Allylboronates tartrate derivatives

To access anti-l,2-diols, indirect methods are required for the preparation of geometrically pure, chiral E-3-alkoxy reagents. To this end, the isomerization of alkenylboronic esters described above (Eq. 41), provides a reliable route to tartrate-derived E-3-siloxy allylboronate 99 (Fig. 7). The latter shows variable enantioselectivities in additions to aldehydes, with cyclohexanecarboxaldehyde affording the highest selectivity (Eq. 70). ... 

The synthesis in Scheme 13.41 features use of an enantioselective allylboronate reagent derived from diisopropyl tartrate. The stereochemistry at the C-2 methyl group is influenced by the C-5 hydroxyl group, with 3 1 stereoselectivity for the desired stereoisomer. 

The tartrate-derived allylboronate reagents in the best cases compare favorably with other allylboration reagents in their reactions with both achiral and chiral aldehydes (e.g. B-Allyldi-... 

The allylation and crotylation of a-alkoxy aldehydes provide chiral 1,2-diol synthons which can be used in the synthesis of polyoj genated small molecules, for example, natural and unnatural sugars The tartrate-derived allylboronates I and 2 provide reagent-controlled selectivity in reaction with chiral glyceraldehyde acetonide 30. The intrinsic selectivity of the aldehyde is estimated by its reactions with pinacol allylboronates 33 and 34. The reagents 1 and 2 overcome the aldehyde s... 

The tartrate-derived allylboronates 25 are best prepared by treatment of triisopropyl borate with the appropriate carbanion. The crude diisopropyl boronate 28 is then treated with either enantiomer of diisopropyl tartrate (DIPT) to effect rapid transesterification, giving the boronates 25, 29, and 30 (Equation 4) [47],... 

We began these studies with the intention of applying this tandem asymmetric epoxidation/asymmetric allylboration sequence towards the synthesis of D-olivose derivative 63 (refer to Figure 18). As the foregoing discussion indicates, our research has moved somewhat away from this goal and we have not yet had the opportunity to undertake this synthesis. This, as well as the synthesis of the olivomycin CDE trisaccharide, remain as problems for future exploration. Because it is the enantioselectivity of the tartrate ester allylboronates that has limited the success of the mismatched double asymmetric reactions discussed here, as well as in several other cases published from our laboratorythe focus of our work on chiral allyiboronate chemistry has shifted away from synthetic applications and towards the development of a more highly enantioselective chiral auxiliary. One such auxiliary has been developed, as described below. 

The most extensively developed allylboron reagents for enantioselective synthesis are derived from tartrate esters.40... 

The chiral allyl- and 2-butenylboronates derived from tartrate esters (Chart 10-5) have been used in combination with a wide variety of chiral aldehydes to produce homoallylic alcohols in high yield and moderate to high enantioselectivity [124], The results obtained from reaction of selected chiral aldehydes (Chart 10-6) with the tartrate-modified allylboronates 195 and 197 (Chart 10-5) are shown in Table 10-20. As with the achiral aldehydes, the highest enantioselectivities are obtained when the chiral aldehydes are combined with allylboronate 197. A strong reagent-induced selectivity is apparent, but is nevertheless dependent on the intrinsic bias of the aldehyde. 

Scheme 11-10). It should be noted that glyeeraldehyde derivatives are outstanding substrates for the tartrate ester-modified allylboronates [118]. Aldehyde 261, derived from 260 in two steps, underwent a highly stereoselective (selectivity = >99 1) allylation reaction with the Brown Ipc2BAllyl reagent 195 [112, 113] (an in-depth discussion of the synthesis and use of this reagent appears in Section... 

By a similar method, the (Z)-crotylborate is synthesized from cA-2-butene in 70-75% yield with a 98% isomeric purity. The tartrate esters of allylboronic acids are an excellent reagent for asymmetric allylboration of carbonyl compounds. Allyl(diisopinocampheyl)borane [51] and the allylic boron derivatives of ester and amide, such as camphordiol [52], pinanediol [53], 1,2-diphenyl-1,2-ethylenediamine [54], have also been successfully used for asymmetric allylboration of carbonyls. 

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