Palladium-Catalyzed Cyclization of <i>o</i>-Alkynylphenols with Allyl Carbonates. A Regioselective Synthesis of 2-Substituted-3-allylbenzo[<i>b</i>]furans
作者:Sandro Cacchi、Giancarlo Fabrizi、Leonardo Moro
DOI:10.1055/s-1998-1758
日期:1998.7
2-Substituted-3-allylbenzo[b]furans 3 can be prepared from o-alkynylphenols 1 and allyl carbonates 2 through a palladium-catalyzed O-allylation/cyclization sequence. Two basic procedures have been developed: a stepwise method based on the isolation of O-allyl derivatives 4 and their subsequent cyclization to 3 (procedure A) and a one-pot reaction omitting the isolation of 4 (procedure B). The cyclization of 4 in the presence of the electron-rich sterically-encumbered ligand tris(2,4,6-trimethoxyphenyl)phosphine (ttmpp)exhibits remarkable regioselectivity in that 3-allylbenzofurans in which the benzofuryl unit is bound to the less substituted allyl terminus are formed almost exclusively. Some loss of the stereochemistry of the carbon-carbon double bond is observed.
通过钯催化的 O-烯丙基化/环化顺序,可从邻炔基酚 1 和烯丙基碳酸酯 2 制备 2-取代的-3-烯丙基苯并[b]呋喃 3。目前已开发出两种基本程序:一种是基于分离 O-烯丙基衍生物 4 并随后将其环化为 3 的分步法(程序 A),另一种是省略分离 4 的单锅反应(程序 B)。在电子丰富的立体位阻配体三(2,4,6-三甲氧基苯基)膦(ttmpp)存在下,4 的环化反应具有显著的区域选择性,几乎只生成苯并呋喃单元与取代程度较低的烯丙基末端结合的 3-烯丙基苯并呋喃。可以观察到碳-碳双键的一些立体化学损失。