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2.3.3.4 Three-Component Hetero-[4 + 2]-Cycloaddition/Allylboration Approach

DOI: 10.1055/sos-SD-211-00113

Szabó, K. J.Science of Synthesis: Multicomponent Reactions, (20142367.

Another efficient procedure for in situ generation of allylboronates has been presented by Hall and co-workers.[‌21‌,‌22‌,‌74‌‌76‌] In this reaction, a [4 + 2] inverse-electron-demand cycloaddition of a heterodienylboronate and electron-poor alkenes is performed to produce cyclic allylboronates 23 (Scheme 13). The allylboronates can be reacted with aldehydes without isolation to give the corresponding homoallylic alcohols 24. A particularly attractive feature of this reaction is that using catalyst 22, which was introduced by Jacobsen and co-workers,[‌77‌] enantiomerically pure allylboronates can be obtained.[‌22‌,‌74‌‌76‌] Thus, the procedure is suitable for the synthesis of chiral dihydro-2H-pyrans 24 (X = O) and tetrahydropyridines 24 (X = NR2) from allylboronates. The reaction has a very broad scope as a large variety of alkenes, including chiral ones,[‌75‌] can be employed (in the latter case, the reaction proceeds without the chromium catalyst).

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