Decarboxylation, allylic acetates

Wylation under neutral conditions. Reactions which proceed under neutral conditions are highly desirable, Allylation with allylic acetates and phosphates is carried out under basic conditions. Almost no reaction of these allylic Compounds takes place in the absence of bases. The useful allylation under neutral conditions is possible with some allylic compounds. Among them, allylic carbonates 218 are the most reactive and their reactions proceed under neutral conditions[13,14,134], In the mechanism shown, the oxidative addition of the allyl carbonates 218 is followed by decarboxylation as an irreversible process to afford the 7r-allylpalladium alkoxide 219. and the generated alkoxide is sufficiently basic to pick up a proton from active methylene compounds, yielding 220. This in situ formation of the alkoxide. which is a... 

Geranyl acetoacetate (685) is converted into geranylacetone (686). On the other hand, a mixture of E- and Z-isomers of 688 is obtained from neryl acetoacetate (687). The decarboxylation and allylation of the allyl malonate or cyanoacetate 689 affords the o-allylated acetate or nitriie[447]. The trifluoromethyl ketone 691 is prepared from cinnamyl 4.4,4-trifluoroacetoace-tate (690)[448],... 

The decarboxylation of allyl /3-keto carboxylates generates 7r-allylpalladium enolates. Aldol condensation and Michael addition are typical reactions for metal enolates. Actually Pd enolates undergo intramolecular aldol condensation and Michael addition. When an aldehyde group is present in the allyl fi-keto ester 738, intramolecular aldol condensation takes place yielding the cyclic aldol 739 as a main product[463]. At the same time, the diketone 740 is formed as a minor product by /3-eIimination. This is Pd-catalyzed aldol condensation under neutral conditions. The reaction proceeds even in the presence of water, showing that the Pd enolate is not decomposed with water. The spiro-aldol 742 is obtained from 741. Allyl acetates with other EWGs such as allyl malonate, cyanoacetate 743, and sulfonylacetate undergo similar aldol-type cycliza-tions[464]. 

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. 

Palladium-catalyzed coupling of allylic acetate 113 (Scheme 23) with diethyl malonate, or with acetylated diethyl tartronate, yielded the Boc-protected allylic amines 114 in 99 1 E stereoselectivity.1 "1 Saponification of 114 (R1 = H) and decarboxylation gave the Tyr- Ala alkene isostere 116 in quantitative yield, whereas 114 (R = OAc) was converted into the Tyr-Gly alkene isostere 115 in a two-step procedure in 62% overall yield. Considering the number of steps and the overall yield, this procedure is among the most efficient to prepare Xaatp, CH=CH]Gly dipeptide isosteres. 

The r)3-allylpalladium formate complex is considered as a model of the intermediate in a catalytic reductive cleavage of allylic formate or allylic acetate combined with formic acid to olefins. The r)3-allylpalladium formate was revealed to be decarboxylated to release olefins upon coupling of the produced palladium hydride with the r)3-allyl ligand (Eq. 7). 

Formates. The decarboxylation reaction of metal formates is a fairly general route for the synthesis of metal hydrides and it has been applied to many transition metals. As an example, allyl palladium formates, which are believed to be intermediates in the catalytic reductive cleavage of allylic acetates and carbonates with formic acid to give monoolefins (Scheme 6.32), have been synthesized. In fact the complexes undergo decarboxylation and the reductive elimination of the allyl hydrido fragments, supporting the catalytic cycle proposed [105]. 

Several 1,3-diene syntheses involving elimination reactions that are catalyzed by Pd(Ph3P)4 have been reported. The first involves the Et3N mediated elimination of HOAc from allylic acetates in refluxing THF. A complementary procedure involves the Pd(Ph3P)4 catalyzed decarboxylative elimination of /3-acetoxy-carboxylic acids (eq 46). The substrates are easily prepared by the condensation of enals and carboxylate enolates irrespective of the diastereomeric mixture, ( )-alkenes are formed in a highly stereocontrolled manner. The geometry of the double bond present in the enal precursor remains unaffected in the elimination and the reaction is applicable to the formation of 1,3-cyclohexadienes. 

