A Simple and General Chiral Silicon Lewis Acid for Asymmetric Synthesis:Highly Enantioselective [3+2] Acylhydrazone-Enol Ether Cycloadditions

摘要

[3+2] Cycloaddition reactions of azomethine imines,diazoal-kanes,and nitrile imines provide access to medicinally important pyrazolidine and pyrazoline derivatives,and both chiral auxiliary-controlled diastereoselective and catalytic enantioselective variants of such reactions have been reported.More recently,Kobayashi has established that related acylhydrazone-olefin cycloadditions may be catalyzed by (chiral) Lewis acids.We have established a program to develop chiral silicon-based Lewis acids for asymmetric synthesis,motivated by the significant practical advantages that could accrue to the use of silicon.Our strategy for inducing Lewis acidity in silanes is ring-strain,and we have documented that simply constraining allyl- and crotylsilanes in a five-membered ring leads to type I allyl-/crotylation reagents for both aldehydes and acyl-hydrazones.In an effort to generalize this strategy beyond allylation chemistry,we recently developed an enantioselective Friedel-Crafts reaction promoted by silanes 1 (Scheme 1).