Decarboxylative elimination

Ailyl enol carbonates derived from ketones and aldehydes undergo Pd-cat-alyzed decarboxylation-elimination, and are used for the preparation of a, /3-unsaturated ketones and aldehydes. The reaction is regiospecific. The regio-isomenc enol carbonates 724 and 726, prepared from 723, are converted into two isomeric enones, 725 and 727. selectively. The saturated aldehyde 728 can be converted into the a,/3-unsaturated aldehyde 730 via the enol carbonate 729[459]. 

The allyl cyanoacetate 731 can be converted into an a, /3-unsaturated nitrile by the decarboxylation-elimination reaction[460], but allyl malonates cannot be converted into unsaturated esters, the protonation and allylation products being formed instead. 

Trost and coworkers have devised a stereocontrolled 1,3-diene synthesis employing a palladium-catalysed decarboxylative elimination procedure from allylic acetates carrying carboxylic acid functionality ji- to the acetate group (equation 18)48. This decarboxylative elimination strategy has been applied to the synthesis of an insect pheromone, codlemone48a and the ethyl ester of vitamin A carboxylic acid (Table 5)48b. 

TABLE 5. Dienes and polyenes through decarboxylative elimination... 

Decarboxylative elimination in 4-hydroxycyclohex-2-enecarboxylic acids with dimethylformamidedineopentylacetal has been shown to result in the formation of dienes in a regioselective manner under neutral conditions (equation 19)49. 

Claisen A Decarboxylation Elimination JK Miscellaneous > Ester Synthesis ... 

Processes such as decarbonylation, decarboxylation, elimination of water, and several other reactions may also occur prior to ionization, i.e., as non-mass spectral reactions, typically as a result of thermal degradation upon heating of the sample to enforce evaporation. In such a case, the mass spectrum obtained is not that of the analyte itself, but of its decomposition product(s). Sometimes, those thermal reactions are difficult to recognize, because the same neutral loss may also occur by a true mass spectral fragmentation of the corresponding molecular ion. 

Ikegami has devised an interesting approach based upon 1,3-cyclooctadiene monoepoxide as starting material (Scheme LX) Transannular cyclization, Sharpless epoxidation, and silylation leads to 638 which is opened with reasonable regioselec-tivity upon reaction with l,3-bis(methylthio)allyllithium. Once aldehyde 639 had been accessed, -amyllithium addition was found to be stereoselective, perhaps because of the location of the te -butyldimethylsilyloxy group. Nevertheless, 640 is ultimately produced in low overall yield. This situation is rectified in part by the initial formation of 641 and eventual decarboxylative elimination of 642 to arrive at 643. An additional improvement has appeared in the form of a 1,2-carbonyl transposition sequence which successfully transforms 641 into 644... 

Substituted cyclopropyl rings conjugated with a triple bond system have recently received attention as C5 building blocks. The procedure described here is a modification of the decarboxylation-elimination reaction for the preparation of a.3 acetylenic acids from enol sulfonates of acyl malonates. Addition of aqueous alkali to the enol sulfonate of diethyl cyclopropyl carbonyl malonate gives cycl opropyl propiol ic acid, but the yield is 1 ow. 

E. Nitroso Decarboxylations (Elimination-Type Reaction). 2-5. Preparation of 2,6-Dibromo-4-nitrosophenol. ... 

The preferred reagent for Hunsdiecker-type reactions is lead(IV) acetate in the presence of an inorganic halide.18,19 The yields are usually good to excellent. Reaction of cyclobutanecar-boxylic acid with lead(IV) acetate in the presence of lithium chloride gave chlorocyclobutane (1) in good yield.18 Other carboxylic acids reacted under similar conditions.18 However, care should always be taken when lead(IV) acetate is used, as the use of this reagent in the absence of the halide results in decarboxylative elimination to give an alkene, which is found as a byproduct.19... 

Many addition and elimination reactions, e.g., the hydration of aldehydes and ketones, and reactions catalyzed by lyases such as fumarate hydratase are strictly reversible. However, biosynthetic sequences are often nearly irreversible because of the elimination of inorganic phosphate or pyrophosphate ions. Both of these ions occur in low concentrations within cells so that the reverse reaction does not tend to take place. In decarboxylative eliminations, carbon dioxide is produced and reversal becomes unlikely because of the high stability of C02. Further irreversibility is introduced when the major product is an aromatic ring, as in the formation of phenylpyruvate. 

Decarboxylative elimination 526 Degree of hydration, effect on metal binding 311... 

In animals all isoprenoid compounds are apparently synthesized from mevalonate, which is converted by the consecutive action of two kinases21 23 into mevalonate 5-diphosphate (Fig. 22-1, step b). Mevalonate kinase is found predominantly in peroxisomes, which are also active in other aspects of steroid synthesis in humans.2124 A deficiency of this enzyme is associated with mevalonic aciduria, a serious hereditary disease in which both blood and urine contain very high concentrations of mevalonate.23 Mevalonate diphosphate kinase, which is also a decarboxylase, catalyzes phosphorylation of the 3-OH group of mevalonate (step c, Fig. 22-1) and decarboxylative elimination of phosphate (step d)25 to form isopentenyl diphosphate. 

ATP facilitates the decarboxylation-elimination the anticipated phosphorylation of the tertiary alcohol to make a better leaving group is apparently not involved... 

Pd-catalysed decarboxylation-elimination offers a useful method for regioselective generation of conjugated dienes. The polyene system of vitamin A derivative 381 is prepared from the /f-acetoxycarboxylic acid 379 by decarboxypalldation, as shown by 380 [172],... 

The E2 Reaction. The stereochemistry of the E2 process is even less well understood. It is exemplified by the decarboxylative eliminations of the vinylogous /3-hydroxy acids 5.16 and 5.18, which are both largely, although not exclusively, syn. The corresponding E2 reaction with /3-hydroxy acids is highly anti selective, in the usual way for /3 eliminations. 

When the anions of these acids are good leaving groups, elimination from their enol esters can be readily induced by comparatively weak bases. This is the case for p-bromobenzenesulphonates and triflates. Thus decarboxylative elimination from enol sulphonates 28 furnishes good yields of 2-alkynoic acids (equation 88), when R... 

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