Boron-tethered cycloaddition

Scheme 10-21 Nicolaou et al. used a boron-tethered cycloaddition to construct the C-ring of taxol.

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

Similar intramolecular cycloaddition of TBDMS-protected oximes, tethered with olefin moieties, in the presence of boron trifluoride-ether generated A-nonsubstituted isoxazolidines in moderate to good yield (eq 52). The olefin geometry of the substrate was retained under the reaction conditions and the cycloaddition proceeded with high diastereoselectivity. 

The same year, Gandon et al. reported the synthesis of fused arylboronic esters 36 via cobalt(0)-mediated cycloaddition of alkynylboronates 33 with diynes 35 (Scheme 1.11) [22], The boronate is first reacted with Co2(CO)g at room temperature for 4 h to generate the corresponding dicobaltatetrahedrane 34. The diyne is then added and the mixture is refluxed for 2 h. To show the utility of the products, one of them was treated with phenyl iodide under Pd catalysis to give 37. Complementary to these investigations, Ru-catalyzed [2 - - 2 - - 2] cycloaddition of tethered alkynyl-boronic esters with alkynes was reported [23]. In this case, the borylated arene could not be isolated but was converted directly in situ by Suzuki-Miyaura coupling. 

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