Allylboration protocol

The conventional procedure for the synthesis of homoallylic amines involves the allylboration of imines (generated in situ from the partial reduction of expensive and toxic nitriles). Recently, a practical and operationally simple protocol has been developed for the stereoselective formation of homoallylic amines starting directly from the... 

The ruthenium-catalyzed olefin cross-metathesis to the preparation of functionalized allyl boronates has resulted in a one-pot three-component coupling procedure for the synthesis of functionalized homoallylic alcohols.617,618 The utility of the protocol was demonstrated in asymmetric allylboration using a tartrate ester (Equation (152)).617... 

In the total synthesis of (+)-trienomycins A and F, Smith et al. used an Evans aldol reaction technology to construct a 1,3-diol functional group8 (Scheme 2.1i). Asymmetric aldol reaction of the boron enolate of 14 with methacrolein afforded exclusively the desired xyn-diastereomer (17) in high yield. Silylation, hydrolysis using the lithium hydroperoxide protocol, preparation of Weinreb amide mediated by carbonyldiimidazole (CDI), and DIBAL-H reduction cleanly gave the aldehyde 18. Allylboration via the Brown protocol9 (see Chapter 3) then yielded a 12.5 1 mixture of diastereomers, which was purified to provide the alcohol desired (19) in 88% yield. Desilylation and acetonide formation furnished the diene 20, which contained a C9-C14 subunit of the TBS ether of (+)-trienomycinol. 

A new protocol has been introduced to improve selectivity in allylboration of aldehydes. The readily available a-substituted allyl or crotyl pinacol boronic esters often give low A/Z-selectivity. Addition of -butyllithium followed by trapping of alkoxide with TFAA generates an allyl borinic acid, which gives very high -selectivity, and in some cases, this is the opposite of the standard conditions. The borinic ester intermediate was characterized by B-NMR. 

Aggarwal performed the stereoselective total synthesis of solandelactones E and F by a stereocontrolled allylboration reaction of the eight-membered lactone 83, which was prepared by cyclization of 81 (Scheme 5.28) [76]. Treatment of the seco-acid 81 under Yamaguchi conditions according to the protocol previously established by Martin et al. [73] and Pietruszka [75] for a similar reaction resulted in the formation of the target lactone 82 in good yield. 

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