Allylic leaving group

The wM-diacetate 363 can be transformed into either enantiomer of the 4-substituted 2-cyclohexen-l-ol 364 via the enzymatic hydrolysis. By changing the relative reactivity of the allylic leaving groups (acetate and the more reactive carbonate), either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. Then the enantioselective synthesis of (7 )- and (S)-5-substituted 1,3-cyclohexadienes 365 and 367 can be achieved. The Pd(II)-cat-alyzed acetoxylactonization of the diene acids affords the lactones 366 and 368 of different stereochemistry[310]. The tropane alkaloid skeletons 370 and 371 have been constructed based on this chemoselective Pd-catalyzed reactions of 6-benzyloxy-l,3-cycloheptadiene (369)[311]. 

To acliieve diastereoselectivity in tlie course of allylic substitution, tlie cnnitoliing cliital inforniation may not only reside in tlie substtate skeleton but may also be pan of tlie allylic leaving group. Tlius, a cliital carbamate bas been developed as a... 

The obvious disconnection in cyclopeptide alkaloids and indeed the strategy employed in most total syntheses of this type of compound is the formation of the aryl ether bond. Many groups chose to form the macrocyclic aryl ether by S Ar reaction. We decided to follow the inverse strategy, i.e., displacement of an allylic leaving group by a phenolate. In case of the natural cyclopeptide alkaloids, this would involve activation of yS-hydroxy-a-amino acids, which is likely to be accompanied by extensive elimination. Elimination is not possible when a-methylene- -hydroxy acids are used. Additionally, the double bond activates the leaving group and provides a handle for a possible later side chain attachment. 

Recently, Lambert and co-workers645,646 have reported the preparation of the trimesityl-substituted Mes3Si+ cation 249 using the so-called allyl leaving group approach [Eq. (4.162)] instead of the hydride transfer reaction applied in earlier studies. The electrophile attacks the double bond and the intermediate carbenium... 

A number of allylic leaving groups shown in Scheme 4.2 are cleaved by Pd catalysts. Mainly allylic esters are used as substrates for the catalytic reactions. In addition, even allylic nitro compounds [29,30] and sulfones [31-33] are known to form 7r-allylpalladium complexes. 

By changing the relative reactivity of the allylic leaving groups, namely acetate and the more reactive carbonate, either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. The Pd-catalysed reaction of the allylic acetate moiety of 150 with malonate affords 151. Acetylation of 151 and Pd-catalysed 1,4-elimination of... 

An alternative allyl leaving group approach was used by Lambert and coworkers38 in the preparation of the sterically hindered trimesitylgermyl and trimesitylstannyl cations (Scheme 3), analogous to their earlier preparation of the trimesitylsilyl ion39. 

As discussed later, others have noted formation of three- and four-membered ring products from phenyl-, silyl or alkynyl-substituted alkenes, which generate more stabilised product organolithiums, or from compounds bearing allylic leaving groups.111... 

Stereochemistry A Pd(0) complex displaces allylic leaving groups (with inversion of configuration) to generate cationic Tr-allyl palladium species A. This complex is electronically deficient and undergoes attack of a suitable soft nucleophile (with inversion) to give a product with overall retention (Scheme 5.8). 

Table 4. Palladium(0)-Catalyzed Reactions of Various Cyclic Substrates bearing an Allylic Leaving Group with Sodium Azide ...

Palladium-catalyzed reaction of conjugated dienes in the presence of a halide anion can be controlled to give a l-acyloxy-4-halo-2-alkene selectively under appropriate reaction conditions. The catalyst for this system is a palladium(II) salt, usually Pd(OAc)2 or Li2PdCl4. The reaction may be intermolecular or intramolecular. In most cases, it is stereoselective and results in a 1,4-cis addition to the diene. The products obtained from such reactions are useful synthetic intermediates since they have two allylic leaving groups with a large difference in reactivity (see below, under Synthetic applications ). 

Although nucleophiles generally substitute allylic leaving groups with retention of configuration, the substitution of allylic carboxylates sometimes proceeds in a nonstereospecific manner due to palladium-catalyzed isomerization of the substrates (Scheme 48). ... 

Allylic leaving group C(sp )-metallicf Catalyst metal Ref. 

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