A highly enantioselective isothiourea‐catalyzed acylative kinetic resolution (KR) of acyclic tertiary alcohols has been developed. Selectivity factors of up to 200 were achieved for the KR of tertiary alcohols bearing an adjacent ester substituent, with both reaction conversion and enantioselectivity found to be sensitive to the steric and electronic environment at the stereogenic tertiary carbinol centre. For more sterically congested alcohols, the use of a recently‐developed isoselenourea catalyst was optimal, with equivalent enantioselectivity but higher conversion achieved in comparison to the isothiourea HyperBTM. Diastereomeric acylation transition state models are proposed to rationalize the origins of enantiodiscrimination in this process. This KR procedure was also translated to a continuous‐flow process using a polymer‐supported variant of the catalyst.
Cycloaddition reactions, in particular Diels-Alder reactions, have attracted a lot of attention from organic chemists since they represent one of the most powerful methodologies for the construction of carbon-carbon bonds. In particular, inverse-electron-demand hetero-Diels-Alder reactions have been an important breakthrough for the synthesis of heterocyclic compounds. Among all their variants, the organocatalytic enantioselective version has been widely explored since the asymmetric construction of diversely functionalized scaffolds under reaction conditions encompassed within the green chemistry field is of great interest. In this review, a profound revision on the latest advances on the organocatalytic asymmetric inverse-electron demand hetero-Diels-Alder reaction is shown.
Described herein is a new visible-light photocatalytic strategy for the synthesis of enantioenriched dihydrofurans and cyclopentenes by an intramolecular nitro cyclopropane ring expansion reaction. Mechanistic studies and DFT calculations are used to elucidate the key factors in this new ring expansion reaction, and the need for the nitro group on the cyclopropane.
A highly enantioselective organocatalytic vinylogous Mukaiyama aldol reaction of silyloxy dienes and isatins under bifunctional organocatalysis is presented. Substituted 3-hydroxy-2-oxindoles are synthesised in good yields and enantioselectivities. These synthetic intermediates are used for the construction of more complex molecules with biological properties such as the formal synthesis of a CB2 agonist presented.
A highly
efficient enantioselective inverse-electron-demand aza-Diels–Alder
reaction between aza-sulfonyl-1-aza-1,3-butadienes and silyl (di)enol
ethers has been developed. The presented methodology allows the synthesis
of benzofuran-fused 2-piperidinol derivatives with three contiguous
stereocenters in a highly selective manner, as even the hemiaminal
center is completely stereocontrolled. Density functional theory (DFT)
calculations support that the hydrogen-bond donor-based bifunctional
organocatalyst selectively triggers the reaction through the ipso,α-position
of the dienophile, in contrast to the reactivity observed for dienolates
in situ generated from β,γ-unsaturated derivatives. Moreover,
the calculations have clarified the mechanism of the reaction and
the ability of the hydrogen-bond donor core to hydrolyze selectively
the
E
isomer of the dienol ether. Furthermore, to
demonstrate the applicability of silyl enol ethers as nucleophiles
in the asymmetric synthesis of interesting benzofuran-fused derivatives,
the catalytic system has also been implemented for the highly efficient
installation of an aromatic ring in the piperidine adducts.
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