Hydrogenation conjugated esters

Another potential mechanistic complication is capture of the intermediate carbenium ion by the conjugate base of the acid. When CF3C02H is used as the acid, this would lead to trifluoroacetate esters. Kursanov et al. showed that, under the reaction conditions for ionic hydrogenations, trifluoroacetate esters can be converted to the hydrocarbon product (Eq. (3)). 

Aminopenicillanic acid (8) was converted to 6(5)-bromopenicillanic acid by trapping of the diazo-intermediate with hydrogen bromide. Esterification of the dicyclohexylamine salt (93) with p-methoxybenzyl bromide, followed by oxidation, afforded the sulphoxide (94) in 60% yield from 6-APA. Elaboration of this sulphoxide to the disulphide (96) was effected by the procedure established by Kamiya et al. [98] the sulphenic acid (95), formed by heating the sulphoxide to reflux in toluene, was intercepted by reaction with 2-mercaptobenzothiazole to yield the disulphide (96). The latter was transformed by base-catalysed double bond isomerization to the conjugated ester disulphide (97) [95% yield from (94)]. Reductive formylation of disulphide (97) then provided the formylthio-derivative (98). Cyclization of the oxalimide (99), obtained by ozonolysis of... 

During the photocycloaddition of 88 with cyclopentene (Reaction 1), the de of the major isomer 89 increased from 30% in nonpolar solvents up to 68% in a mixture of methanol and acetic acid. When prochiral enone 91 was irradiated in the presence of a cyclopentene linked to the 8-phenylmenthol (Reaction 2), the best selectivity was now obtained in nonpolar solvents. To explain this effect, it was proposed that the facial selectivity is high in every case and that the diastereoselectivity depends on an s-cis s-trans ratio of the conjugated esters influenced by hydrogen bonding [65]. Similar results were obtained with chiral... 

THF is essential for these reactions in its role as the hydrogen donor and supplies the requisite hydrogen atom. This feature was established by deuterium labeling studies. However, these radical-mediated reactions fail to add to conjugated esters and cyclohexene acceptors and require an aniline substituent for successful radical-mediated ring closure. A bridged ring example is illustrated in eq 11. ... 

Borane reacts with carboxylic acids first of all to form triacylborates, with evolution of hydrogen gas. Esters are usually less electrophilic than ketones because of conjugation between the carbonyl group and the lone pair of the sp hybridized oxygen atom—but, in these boron esters, the oxygen next to the boron has to share its lone pair between the carbonyl group and the boron s empty p orbital, so they are considerably more reactive than normal esters. 

Reaction of ethyl crotonate and nitroethane in the presence of the basic 1,8-diaza-bicyclo[5.4.0]undec-7-ene (DBU) led to nitro-ester 3.49 in 74% yield.31 Catalytic hydrogenation of the nitro group and acid hydrolysis produced 3-methyl-4-amino-butanoic acid, 3.50. Choosing different conjugated esters as starting materials led to formation of 3-ethyl- (59% overall yield) 3-propyl- (51% overall yield) 3-isopropyl-(62% overall yield) 3-butyl- (56% overall yield) 3-sec-butyl- (60% overall yield) 3-isobutyl- (58% overall yield) and, 3-r-butyl-4-aminobutanoic acid (31% overall yield).31 The yields observed in this study clearly show that conjugate addition of nitro enolates is a synthetically useful process. 

Diphenylketene (253) reacts with allyl carbonate or acetate to give the a-allylated ester 255 at 0 °C in DMF, The reaction proceeds via the intermediate 254 formed by the insertion of the C = C bond of the ketene into 7r-allylpalla-dium, followed by reductive elimination. Depending on the reaction conditions, the decarbonylation and elimination of h-hydrogen take place in benzene at 25 °C to afford the conjugated diene 256(155]. 

In the case of the cyclohexane derivative 7 however, that bears an equatorial acetate group, two axial cis-/3-hydrogens are available, and elimination in both directions is possible. The pyrolysis of 7 yields the two elimination products 8 and 6. Formation of product 8 is strongly favored, because the new double bond is in conjugation to the ester carbonyl group. ... 

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