Decarboxylation stereospecific

The regioselective and stereospecific construction of C-20 stereochemistry is explained by the following mechanism. The Pd(0) species attacks the ( )-/3-carbonate 616 from the a-side by inversion to form the Tr-allylpalladium species 620, which has a stable syn structure[392]. Then concerted decarboxylation-hydride transfer as in 621 takes place from the a-side to give the unnatural configuration in 617. On the other hand, the Tr-allylpalladium complex 622... 

Decarboxylation. Decarboxylation of linear and aromatic carboxyUc acids and of amino acids is common and of practical interest. L-Lysine [56-87-1] (48) can be synthesized by stereospecific decarboxylation of meso- (but not DL-) aa -diaminopimehc acid [2577-62-0] (49). The reaction is catalyzed by Bacillus sphaericus and proceeds in quantitative yields (92). 

When ethyl trifluoroacetylacetate is treated with an allylic alkoxide, tran-sesterification is followed by ester enolate Claisen rearrangement m a process that on decarboxylation yields stereospecifically the tnfluoromethyl ketone product [22] (equation 19)... 

In the electrolysis of quinuchdine-2-carboxylic acid decarboxylation occurs without rearrangement (Table 12, No. 5). Stereospecifically substituted cyclopentanes, that... 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

Isatin derivatives and proline react to give stereospecific formation of an azo-methine ylide intermediate via the decarboxylation route. The resulting 1,3-dipole undergoes a cycloaddition reaction with N-substituted maleimide (6), as a dipolaro-... 

In another report of Singh and Han [61], Ir-catalyzed decarboxylative amidations of benzyl allyl imidodicarboxylates derived from enantiomerically enriched branched allylic alcohols are described. This reaction proceeded with complete stereospecificity-that is, with complete conservation of enantiomeric purity and retention of configuration. This result underlines once again (cf. Section 9.2.2) that the isomerization of intermediary (allyl) Ir complexes is a slow process in comparison with nucleophilic substitution. 

Hence, Stobbe-like condensation with dimethyl-isopropylidene malonate and saponification of malonic acid, half-esters afforded the corresponding 14-carboxyretinoic acids, as a mixture of all E and 9Z isomers (80/20). The all E diacid was easily removed by crystallization from MeCN or ether, Fig. (36). A stereospecific decarboxylation in 2,6-dimethylpyridine led to isotretinoin. 

The dihydropyridazine (764) is unstable and loses nitrogen rapidly to yield a 1,3-diene (765) (69JA777, 72CC1260). Since (764) is generated by hydrolysis and oxidation of the cycloadduct (763) prepared from azodicarboxylate and2,4-hexadiene, it has proven possible to functionalize the double bond of (763) prior to nitrogen extrusion. Cyclopropanation, hydrolysis, decarboxylation and oxidation of (763) produce the 2,4-heptadiene (767) in a stereospecific manner via a concerted, orbital symmetry allowed retro-Diels-Alder process (Scheme 177). 

It may be protested that the reaction of the citric acid cycle by which oxaloacetate is converted to oxo-glutarate does not follow exactly the pattern of Fig. 17-18. The carbon dioxide removed in the decarboxylation step does not come from the part of the molecule donated by the acetyl group but from that formed from oxaloacetate. However, the end result is the same. Furthermore, there are two known citrate-forming enzymes with different stereospecificities (Chapter 13), one of which leads to a biosynthetic pathway strictly according to the sequence of Fig. 17-18. 

The sulfone (18) (see Section VI,P) has been resolved.214,215 Unlike open-chain a-sulfonyl carbanions, whose generation and proton capture proceeds with high retention of configuration, Corey et Z.214, 216 found that the carbanion generated by base-catalyzed decarboxylation of (+ )-18 gave a completely racemic sulfone (19). It was concluded that the lack of stereospecificity of the reaction is evidence for a planar cyclic a-sulfonyl carbanion intermediate. Cram and Whitney216,218 have studied this reaction in some detail their results indicate that symmetrical (planar) a-sulfonyl carbanions in asymmetric environments are involved as discrete reaction intermediates in the decarboxylation reaction. 

Indirect methods, which are primarily essentially of academic interest, nevertheless may find use under special circumstances. One of these, involving the brominative decarboxylation (Borodin-Huns-diecker reaction) of silver a-fluorocyclopropanecarboxylate, provides a convenient and stereospecific route to the synthesis of fluorobromocyclopropyl compounds (equation 26).96 Another approach makes use of the Diels-Alder reaction between tetrachlorocyclopropene and dienes (equation 27).97,98 The adducts can undergo facile dehydrohalogenation or ring opening. 

Most of the bimolecular absolute asymmetric syntheses are limited to 2+2 cyclobutane formation or polymerization of olefins. Koshima et al. reported a unique example of bimolecular reaction whereby acridine 20 and diphenylacetic acid are assembled in a 1 1 molar ratio by hydrogen bonding, and crystallized in a chiral space group, P2i2i2i.[18] Irradiation of the crystals caused stereospecific decarboxylating condensation to give chiral 21 in 33-39% ee. 

When an alkene is the dipolarophile it is thought that the intermediate 327 is formed and this then undergoes decarboxylation. The cycloaddition step is concerted, rate determining, and stereospecific. A detailed discussion of the investigations of the mechanism of the process is available <2001J(P 1) 1270>. 

On the other hand, cation formation by decarboxylation of an acyloxonium cation RC02+ is supported by the partial stereospecificity observed in the electrolysis of cis- and trans-bicyclo [3.1.0] hexane-3-carboxylic acid 2°5 and the electrocyclic ring opening in the anodic oxidation of 3-methyl-2-phenylcyclopro-panecarboxylate (22, 23) to cyclopropyl methyl ether (24, 25) and allylic ethers (26, 27) (Eq. (97)) 206). 

Saponification of the isoxazoline ring and subsequent decarboxylation of compound (243) afforded the 5-cyano-l,4-dihydropyridine-3-carboxylic acid ester (244) <89JOC5585>. If the acid (243) was first reesterified with ( — )-menthol, the resulting diastereomeric esters were separable. Saponification of the individual enantiomers followed by decarboxylation afforded a method to prepare 3-cyano-l,4-dihydropyridines (244) stereospecifically. 

Dopa and dopamine are important compounds because they are the precursors oxidase (Chapter 50) hydroxyiates stereospecifically at the benzyiic position to to adrenaline in humans. Decarboxylation of dopa gives dopamine, which an give noradrenaline (norepinephrine). 

The rest of the synthesis (Scheme 13) is completely stereospecific and most of the steps are known (20). The bicyclic acid was oxidatively decarboxylated with lead tetraacetate and copper acetate (21). The resulting enone was alkylated with methyllithium giving a single crystalline allylic tertiary alcohol. This compound was cleaved with osmium tetroxide and sodium periodate. Inverse addition of the Wittig reagent effected methylenation in 85% yield. Finally, the acid was reduced with lithium aluminum hydride to grandisol. 

Though the pertinent experiment in the corrin series has not yet been reported, a record of the analogous experiment in the porphyrin series has appeared [84], In contrast to corrins, in haems all four acetate side chains of the parent urogen III have been decarboxylated to methyl groups. In principle, these could be carved out as acetic acid and analysed for chirality if the starting succinate had been stereospecifically labelled with deuterium as well as with tritium. The synthesis of (2/ )-[2-2H,2-3 H]succinic acid was achieved by incubating (37 ,S)-[3-3 H]-... 

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