ラジカル環化反応の位置化学の制御と生理活性化合物合成への応用
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Radical cyclizations are now widely used in organic synthesis for the construction of carbo- and heterocyclic compounds, including natural products. In this review, results of our recent studies on the control of regiochemistry of radical cyclizations are summarized. One factor determining the course (regiochemistry) of radical cyclizations is the reaction temperature used. For example, at a low temperature, <I>i.e</I>., under kinetically controlled conditions, 4-<I>exo</I>-<I>trig</I> cyclization predominated over another possible cyclization, 5-<I>endo</I>-<I>trig</I> cyclization, whereas at a high temperature, <I>i.e</I>., under thermodynamically controlled conditions, 5-<I>endo</I> cyclization products were obtained. Another factor determining the course of radical cyclizations involves the nature of the prefered conformation of the radical precursors. In general, a 5-<I>exo</I>-<I>trig</I> cyclization is prefered over a 6-<I>endo</I>-<I>trig</I> ring closure in systems having an alkenic bond at the 5-position relative to the aryl radical center. We found, however, that <I>N</I>- (ο-bromobenzyl) enamides underwent cyclization in a 6-<I>endo</I>-<I>trig</I> manner to give exclusively a tetrahydroisoquinoline derivative. The mode of the cyclization can be changed to 5-<I>exo</I> by introducing a (<I>Z</I>) -phenylthio group into the terminus of the <I>N</I>-vinylic bond, indicating that the stabilizing effect of the sulfur atom on an adjacent radical center is highly effective for controlling regiochemistry. The introduction of a phenylthio group into the terminus of the <I>N</I>-vinylic bond is also effective for achieving 6-<I>exo</I>-<I>trig</I> cyclization of aryl radicals. Applications of these radical cyclizations to the synthesis of some therapeutically important materials are also presented.
- 有機合成化学協会の論文
- 2004-04-01
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