Alkenyl pinacol boronate

In the intermolecular process, cross-metathesis offers a usefid and versatile alternative to alkyne hydroboration for the synthesis of functionalized alkenyl pinacol boronates with a moderate to high -selectivity. Styrenes, allylsilanes, protected alcohols and amines are good partners as 1,1-disubstituted olefins [105-107]. This reaction has been exploited recently as part of a macrocyclization strategy in the epothilone series (Scheme 9.52) [108]. 

Danishefsky has applied the cross-metathesis reaction for the synthesis of highly functionalized alkenyl boronate precursor 118 from the terminal olefin 119 and vinyl pinacol boronate in the presence of first generation Grubbs-catalyst 121 [95]. The metathesis reaction was driven to completion by increasing the amount of vinyl boronate ester 120 affording the polyfunctional boronic ester 118 in 93% yield and with exclusive trans-stereochemistry. The alkenyl boronic ester 118 was subsequently used in the synthesis of epothilone 490 via intramolecular Suzuki macrocydization (Scheme 3.62). 

Palladium-mediated cross-coupling of 2-pyrone-5-boronate (63), prepared by coupling of 5-bromo-2-pyrone with pinacol borane (30), with a range of androster-one-derived alkenyl triflates 64 affords bufadienolide type steroids 65 in high yields (Scheme 3.29) [47]. 

Srebnik has reported the synthesis of phosphono boronates by hydroboration with pinacol borane (30) [92]. The reaction proceeds well with terminal alkenyl phosphonates whereas internal alkenyl phosphonates gave complex mixtures. Hydroboration of the corresponding alkynyl phosphonates under identical conditions gave alkenyl phosphonates that were difficult to isolate and were in situ subjected to Suzuki coupling with phenyl iodide to give trisubstituted phosphonates providing a new one-pot synthesis of this class of compounds (Scheme 3.60). 

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