Brønsted acids - phosphoramides - Diels-Alder reaction
Significance
<P>An organocatalyzed asymmetric Diels-Alder reaction of ethyl vinyl ketone with silyloxydiene
is described. For this purpose, the authors have developed a
N-triflyl phosphoramide as a strong chiral Brønsted acid catalyst. This chiral catalyst
allows the activation of unsaturated ketones, leading to substituted cyclohexene derivatives
in moderate to good yields and good enantioselectivities. </P>
Comment
<P>Although metal Lewis acid catalyzed asymmetric Diels-Alder reactions are well established,
reactions of α,β-unsaturated ketones are still challenging. The authors have recently
shown that an achiral metal-free Brønsted acid may function as a catalyst in a non-asymmetric
variant of this reaction and they now extend it to an asymmetric variant. Recently,
several examples of asymmetric Brønsted acid catalysis have been reported, using chiral
alcohols, thioureas, ammonium salts and phosphoric acids as appropriate catalysts.
These catalysts are suitable for the activation of imines and in some cases of carbonyl
compounds, although most reports are confined to reactive substrates. By modifying
the well-established phosphoric acid motif, the authors overcome this limitation.
The utilized
N-triflyl catalysts are stronger Brønsted acids and are suitable catalysts for the
asymmetric Diels-Alder reaction of α,β-unsaturated ketones in good enantioselectivities.</P>
