only when using the expensive Eu(hfc)3 catalyst. In addition,
the diene itself requires extremely careful handling and is
unstable during storage. A resin-bound derivative of Bras-
sard’s diene has been reported as being stable for extended
periods, but its reactions come with reduced yields and other
limitations inherent to solid-bound reagents.7 The various
other routes toward 5,6-dihydro-2-pyrone moieties suffer
similarly from low efficiencies.8
In addition to exhibiting biologically important functions,
4-alkoxy-5,6-dihydro-2-pyrones are also synthetically use-
ful: theycanbeefficientlytransformedintohydroxypyrazoles,9a
reduction of the carbonyl group provides a hydroxylated
anomeric carbon for further functionalization,9c,d and they
are converted to disubstituted 2,3-dihydro-4-pyrones upon
addition of a nucleophile followed by acid hydrolysis.9e The
synthetic utility of 5,6-dihydro-2-pyrones has also been
demonstrated in several syntheses of polyether ionophore
antibiotics.9b
Considering the many versatile and efficient methodologies
that have emerged featuring phosphine-catalyzed annulations
of electron-deficient allenes,10 we suspected that it might be
possible to synthesize disubstituted dihydropyrones from
allenoates. Previously, we reported the preparation of diox-
anes and pyrones through nucleophilic phosphine-catalyzed
additions of allenoates to aldehydes as effective electrophiles
(Figure 1).11
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Figure 1. Divergent pathways for phosphine-catalyzed annulations
of aldehydes with 2,3-butadienoates.
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interaction between the partially anionic carbonyl group and
the phosphonium center (Scheme 1).12 Dioxanes result when
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Scheme 1. Possible Reaction Pathways
an aldehyde adds to this s-trans-dienolate, followed by the
addition of a second molecule of aldehyde and subsequent
6-exo-trig cyclization.11a Use of a more sterically bulky
phosphine destabilizes the s-trans-phosphonium dienolate
and shifts the equilibrium in favor of the s-cis isomer 2.11b
Upon addition of an aldehyde, the intermediate â-phospho-
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6722 and references cited therein.
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