Stereochemically homogeneous allylic

Yamamoto and coworkers [1] have developed a methodology for the preparation of stereochemically homogeneous allylic metals (Mg and Ba) from the corresponding allylic chlorides. These allylic metals react in high diastereose-lective-y-allylation with aldehydes in the presence of B-methoxy-9-BBN as an additive. With this method threo homoallylic alcohols are selectively obtained from ( )-allylic metal compounds and erythro isomers from (Z)-allylic metal compounds (Scheme 6.27) in excellent yields [1]. 

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

Complete chirality transfer has been achieved in syntheses of 1-amino-2-hydroxy-3-alkenyl derivatives from (Z)-/ -substituted ally] sulfoxides, due to 1,3-allylic strain as an additional controlling factor. Stereochemically homogeneous 3,6-dihydro-l,2-thiazine oxides are easily obtained by hetero Diels-Alder reaction of A-sulfinyl carbamates and ( , )-2,4-hexadienes37 (see Section D.1.6.1.1.1.1.2.2). 

The role of allylic sulfoxides as homoaldol equivalents in the synthesis of ( )-allylic alcohols was summarized earlier. A more recent finding is that allylic sulfoxides are precursors for conjugated dienes (equation 70). The elimination is regioselective but stereorandom. The 2,3-rearrangement of propar-gylic sulfenates gives allenyl sulfoxides. Allenyl sulfoxides are valuable synthetic intermediates. They can be converted into stereochemically homogeneous allenes, e.g. (205 equation 71). ... 

Within the last ten years, a large number of papers have appeared in which the titanium/tartrate-catalyzed asymmetric epoxidation of allylic alcohols was used as a key step in the synthesis of carbohydrates, amino acids, pheromones, leucotrienes and other stereochemically homogeneous compounds. This development has been discussed in several reviews [20],... 

According to Eq. (99), stereochemically homogeneous 3-carbonyl-substituted tetrahydrofurans are constructed in a brick-box system by sequential ho-moaldol and aldol reaction. The metallated allyl carbamate serves as an equivalent for the chiral dianion A, which accepts two different aldehydes B and C in a highly controlled manner [226]. 

These results support the intermediate allylic boron ate complex formed from stereochemically homogeneous allylmetal and il-OMe-9-BBN, with a plausible chair-like transition state Tj. This transition state is further stabilized by the adjacent four-membered ring formed by the second metal [2]. The alternative transition state Tj is destabilized by the 1,3-diaxial repulsion of the bulky cyclooctyl ring of 9-BBN (Fig. 6.1). erythro-threo Assignments are made... 

The alkene isomerization B — iso-B also may have another stereochemical consequence (part II of Figure 17.75) the destruction of the configurational homogeneity of a stereocenter C HR5R6 in the allylic position of the substrate. In that case, the hydrogenation results in a mixture containing the stereoisomers A and iso-A. 

The mechanistic and stereochemical options available to allylic carbanions are fully as complex as those available to allylic carbonium ions. Much that is known concerning the mechanisms of base-catalyzed alkene isomerizations was obtained from stereochemical, isotope exchange, and product composition studies. The present discussion is limited to alkene isomerizations whose kinetics have been studied in homogeneous systems. Isomerizations involving allylic carbanion intermediates have been reviewed by Cram . ... 

---