Diastereo- and Enantioselective anti-Alkoxyallylation Employing Allylic gem-Dicarboxylates as Allyl Donors via Iridium-Catalyzed Transfer Hydrogenation

摘要

Enantioselective carbonyl allylation represents an important classnof C C bond-forming reactions that have found broad use in organicnsynthesis.n1nAmong allylation protocols, enantioselective aldehyden-alkoxyallylation provides an efficient means of generating allylicnvicinal diol substructures. To date, asymmetric syn-additions of thisntype have employed chirally modified alkoxy-substituted allylmetalnreagents based on boron,n2naluminum,n3ntitanium,n4nindium,n5nor tin.n6nIndirect enantioselective aldehyde -alkoxyallylation can be achievednusing chirally modified reagents that promote aldehyde -silylallylationnor -borylallylation followed by oxidation of the C Si or C B bond,nrespectively.n7 9nFor direct methods, stereocontrolled access to anti-nalkoxyallylation products is problematic because of difficulties en-ncountered in the synthesis of the requisite E-configured allylmetalnreagents.n10nThis fact has motivated alternate approaches to the anti-n1-ene-2,3-diol functional group array. For example, enantioselectivencatalytic dihydroxylation of conjugated dienes was attempted,n11nbutnsyn-1-ene-2,3-diols were formed. To our knowledge, only Duthaler’snchiral allyltitanium reagentn4nhas been employed in direct enantiose-nlective anti-alkoxyallylation; however, only a single highly stereose-nlective example was reported, and use of this allylmetal reagent isnattended by considerable “preactivation”.n12nTo date, catalytic enanti-noselective methods for aldehyde syn-or anti- -alkoxyallylation remainnelusive.