Reversible malonate esters

The cyclization involves a nucleophilic attack of the malonic ester car-banion on the carbonyl carbon atom of the aldehyde, and the substituted malonic ester carbanion attacks the electron-deficient carbon atom bearing the iodine atom, or in the reverse order, to give 119. The hydroxyl group generated in the first step of the reaction attacks the carbon atom, giving the pyranose product. 

Unlike the reversed shift of the emission band compared to the dihydrogen addend type, the singlet lifetime in the bis- and tris(bis-(ethoxycarbonyl)-methylene) derivatives is increased comparable to the former multiple adducts. The values range from 1.7 to 3.1 ns (tris-adduct), depending on the distorted T7-electron system of the fullerene core [67,108], In comparison to C6o, the fluorescence quantum yield for the malonic ester hexaadduct is increased by the factor 10 (30 X 10 4) [67,111], In the case of both pyrrolidino hexa-adducts (Th 14 and D3 15, Fig. 13), the effect is remarkably higher. The fluorescence quantum yields are increased about 100-fold (-0.02) compared to C6o. On the other hand, the singlet lifetime is only partly increased with -3.5 ns [111,112],... 

First all three ester bonds and both amide bonds are hydrolyzed to carboxylic acid groups by the aqueous acid. The mechanisms for these reactions are discussed in Section 19.5. The ester hydrolyses follow the exact reverse of the Fischer esterification mechanism shown in Figure 19.3, and the amide hydrolysis occurs by a very similar mechanism. The product of these hydrolysis steps has three carboxylic acid groups and one amino group. Two of these acid groups are attached to the same carbon so that one can be eliminated as carbon dioxide by the cyclic mechanism described in Section 20.4 for the malonic ester synthesis ... 

The stereochemical course of the subsequent Michael addition of malonic ester to the unsaturated ketone (23) proved to be unexpected. The kinetically controlled product 27 of addition was obtained in the presence of sodium methoxide and an excess of dimethyl malonate however, the thermodynamically preferred ester 28, also obtainable by base-catalyzed equilibration of 27, was the major product of the reaction. According to the IR (absence of Bohlmann bands) and NMR spectra, both 27 and 28 contained cis-quinolizidine ring systems formed possibly by reversible retro-Michael cleavage of the C-3 to Aj, bond in 23. This possibility explains the observed rapid destruction of 23 in the presence of very strong base with simultaneous appearance of a UV maximum at 410 nm presufiaably due to the conjugated enone system present in 29. 

The step marked with an asterisk is reversible and, in fact, is an unfavorable equilibrium, because the product (a simple ketone enolate) is a less stable anion than is the doubly stabilized malonate anion. However, the next step, reaction with more malonic ester to make a new malonate anion, drives the equilibrium to product. The reaction is catalytic in base because malonate is regenerated in this last step. 

Nucleophilic addition to the cyclopropylmethylidenemalonate (468) gives only the 1,4-adduct (469). This contrasts with the addition of butanethiol to (470X which affords adduct (471), probably because PP-substitution hinders Michael addition in the latter case. Compound (468) also reacts with 1-pyrrolidinocyclohexene by Michael addition, the major product being formed by a subsequent loss of malonic ester in a reverse Michael reaction. ... 

All the steps are reversible. With a catalytic amount of base, the reaction proceeds with thermodynamic control of enolate formation. The most effective nucleophiles under these conditions are carbanions derived from relatively acidic compounds such as j8-ketoesters or malonate esters. Scheme 1.10 provides some examples. 

Write out, in full detail, the mechanism of the Michael addition of malonic ester to 3-buten-2-one in the presence of ethoxide ion. Be sure to indicate all steps that are reversible. Does the overall reaction appear to be exo- or endothermic Explain why only a catalytic amount of base is necessary. 

The point of addition of the radical to a phenylated compound can be reversed when another ester group is introduced into the substrate molecule. Thus, benzal diethyl malonate reacts with formamide under ultraviolet irradiation in the presence of benzophenone to give derivatives of l-phenyl-l,2,2-tricarboxyethane [61),... 

The value of /-butyl esters in synthesis is that this hindered ester group is resistant to saponification but can be removed when desired by acid-catalyzed reversal of the reaction of formation with liberation of isobutene. Thus a general method for the synthesis of 8-keto ethyl esters involves acylation of ethyl /-butyl malonate... 

The reactive C-H site of malonates (100) can be masked by conversion to the corresponding tri-ester (101). Reversion back to the malonate can be achieved either by mild basic alcoholysis (cat.NaOMe, MeOH, 20°C) or by treatment with KCN in warm DMF. ... 

The rate of ceric oxidation of malonic add and its diethyl ester in acetic acid/sul-furic acid solutions has recently been reported by Vaidya et al. (1987). They find no evidence for precursor complex formation in either system. The reactive Ce(IV) species appear to be Ce(S04)2 ( 2) and CefSO ) " k 2). The second-order rate parameter for the oxidation of malonic add is 40 times greater than that for the ester. Oxidation of the ester is proposed to occur through the enol form yielding a malonyl radical analogous to that identified by Amjad and McAuley. Foersterling et al. (1987) find that the second-order rate constant for malonic acid oxidation by Ce(lV) in sulfuric acid is in excellent agreement with the value of Vaidya et al. They observe that Ce(III) does inhibit the reaction in sulfuric add, which they attribute to a reversible Ce(IV) malonic acid rate-controlling step. 

Distillation columns are occasionally used as chemical reactors. The advantage of this approach is that distillation and reaction can take place simultaneously in the same vessel, and the products can be removed to drive the reversible reaction to conpletion. The most common industrial application is for the formation of esters from a carboxylic acid and an alcohol. For exanple, the manufacture of methyl acetate by reactive distillation was a major success that conventional processes could not conpete with fBiegler etal.. 1997T Reactive distillation was first patented by Backhaus in 1921 and has been the subject of many patents since then (see Doherty and Malone. 2001 Doherty et al.. 2008 and Siirola and Barnicld. 1997. for references). Reaction in a distillation column may also be undesirable when one of the desired products deconposes. 

Examples of application of the iert-dodecanethiol/silane couple include typical reductive dehalogenations, but also Barton-McCombie deoxygenations of alcohols, through conversion of the latter into xanthate esters (eq 6), hydrosilylations of alkenes (eq 7), and preparation of silanethiols. The thiol can also be used as a polarity reversal catalyst in conjunction with tris(trimethylsilyl)silane and hexabutyldistannane/malonic acid. ... 

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