Abstract
EWG-activated and polar quaternary ammonium salt-tagged ruthenium metathesis catalysts have been applied in a two-step one-pot metathesis-oxidation process leading to functionalized α-hydroxyketones (acyloins). In this assisted tandem process, the metathesis catalyst is used first to promote ring-closing metathesis (RCM) and cross-metathesis (CM) steps, then upon the action of Oxone™ converts into an oxidation catalyst able to transform the newly formed olefinic product into acyloin under mild conditions.
Highlights
In light of the above, a two-step protocol composed of olefin metathesis (OM) and oxidation reactions offers an interesting level of retrosynthetic orthogonality (Scheme 1a)
We examined the efficiency of classical Ru1–Ru2 and tagged Ru3–Ru5 Rucatalysts in a model tandem ring-closing metathesis (RCM)/α-ketohydroxylation sequence of diethyl diallylmalonate (S1) (Scheme 2)
On the basis of Plietker’s and Snapper’s observations, the oxidation step was performed by OxoneTM as a stoichiometric oxidant in a 6:6:1 v/v/v mixture of MeCN/EtOAc/H2 O in the presence of NaHCO3 ; we performed this step at room temperature (RT) and using ten- to five-times lower
Summary
In addition to acyloin condensation [7,12,13,14]—an established methodology in α-hydroxyketones synthesis—other transformations like, e.g., oxidation [15,16], epoxide opening [17,18], the Barbier reaction [19], reduction of 1,2-diketones [20,21], asymmetric reductive coupling of alkynes and aldehydes [22], or CO2 -promoted regioselective hydration of propargylic alcohols [23], can be used.
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