Allylic carbonates are more reaetive than acetates. In addition, reaction of carbonates proceeds in the absence of bases [6]. Formation of jr-allylpalladium 9 from allyl methyl carbonates 8 proceeds by oxidative addition, followed by decarboxylation, and TT-allylpalladium methoxide 9 is generated at the same time, which abstracts a proton from a pronucleophile to form 10. In situ formation of methoxide is a key in the allylation under neutral conditions. Allylation under neutral conditions is useful for the reaction of base-sensitive compounds. For example, exclusive chemoselective reaction of the carbonate group in 4-acetoxy-2-butenyl methyl carbonate (11) occurred in the absence of a base to yield 12. Similar chemoselective reaction of the allyl carbonate group in the chiral cyclopentenyl methyl carbonate 13 with the jS-keto ester 14 without attacking the allylic acetate group to give 15 was observed even in the presence of NaH. As expected, retention of stereochemistry (see Chapter 4.2.1) was observed in this substitution [7]. 

Reactions that proceed under neutral conditions are highly desirable. An important event in TT-allylpalladium chemistry is the introduction of highly reactive allylic carbonates (Sect. V.2.1.3), Their reactions can be carried out under mild neutral conditions. " Also, reactions of allylic carbamates, " allyl aryl ethers, and vinyl epoxides proceed without addition of bases. As shown by the mechanism in Scheme 6, the oxidative addition of allyl methyl carbonates is followed by decarboxylation as an irreversible process to afford TT-allylpalladium methoxide, and the generated methoxide picks up a proton from pronucleophiles (NuH), such as active methylene compounds. This in situ formation of the alkoxide is the reason why the reaction of aUyl carbonates can be carried out without addition of bases from outside. Alkoxides are rather poor nucleophiles, and alkyl allyl ethers are not formed from them. In addition, formation of TT-allylpalladium complexes from allylic carbonates involving decarboxylation is irreversible. In contrast, the formation of TT-allylpalladium acetate from allyl acetate is reversible. 

Unsaturated carboxylic adds (23) can be electrolytically decarboxylated to allylic acetates (24) in 80—90% yield. a-Phenylthiocarboxylic acids can also be decarboxylated electrolytically to give high yields of aldehyde acetals (c/. 3, 49). Electrolytic procedures are also useful for the conversion of malonic acids into ketones. " This method has been used to prepare valerolactones from cyclic ethers (25), themselves prepared by Diels-Alder reactions between dienes and ketomalonates. ... 

In an extension of previous work, it has been found that Pd(0)-catalysed intramolecular cyclization of allylic acetates (21) can be used to prepare the chrysanthemic acid analogues (22). The potentially useful cw-cyclopropane (23) can be simply obtained by base-induced addition of cyanoacetate to ethyl 2-bromo-3,3-dimethylacrylate followed by decarboxylation oddly, a similar reaction using malonate fails to give a cyclopropane. Optically pure dichloro cw-chrysanthemic acid (26) has been obtained by a Favorskii rearrangement of the chiral cyclobutanone (25) prepared from the keten (24) by sequential [2 + 2]cycloaddition, cine-rearrangement, and resolution (Scheme 3). ... 

Allylic acetates afford olefins by elimination of acetic acid whereas carboxylic acids are readily decarboxylated with Pd catalysts. 

Co-adsorption experiments show a complex role of the nature and concentration of chemisorbed ammonia species. Ammonia is not only one of the reactants for the synthesis of acrylonitrile, but also reaction with Br()>nsted sites inhibits their reactivity. In particular, IR experiments show that two pathways of reaction are possible from chemisorbed propylene (i) to acetone via isopropoxylate intermediate or (ii) to acrolein via allyl alcoholate intermediate. The first reaction occurs preferentially at lower temperatures and in the presence of hydroxyl groups. When their reactivity is blocked by the faster reaction with ammonia, the second pathway of reaction becomes preferential. The first pathway of reaction is responsible for a degradative pathway, because acetone further transform to an acetate species with carbon chain breakage. Ammonia as NH4 reacts faster with acrylate species (formed by transformation of the acrolein intermediate) to give an acrylamide intermediate. At higher temperatures the amide may be transformed to acrylonitrile, but when Brreform ammonia and free, weakly bonded, acrylic acid. The latter easily decarboxylate forming carbon oxides. 

The stereoselective total synthesis of (+)-epiquinamide 301 has been achieved starting from the amino acid L-allysine ethylene acetal, which was converted into piperidine 298 by standard protocols. Allylation of 297 via an. V-acyliminium ion gave 298, which underwent RCM to provide 299 and the quinolizidine 300, with the wrong stereochemistry at the C-l stereocenter. This was corrected by mesylation of the alcohol, followed by Sn2 reaction with sodium azide to give 301, which, upon saponification of the methyl ester and decarboxylation through the Barton procedure followed by reduction and N-acylation, gave the desired natural product (Scheme 66) <20050L4005>. 

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