Diastereoselective Allylations with Chiral Boron Reagents

5 Diastereoselective Allylations with Chiral Boron Reagents 

Numerous notable applications of Brown s chiral borane reagents have been documented [32, 78], with the stereoselective allylation of meso-dialde-hyde 82 serving as an elegant example [79], Treatment of 82 with 16 furnished the bisallylated product 83 in 98 % ee and 15 1 dr. This example represents an early illustration of the concept of two-directional chain synthesis for the construction of complex molecules [11, 80], In this approach, the symmetry of a given targeted fragment may permit synthetic elaboration to be carried out concurrently at two ends of molecule. Ultimately, such an approach may demand a subsequent step in the sequence to differentiate the two ends of the chain and to break the symmetry. 

6 Mechanistic Aspects - Lewis Acid-Promoted Addition of Allylsilanes and Allylstannanes to Aldehydes 

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Allylic Boron Reagents 361 Table 10-18. Double diastereoselection addition with chiral 2-butenylboranes 192. 

The use of chiral Br0nsted acids is illustrated in Eq. 93 as a method for catalyst-controlled double diastereoselective additions of pinacol allylic boronates. Aside from circumventing the need for a chiral boronate, these additions can lead to very good amplification of facial stereoselectivity. For example, compared to both non-catalyzed (room temperature, Eq. 90) and SnCU-catalyzed variants, the use of the matched diol-SnCU enantiomer at a low temperature leads to a significant improvement in the proportion of the desired anti-syn diastereomer in the crotylation of aldehyde 117 with pinacolate reagent (Z)-7 (Eq. 93). Moreover, unlike reagent (Z)-ll (Eq. 91) none of the other diastereomers arising from Z- to E-isomerization is observed. 

In order to explain the chemistry of allylic metals, the reactions of allylic boron compounds [8,12-14] are covered in detail. The boron chemistry is divided into four parts simple enantioselectivity (addition of CH2=CHCH2-, creating one new stereocenter), simple diastereoselectivity of crotyl additions (relative configuration after CH3CH=CHCH2- addition, where neither reagent is chiral), single asymmetric induction with chiral allyl boron compounds (one and two new stereocenters), and double asymmetric induction (both reactants chiral, one and two new stereocenters). Then follows a brief discussion of other allyl metal systems. 

Chiral boronates incorporating tartrate controlling groups were utilized by Roush in the synthesis of the trioxadecalin portion of mycalamide A (Scheme 5.15) [81]. The sequence involves two diastereoselective allylations to give 86 and 91 with excellent diastereoselectivity (dr>98 2). The route also showcases the use of Brown s allylation reagent 16 to access fragment 88 (dr>98 2). 

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