Isomers double-bonded

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-... 

Synthetic work in the imidazole series is complicated by the ready tautomeric equilibrium that can be set up among the double bond isomers (A, B). Thus, even i a reaction designed to afford the 5 isomer (A) is accomplished rcgiospecifically, tautomeriza-tion of the product affords either the 4 isomer (B) or a mixture of the two. (This tautomerism is, of course, no problem in azomycin because of the symmetry of that molecule.)... 

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). 

Problem 23,4 Aldol condensation of 3-methylcyclohexanone leads to a mixture of two enone products, not counting double-bond isomers. Draw them. 

For product 1 (X = Y = H) an equilibrium between the two possible double-bond isomers has been detected by NMR spectroscopy. For the dibromo derivative (X = Y = Br) a tub conformation has been determined by X-ray structural analysis. 

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. 

The regiochemistry of the Heck reaction is determined by the competitive removal of the (3-proton in the elimination step. Mixtures are usually obtained if more than one type of (3-hydrogen is present. Often there is also double-bond migration that occurs by reversible Pd-H elimination-addition sequences. For example, the reaction of cyclopentene with bromobenzene leads to all three possible double-bond isomers.146... 

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). 

The main products correspond in most cases to a formal [4+l]-cycloaddition. With butadiene, isoprene or a,/3-unsaturated ketones small amounts of a double bond isomer (a,/3 to Si) are observed. With a,/J-unsaturated amines the latter type of isomer is the main product. The nature of both isomers is consistent with a primary [2+l]-cycloaddition, preferably at a C=Y unit (Y = O, NR), followed by a ring-opening isomerization. 

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. 

Examples of the use of this procedure to prepare vinyl triflates from ketones are provided in Table II. Often mixtures of cis and trans isomers as well as the various double bond isomers of vinyl triflates are obtained by this procedure, and the amounts of these isomers produced may vary with the base and solvent used. Also, small amounts of unchanged ketone may contaminate the initial crude product. Consequently, separation procedures such as preparative gas chromatography or efficient fractional distillation may be required to obtain a single vinyl triflate isomer. 

The hydrido-cobalt-tetracarbonyl complex (I) undergoes a CO-dissocia-tion reaction to form the 16-electron species HCo(CO)3 (II). This structure forms a 7r-complex (III) with the substrate and is a possible explanation for the formation of further (C = C)-double bond isomers of the substrate. In the... 

H- and C-labeling revealed that the decomposing molecular ions of 1-, 2-, 3-, and 4-octene isomerize to a mixture of interconverting structures within 10 s after ionization. The equilibration of double bond isomers is mainly due to radical-... 

The sulfone moiety was reductively removed and the TBS ether was cleaved chemoselectively in the presence of a TPS ether to afford a primary alcohol (Scheme 13). The alcohol was transformed into the corresponding bromide that served as alkylating agent for the deprotonated ethyl 2-(di-ethylphosphono)propionate. Bromination and phosphonate alkylation were performed in a one-pot procedure [33]. The TPS protecting group was removed and the alcohol was then oxidized to afford the aldehyde 68 [42]. An intramolecular HWE reaction under Masamune-Roush conditions provided a macrocycle as a mixture of double bond isomers [43]. The ElZ isomers were separated after the reduction of the a, -unsaturated ester to the allylic alcohol 84. Deprotection of the tertiary alcohol and protection of the prima-... 

Involvement of 625 in a Wittig reaction with 4-carboxybutyl(triphenyl)phosphorane dissolved in dimethyl sulfoxide generates a mixture of double bond isomers rich in 623. 

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