Allylic configurationally stable

Alternative conditions for reductive decyanations can be used. The allylic ether in compound 26, an intermediate in a total synthesis of (-)-roxaticin, was prone to reduction when treated with lithium in liquid ammonia. Addition of the substrate to an excess of lithium di-ferf-butylbiphenylide in THF at -78°C, and protonation of the alkyllithium intermediate provided the reduced product 27 in 63% yield, as a single diastereomer (Eq. 7). a-Alkoxylithium intermediates generated in this manner are configurationally stable at low temperature, and can serve as versatile synthons for carbon-carbon bond forming processes (see Sect. 4). 

A second route to nonracemic /-oxygenated allylic stannanes utilizes an enantioselective deprotonation of allylic carbamates by BuLi in the presence of (—)-sparteine. The configurationally stable a-lithio carbamate intermediate undergoes enantioselective S/,-2 reaction with Bu3SnCl and Mc SnCI (Scheme 28)65. Once formed, the /-carbamoyloxy stannanes can be inverted by successive lithiation with. s-BuLi and stannation with R3SnCl (Scheme 29)65. The former reaction proceeds with S/.-2 retention and the latter by Sf2 inversion. Nonracemic allylic carbamates can also be used to prepare chiral stannanes. Deprotonation with. s-BuLi TMEDA proceeds stereospecifically with retention (Scheme 29)65. 

The methylation product 245a, on deprotonation with n-BuLi/TMEDA, provides the configurationally stable anion (5 )-246, which was allylated with inversion of the configuration to yield (R)-247 with 97% ee (equation 58). The enantiomer (R)-247 is available via the tin compound 245d, which is cleaved with n-BuLi/11 with retention of the configuration to epi-244, followed by the same sequence of alkylation steps. ... 

The titanium compounds, derived from the configurationally stable lithium-(—)-sparteine complexes 349a,b prepared from primary allyl carbamates, undergo lithium-titanium exchange with chlorotriisopropoxytitanium to form the allyltitanates... 

Treatment of the enantiomerically enriched acyclic allylic carbonate (S)-l (97% ee) under the standard reaction conditions furnished the allylic alkylation product (S)-14 (95% ee) in 86% yield, with net retention of absolute configuration (Eq. 3). This result implies that the displacement occurs via a classical double inversion process, albeit through a configurationally stable distorted u-allyl or enyl ff+n) organorhodium intermediate. This is supported by the fact that the achiral ff-spedes iii would undoubtedly have afforded the racemate of 14 (Scheme 10.3). Additionally, the enyl (a+n) organo-metallic intermediate provides a model for the regio- and enantiospedfidty observed in the reaction. 

Oxidation of the 7/3-phenylselenyl-A -steroids (135) with H2O2 gave the two configurationally stable R- and 5-selenoxides (136) and (137) respectively which react by independent pathways. The R-isomer (136) underwent the familiar 2,3-sigmatropic shift leading, after solvolysis, to the allylic alcohol (138) whereas the 5-isomer (137) gave the A -diene owing to pronounced steric hindrance to the... 

The true stereochemistry of 1, 3-chloropalladation is revealed in the reactions of cis-9-methylenebicyclo[6.1.0]nonane to give a single allylic complex (equation 324), and of trans-9-methylenebicyclo[6.1.0]nonane to give selectively a 4 1 mixture of syn and anti stereoisomeric allylic complexes (equation 325)389 393. In contrast to the noncyclic allylic complexes described above which interconvert under the reaction conditions, these mono-cyclic allylpalladium complexes are configurationally stable even under reflux in benzene for 8 h in the presence of 5 mol% PPh3. 

Allyl sulfides are readily lithiated, and give species 113 which are configurationally stable about the allyl system at <0 °C.1... 

On the other hand, a-alkoxyorganolithiums are not configurationally stable on a macroscopic timescale when they are secondary and allylic or benzylic. For example, despite the known (see section 5.2.1) stereospecificity of the tin-lithium and lithium-tin exchanges of similar compounds, tin-lithium exchange of 110 with rc-BuLi/TMEDA at -78 °C gives an organolithium 111 which has completely racemised after 10 min stannylation returns racemic stannane 112.55 Similarly, 111 racemises rapidly at -70 °C in pentane/cyclohexane in the... 

Facing all the mechanism-related peculiarities of the thermally induced rearrangement it was consequential to try to shift the reaction toward one end of the mechanistic spectrum. The first steps in that direction were undertaken by Hiroi et ah in 1984 they reported on a palladium-catalyzed variant of the reaction [49]. With enantiomerically enriched sulfinates 61 (Scheme 16) they found a much faster reaction as compared to the uncatalyzed one, allowing a reduction of the reaction temperature down to - 78 °C ( ). The stereospecificity of the rearrangement depended heavily on the substitution pattern (between 28 and 92%) and was traced back to the intermediacy of a configurationally stable ri -jt-allylpalladium species whose configuration was influenced by the S centro chirality. Unfortunately, due to difficulties in the preparation of enantiomerically pure 2-alkenylsulfinates (see above), the ee values of the resulting allylic sulfones were quite low. 

The palladium(n) catalyst, because of its Lewis acidity, may play a role in the addition of allylic tin to the ketone however, acylation of crotyltin was not reported to form a tertiary alcohol using palla-dium(II). It appears that solvent effects dominate in these cases. As part of the same study, substitute vi-nylstannanes were shown to undergo acylation with retention of configuration however, the resulting a, -unsaturated ketones were not configurationally stable to the reaction conditions. Isomerically pure (Z)-l-propenylstannane was acylated to afford a 50 50 mixture of alkenes (equation 87). The (Z)-a, -un-saturated ketone was shown to isomerize to a mixture of (Z)- and ( )-isomers under the reaction conditions. Mixtures of (Z)- and ( )-2-substituted vinylstannanes were acylated to afford mainly the ( )-a, -unsatuTated ketone (equation 88). ... 

A beautiful illustration of a delicate balance between a stepwise and a concerted reaction has been found in the reactions of 1,1-dimethylbutadiene 6.133.716 This diene rarely adopts the s-cis conformation necessary for the Diels-Alder reaction with tetracyanoethylene giving the cyclohexene 6.136. However, it can react in the more abundant s-trans conformation in a stepwise manner, leading to a moderately well stabilised zwitterion 6.134. The intermediate allyl cation is configurationally stable, and a ring cannot form to C-l, because that would give a trans double bond between C-2 and C-3 in the cyclohexene 6.137. Instead a cyclobutane 6.135 is formed. All this is revealed by the solvent effect. In the polar solvent acetonitrile the stepwise ionic pathway is favoured, and the major product (9 1) is the cyclobutane 6.135. In the nonpolar solvent hexane, the major product (4 1) is the cyclohexene 6.136 with the Diels-Alder reaction favoured. 

Again, the diene does not need to be in the s-cis conformation—as long as the substituents stabilise the radicals well enough, as they do here, the first bond can form while the diene is still in its more abundant s-trans conformation. The allyl radical 6.139 produced from the s-trans diene is configurationally stable, just as the allyl cation 6.134 was, and it will not be able to cyclise to give a frara.v-cyclohexene. Rotation about the bond between C-2 and C-3 is evidently too slow to compete with the radical combination giving the cyclobutane 6.140. 

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