Catalytic enantioselective three-component hetero-[4+2] cycloaddition/allylboration approach to alpha-hydroxyalkyl pyrans: scope, limitations, and mechanistic proposal.
This article describes the design and optimization of a catalytic enantioselective three-component hetero-[4+2] cycloaddition/allylboration reaction between 3-boronoacrolein, enol ethers, and aldehydes to afford alpha-hydroxyalkyl dihydropyrans. The key substrate, 3-boronoacrolein pinacolate (2) was found to be an exceptionally reactive heterodiene in the hetero-[4+2] cycloaddition catalyzed by Jacobsen's chiral Cr(III) catalyst 1. The scope and limitations of this process were thoroughly examined. The adduct of 3-boronoacrolein pinacolate and ethyl vinyl ether was obtained in high yield and with over 95 % enantioselectivity. This cyclic alpha-chiral allylboronate adds to a very wide variety of aldehyde substrates, including unsaturated aldehydes and alpha-chiral aldehydes to give diastereomerically pure products. Acyclic 2-substituted enol ethers can be employed, in which case the catalyst promotes a kinetically selective reaction that favors Z enol ethers over the E isomers. Surprisingly, 3-boronoacrolein pinacolate was found to be a superior heterodiene than ethyl (E)-4-oxobutenoate, and a mechanistic interpretation based on a possible [5+2] transition state is proposed.
Gao
Xuri
X
Department of Chemistry, Gunning-Lemieux Chemistry Centre, University of Alberta, Edmonton, AB T6G 2G2, Canada.
Hall
Dennis G
DG
Deligny
Michael
M
Favre
Annaick
A
Carreaux
François
F
Carboni
Bertrand
B
D000498:Allyl Compounds / Q000737:chemistry*; D001895:Boron / Q000737:chemistry*; D002384:Catalysis; D011714:Pyrans / Q000737:chemistry*; D013237:Stereoisomerism
D000498:Allyl Compounds; D011714:Pyrans; D001895:Boron