A Chiral N,N′‐Dioxide–Zn^II Complex Catalyzes the Enantioselective [2+2] Cycloaddition of Alkynones with Cyclic Enol Silyl Ethers

Abstract

A highly efficient enantioselective [2+2] cycloaddition between alkynones and cyclic enol silyl ethers was developed by using a chiral N,N'-dioxide-zinc(II) complex as a catalyst. This method functions well for a variety of terminal alkynes as well as cyclic enol silyl ethers, with good to excellent enantioselectivity (up to 97 % ee). This is also the first successful example for the catalytic enantioselective [2+2] cycloaddition of internal alkynes with cyclic enol silyl ethers to give fully substituted cyclobutenes. Meanwhile, the desired cyclobutene product can easily be transformed into fused cyclobutane derivatives.

Au-Catalyzed Intermolecular [2+2] Cycloadditions between Chloroalkynes and Unactivated Alkenes

“…The reactions between alkynes and alkenes have been extensively studied in intramolecular settings in homogeneous gold catalysis, 8,9 some leading to cyclobutene formations. 7e–g However, the intermolecular version 5b,7l,m,10 en route to cyclobutenes has only been reported by Echavarren and co-workers, where 1,1-disubstituted or trisubstituted alkenes are almost exclusively employed (e.g., eq 1). 5b,7l,m With the only exception of cyclooctene, 7l alkenes with 1,2-dialkyl or monoalkyl substitutions have not be reported as suitable alkene partners and appear to be electronically not activated enough; moreover, the necessity of arylacetylenes 5b,7l,m and terminal diynes 7l as alkyne partners in these gold catalysis prevents access to various other types of cyclobutene structures.…”

Chiral Zinc(II)‐Catalyzed Enantioselective Tandem α‐Alkenyl Addition/Proton Shift Reaction of Silyl Enol Ethers with Ketimines

“…Silyl enol ethers are isolable and versatile intermediates serving as convenient carbonyl equivalent donors in organic synthesis because of their high reactivity and operability, which have enabled broad synthetic utilization . In our previous studies on silyl enol ethers, several types of reactions were realized by using chiral Lewis acid catalysts, including [2+2] cycloadditions between alkynones and cyclic silyl enol ethers, [4+2] cycloadditions of silyloxyvinylindoles with β,γ‐unsaturated α‐ketoesters, and so on . To further expand the application of silyl enol ethers, we explored a reaction of silyl enol ethers with imines and anticipated a Mukaiyama–Mannich reaction to afford the corresponding β‐amino carbonyl compounds .…”

Chiral Zinc(II)‐Catalyzed Enantioselective Tandem α‐Alkenyl Addition/Proton Shift Reaction of Silyl Enol Ethers with Ketimines

“…[1] In our previous studies on silyl enol ethers,several types of reactions were realized by using chiral Lewis acid catalysts,i ncluding [2+ +2] cycloadditions between alkynones and cyclic silyl enol ethers, [2] [4+ +2] cycloadditions of silyloxyvinylindoles with b,gunsaturated a-ketoesters, [3] and so on. [1] In our previous studies on silyl enol ethers,several types of reactions were realized by using chiral Lewis acid catalysts,i ncluding [2+ +2] cycloadditions between alkynones and cyclic silyl enol ethers, [2] [4+ +2] cycloadditions of silyloxyvinylindoles with b,gunsaturated a-ketoesters, [3] and so on.…”