C(3) [or C(4)] leads to enantiospecific formation of new stereogenic centers formed at C(l) [or C(6)].203 These relationships are illustrated in the example below. Both the configuration of the new stereocenter and the new double bond are those expected on the basis of a chairlike TS. Since there are two stereogenic centers, the double bond and the asymmetric carbon, there are four possible stereoisomers of the product. Only two are formed. The Zs-double bond isomer has the 5-con figuration at C(4) and the Z-isomer has the -configuration. These are the products expected for a chair TS. The stereochemistry of the new double bond is determined by the relative stability of the two chair TSs. TS B is less favorable than A because of the axial placement of the larger phenyl substituent. 

On further exploration it could be shown that the desilylated precursors 1-90 (Scheme 1.24) permit formation of the aromatized products 1-91 and its double bond isomer in up to 90 % yield, starting from the E-compound as a mixture of two diastereomers with a (Z)-configuration of the double bond, 1-90 gave 50% yield of 1-91. 

A combination of a metathesis and a Diels-Alder reaction was published by North and coworkers [263]. However, this is not a true domino reaction, as the dienophile (e. g., maleic anhydride) was added after the in situ formation of the his-butadiene 6/3-89 from the fois-alkyne 6/3-88 and ethylene. The final product is the fois-cycloadduct 6/3-90, which was obtained in 34% yield. Using styrene as an un-symmetrical alkene instead of ethylene, the mono-cycloadduct 6/3-91 was formed as a mixture of double-bond isomers, in 38% yield (Scheme 6/3.26). 

An interesting synthesis of enantiopure cu-decahydroquinolines, which involves enol ether hydrolysis, double bond isomerization, and intramolecular 1,4-addition of an amino group across a cyclohexenone has been reported <06T9166>. The process is stereoselective, with the exclusive formation of both cu-isomers 176 (43% over 3 steps) and 177 (17% over 3 steps) of the decahydroquinoline ring. 

Similar to the Pd-catalyzed pyrrole and thiophene annulations, an intramolecular Heck reaction of substrate 91 resulted in benzofuran 92 [80], Such an approach has become a popular means of synthesizing fused furans. Muratake et al. exploited the intramolecular Heck cyclization to establish the tricyclic core structure en route to the synthesis of a furan analog of duocarmycin SA, a potent cytotoxic antibiotic [81]. Under Jeffery s phase-transfer catalysis conditions, substrate 93 was converted to tricyclic derivatives 94 and 95 as an inseparable mixture (ca. 4 1) of two double bond isomers. 

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