Allylic barium reagents

REGIO- AND STEREOSELECTIVE CARBOXYLATION OF ALLYLIC BARIUM REAGENTS (E)-4,8-DIMETHYL-3,7-NONADIENOIC ACID... 

REGIO- AND STEREOSELECTIVE CARBOXYLATION OF ALLYLIC BARIUM REAGENTS... 

Regio- and Stereoselective Carboxylation of Allylic Barium Reagents (E)-4,8-Dimethyl-3,7-nonadienoic Acid. 

Homo- and cross-coupling reactions of allyhc halides and reactions of conjugated dienes with dichloroalkanes using reactive barium are reviewed in Section 5.2. The next section covers methods of generating allylic barium reagents and reactions of these carbanions with a variety of electrophiles. The last section describes examples of other carbon-carbon bond forming reactions promoted by barium compounds. 

Preparation of Allylic Barium Reagents and Reactions of these Carbanions with Electrophiles 177... 

Allylic barium reagents prepared in this way can realize highly a-selective reactions with different electrophiles, e.g. cross-coupling reactions with allylic halides or allylic phosphates, additions to carbonyl compounds or imines, and ring opening of epoxides. A selective Michael addition reaction with an a,/ -unsaturated cy-cloalkanone can also be performed by use of an allylic barium reagent. 

Yamamoto and coworkers reported that the com-plexation of allylic barium reagents with crown ethers, in particular 18C6, substantially improved the reactivity toward electrophiles, e.g., aldehydes, and a-regioselec-tivity of these organoalkyl derivatives in (Eq. 20). By contrast, only a negligible effect of acyclic poly ethers on the reaction was observed. 

Alvarez-IbaiTa. C. Csaky. A.G. Martin. M.El. Quiroga. M.L. Study of the asymmetrie synthesis of (Z)-y-substituted-0(,P-didehydroglutamate.s from V-alkylidene-glycinates. Tetrahedron 1999. 55 (23). 7319-7330. Yanagisawa, A. Yamada. Y. Yamamoto. H. Effect of crown ethers on the regioselectivity of allylation of benzaldehyde with allylic barium reagents. Synlett 1997. (9). 1090-1092. 

Similarly, the reaction of monosubstituted allylic barium reagents give rise to exceedingly high diastereoselectivity (Table 6.25) [1]. 

Table 6.25 Diastereoselective y-allylation of various aldehydes by allylic barium reagents using B-OMe-9-BBN as an additive [1]... 

Oxidative Addition of Rieke Barium to Allylic Halides Preparation of Stereochemically Homogeneous Allylic Barium Reagents... 

Corey and coworkers successfully applied this cross-coupling method to the synthesis of (3S)-2,3-oxidosqualene (Scheme 10.2 5) [3]. The utility of allylic barium reagents for nucleophilic substitution reactions was further demonstrated by the synthesis of cembrol A (7), in which ring closure of the epoxy compound occurred regioselectively (Scheme 10.3) [4]. 

Scheme 10.1 Various allylation reactions using allylic barium reagents.

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