Temporary boronate tether

Inspired by these results, Nicolaou et al. used a temporary boronate tether to overcome the undesired regiochemical bias in the intermolecular Diels-Alder reaction of diene 56 and dienophile 57, required for their approach to the C-ring of taxol 58 [21a, b, 24], Thermolysis of a benzene solution of 56 and 57 in the presence of PhB(OH)2 under dehydrating conditions provided the cycloadduct 59 in 79% yield (77% conversion) after transacetalization of the boron tether with 2,2-dimethylpropane-l,3-diol and intramolecular acyl transfer to the less strained [4.3.0]bicyclic system. This was further elaborated to an advanced intermediate in the ultimately successful synthesis of taxol (Scheme 10-21) [21c]. 

Chemo- and regioselective ruthenium-catalyzed cyclotrimerization of alkynes was accomplished through a temporary boron tether. Crude aryl boronates were obtained in a one-pot procedure in which they were subjected to a Suzuki-Miyaura coupling with aryl halides (Scheme 9.50) [103]. 

Scheme 2.51 Ru-catalyzed cycloaddition using a temporary boron tether.

Y. Yamamoto, J.-I. Ishii, H. Nishiyama, K. Itoh, J. Am. Chem. Soc. 2005, 127, 9625-9631. Cp RuCl-catalyzed formal intermolecular cyclotiimerization of three unsymmetrical alkynes through a boron temporary tether regioselective four-component couphng synthesis of phthalides. 

Yamamoto, Y., Ishii, J., Nishiyama, H. and Itoh, K. (2004) Ru(II)-catalyzed chemo-and regioselective cyclotrimerization of three unsymmetrical alkynes through boron temporary tether. One-pot four-component coupling via cyclotrimerization/Suzuki-Miyaura coupling. Journal of the American Chemical Society, 126(12), 3712-3713. 

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