Rearrangement stabilized carbanions

Solanesol and other prenyl alcohols are important as metabolites in mulberry and tobacco leaves and in the synthesis of isoprenoid quinones. Hence, Sato and collaborators107 have developed a stereoselective synthesis of all-trans-polyprenol alcohols up to C50. Construction of the requisite skeletons was accomplished by the alkylation of a p-toluenesulphonyl-stabilized carbanion, followed by reductive desulphonylation of the resulting allylic sulphonyl group. This was achieved most efficiently by the use of a large excess of lithium metal in ethylamine (equation (43)), although all reaction conditions led to mixtures. The minor product results from double bond rearrangement. 

This reaction, for which the termprototmpic rearrangement is sometimes used, is an example of electrophilic substitution with accompanying allylic rearrangement. The mechanism involves abstraction by the base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more... 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Although chlorobenzene is rather inactive in usual reactions, its activity is enhanced by complex formation, and two products are formed by the reaction of stabilized carbanions on the complexed chlorobenzene 207, depending on the conditions [44], The anion of a-methy l propionitrile reacts at the meta position at —78 °C, and the mete-substituted product 208 is obtained by oxidation with I2. However, equilibration (rearrangement) of the carbanion occurs at 25 °C, because the attack of the carbanion is reversible, and the substitution product 209 of the chlorine is obtained. The fluorobenene 210, coordinated by Cr(CO)3, is very reactive. Reaction of y-butyrolactone to the o-lithiated fluorobenzene 211 gives rise to the alkoxide 212, which displaces the fluoride intramolecularly to give the cyclic ether 213 [52], In other words, the complex 211 can be regarded as the 1,2-dipolar synthon 214. However, Cr(CO)3-complexed aromatic bromide and iodide can not be used for the nucleophilic substitution. 

The n-BuLi cleavage of the axial C-Se bond of bis(phenylseleno)acetals, derived from 4-ferf-butylcyclohexanone, always occurs under kinetic control at -78°C [14]. In the case of a mixed acetal, having respectively the phenylseleno and methylseleno groups in axial and equatorial positions, the cleavage of the equatorial C-SeMe bond was predominantly observed. The resulting stabilized carbanion rearranges almost instantaneously leading to the a-phenylselanyl-cyclohexyllithium with the C-Li bond in the axial position [14] (Scheme 14). 

Sulfone stabilized carbanions attached at C-2 substantially accelerate the Claisen rearrangement (Scheme 22) these species may be generated from either regioisomeric precursors [142]. These processes take place with high levels of internal asymmetric induction [ 143], and the scope of the process is broad and it has been extensively studied. This acceleration was also noted for the related sulfinyl carbanions, albeit with low yields and in the case of sulfilimines no internal asymmetric induction was observed [144]. 

The ability of sulfur to stabilize carbanions is exemplified in the following cases. The pentadienyldi-thiocarbamate (144 Scheme 11) can be alkylated at the methylene group to afford the methylated product (145). At 110 C, the dithiocarbamate unit walks its way to the other end of the pentadienyl chain via tandem [3,3] sigmatropic rearrangements. The formation of (14d) is thermodynamically controlled as the more-substituted pentadienyl unit is formed. Dithiocarbamate (146) can be alkylated and eventually transformed into sulfur-free products the sequence giving (148) from (146) is but one of these processes. 

The arylsulfonyl carbanion accelerated Claisen rearrangement is completely regioselective and has also been found to be highly diastereoselective (Scheme 2). The stereochemical course of the reaction conforms to the familiar chair-like transition state model usually invoked for the classic thermal process. Recently, high degrees of asymmetric induction have been observed in tlie rearrangements of chiral cyclic phosphoramidate stabilized carbanion derivatives. ... 

The isocitrate alcohol can now be oxidized with NAD by Isocitrate DH to give an enzyme bound intermediate. The intermediate has a carboxyl group beta to a carbonyl carbon, so it has an excellent leaving group, CO2, attached to a stabilized carbanion. Thus it immediately rearranges to lose carbon dioxide ... 

In contrast, attack of cyanocarbanions (RCHCN) on 2,4,6-triphenyl-l,3,5-thiadiazinium tetra-fluoroborates (65) takes place at C-4 to give ultimately the pyrimidine-4-thiones (67) via acid-catalyzed rearrangement of the initially formed 6-imino-l,3-thiazines (66) as outlined in Scheme 6 <82BCJ2158>. In a similar manner, with carboxyamide stabilized carbanions, 5-substituted-pyrimidin-4(3/7)-ones (68) are produced. 

---