Allylic inversion

Cations which are covalently attached to the allyl anion part by a cr-bond and have sufficient Lewis acid properties offer the broadest versatility and highest levels of stereocontrol, since the C—C bond-forming step can occur in a pericyclic process9 accompanied by allylic inversion. It is reasonable to assume the prior assembly of both reaction partners in an open-chain complex, in which usually the (F )-oxonium ion, avoiding allylic 1,3-strain10, is predominant. 

Lewis acid catalyzed carbonyl addition of allylsilanes (Section D.l.3.3.3.5.) and allylstannanes (Section D.l.3.3.3.6.) usually proceed with clean allylic inversion (Section D.l.3.3.1.2.). Since these compounds are prepared by several routes and are also stable enough to be purified, each regioisomer can be approached. 

For the stereoselective construction of a structural unit of type 1 from aldehydes or ketones, the synthon 2 is required. It is delivered by the /-hetero-substituted allylmetal (E)- or (Z)-3 with allylic inversion. 

Similar stereoselectivities are achieved in the allylation of enantiomerically pure proline-derived a-oxoamides47. l-Bromo-3-methy]-2-butcne reacts with clean allylic inversion. Since pinacol-type coupling products are also produced under the reaction conditions, this was taken as evidence for a radical addition mechanism47. 

The stereohomogeneous (Z)-crotyltrimethylsilane reacts with 1-hexene to give 3-methyl-4-(trimethylsilyl)methyl-l-octene as a mixture of two diastereomers in 25% yield [Eq. (4)]. The diastereomeric products possess a methyl group at the carbon a to the double bond, indicating that an allylic inversion occurs during the allylsilylation. 

Allyltrimethylsilane (la) reacts with cycloalkenes such as cyclohexene and cyclo-pentene at room temperature to give stereospecifically tra j -l-trimethylsilyl-2-al-lylcycloalkanes. These products are formed through trans addition and an allylic inversion. 

Cyclopentanone synthesis. The palladium reagent (I), as well as tetrakis(tri-phenylphosphine)palladium, promotes a 1,3-alkyl shift from oxygen to carbon with no allylic inversion. Two typical examples are formulated.1... 

In the palladium-catalyzed addition of allylstannanes to allyl acetates, complete allyl inversion of the allylstannane is observed.137 Symmetrical coupling is also possible, for example, by treatment of cinna-myl acetate with hexa-n-butyldistannane.137 In this reaction, cinnamyl tri-n-butylstannane is generated in situ by Pd catalysis and it then couples with the allyl acetate with allyl inversion (equation 32). 

The mechanism of coupling of allyltin and allylpalladium can be altered by the addition of maleic anhydride to the reaction mixture (vide infra).m In this modified procedure, reaction of the allylpalladium chloride dimer with allyl-Bun3Sn now gives coupling without allyl inversion of the allyltin reagent. 

Allyltin reagents were suggested to add to ir-allylpalladium complexes directly on the allyl ligand,137138 although a definitive stereochemical experiment could not be successfully carried out. The fact that these reactions proceed with allyl inversion of the stannane (equation 168) would require an unlikely, sterically unfavorable allylpalladium-R intermediate if metal attack was followed. 

Chiral (E)-crotylsilanes have been utilized with Phi = NTs for Cu(I) - catalyzed syntheses of olefinic dipeptide isosteres, examples of which are shown in Scheme 70 [191]. In this case, tosylamidation occurs with allylic inversion,probably via asymmetric tosylaziridination of the C,C-double bond. The diastereos-electivity of product formation is high (>30 1) and appears to be strongly influenced by the hydroxyl group in the starting compounds. 

The treatment of various sulfides with Phi = NTs in the presence of cuprous triflate leads to the corresponding N-tosylsulfimides (N-tosylsulfilimines) 21 [30]. The presence of the chiral bis(oxazoline) ligand 22 in the reaction medium results in coordination of the copper(III)-nitrene intermediate, L Cu(III) = NTs, and enables the enantioselective production of 21 (Scheme 12). Similar copper-catalyzed reactions of allylic sulfides with Phi = NTs lead to formal insertion of the NTs group into the carbon-sulfur bond of the substrates, and proceed via a [2,3]-rearrangement with allylic inversion, to give sulfonamides 23 [30]. 

When this type of reaction was applied to the preparation of the ester 2, phosphorus trichloride was used as the phosphorus (III) halide. Treatment of this with one mole equivalent of 3,3-dimethoxyprop-l-ene yields mainly the enol ether 4, with smaller amounts of the isomeric a-chloroallyl methyl ether 5. This mixture is not very stable and has to be treated with trimethyl phosphite rapidly, in order to provide 2 in good yield, as shown in Scheme 1. The significance of this sequence is that it reveals that since 2 is the only product, the reactions leading to 2 via 4 must proceed by two allylic inversions, whilst those via 5 involve direct substitution twice at the original acetal carbon. 

Indium trichloride-mediated addition of (i )-a-(methoxymethoxy)allylic stannane (>95% ee) to cyclohexanecarbox-aldehyde affords the anti-adduct predominantly (anti syn = 98 2) and stereoselectively (>95%ee) (Equation (12)). Production of a transient allylic indium reagent is postulated via a stereosepecific anti-Se2 transmetallation. This a-(methoxymethoxy)allylic stannane reacts without allylic inversion, whereas the reaction of crotylstannane in Equation (5) (Section 9.14.3.3.1) proceeds with net allylic inversion. <5-Oxygenated allylic stannane also undergoes transmetallation with InCl3, and in situ addition to a-ODPS acetaldehyde leads mainly to the //-adduct, which is a potential precursor to D-(+)-altrose (Scheme 31).149,150... 

All the rearrangements in the quinoline series were effected neat without solvents at 200°. Yields were almost quantitative in almost all the instances although more than one product often resulted from a single starting material. The common feature of allylic inversion was clearly established in these migrations. The intermediacy of the cyclo-hexadienone-type structure was also demonstrated. 

In organic syntheses allylsilanes and allylstannanes have been used extensively as allyl anion equivalents during the last two decades [187-190]. The regioselective attack of electrophiles, which finally yields products with allylic inversion (Scheme 43), has been explained by the hyperconju-gative stabilization of carbenium centers by the carbon-silicon or carbon-tin bond in the j3-position [191-196], which has initially been derived from solvolytic experiments [197-199]. 

On treatment with a palladium(O) catalyst, vinyl epoxides undergo facile unimolecular rearrangement to give dienols or enones depending on the substitution pattern of the substrate. In the presence of an active methylene compound in the reaction system, however, a single alkylation product is formed. Cyclic and acyclic vinyl epoxides participate equally well. The reaction proceeds with clean alkylation from the same face as the oxygen of the epoxide, and proceeds with allyl inversion (Scheme 22). ... 

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