Reactions with allylboronates stereochemistry

Allylboron compounds have proven to be an exceedingly useful class of allylmetal reagents for the stereoselective synthesis of homoallylic alcohols via reactions with carbonyl compounds, especially aldehydes1. The reactions of allylboron compounds and aldehydes proceed by way of cyclic transition states with predictable transmission of olefinic stereochemistry to anti (from L-alkene precursors) or syn (from Z-alkene precursors) relationships about the newly formed carbon-carbon bond. This stereochemical feature, classified as simple diastereoselection, is general for Type I allylorganometallicslb. 

The reactions of allylboronates 1 (R = H or CH3) may proceed either by way of transition state 3, in which the a-substituent X adopts an axial position, or 4 in which X occupies an equatorial position. These two pathways are easily distinguished since 3 provides 7 with a Z-olefin, whereas 4 provides 8 with an E-olefinic linkage. There is also a second fundamental stereochemical difference between these two transition states 7 and 8 are heterochirally related from reactions in which 1 is not racemic. That is, 7 and 8 arc enantiomers once the stereochemistry-associated with the double bond is destroyed. Thus, the selectivity for reaction by way of 3 in preference to 4, or via 6 in preference to 5 in reactions of a-subsliluted (Z)-2-butenylboronate 2, is an important factor that determines the suitability of these reagents for applications in enantioselective or acyclic diastereoselective synthesis. 

Relatively few studies of the reactions of allylboronates and ketones have appeared. The reaction of (85) and ethyl pyruvate, for example, was conducted under 6 kbar pressure at 45 C for 80 h to give a 9 1 mixture of diastereomers (86a) and (86b). The stereochemistry of this reaction parallels that seen with crotyl-9 BBN (Figure 10) in that the structure of the major isomer is consistent with a transition state in which the —C02Et unit adopts an equatorial position. The same result could occur, however, via a boatlike transition state with an axial —C02Et group. 

Roush, W R, Adam, M A, Walts, A E, Harris, D J, Stereochemistry of the reactions of substituted allylboronates with chiral aldehydes. Factors influencing aldehyde diastereofacial selectivity, J. Am. Chem. Soc., 108, 3422-3434, 1986. 

The stereochemistry of the reactions of oxime ethers and crotylboronates (22) and (23) have been discussed earlier (Scheme 8). The reactions of the corresponding oximes with (22) and (23) appear to follow a similar stereochemical course (Scheme 14). Stereoselectivity, however, is not as high with the isobutyraldehyde and pentanal oximes as it is with phenylaldoxime. The reaction of MeaSi-substituted allylboronate (90) and acetaldehyde oxime performed in refluxing CCU similarly provides a 79 21 mixture of the anti and syn product diastereomers (MeaSi replacing Me in 25 and 26). Excellent stereoselectivity for syn-homoallylamines has been achieved via the Lewis acid catalyzed reactions of aldimines and crotyltributylstannane. ... 

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