Chiral
bifunctional urea-catalyzed asymmetric direct hetero-Diels–Alder reaction between alkylidene azlactone-derived
dienes and trifluoromethyl aryl ketones is reported for the first
time. The direct hetero-Diels–Alder reaction followed
by ring opening results in densely functionalized α,β-unsaturated
δ-lactones featuring a CF3-substituted quaternary
stereocenter in high yields with excellent enantioselectivities. The
method is compatible over a range of substrates. Moreover, the reaction
is scaled up and the α,β-unsaturated δ-lactones
were converted to amino acid derivatives decorated with trifluoromethylated
carbinol functionality.
We
present protocols for the highly regioselective hydroxyselenylation
and silanoxyselenylation of allylic silanols. N-(Phenylseleno)phthalimide
acts as the selenylating agent for both transformations. Under basic
conditions, hydroxyselenylation proceeds with >20:1 regioselectivity,
and the products are valuable synthons for further transformations.
We show that the silanol plays a critical role in maintaining the
yield and regioselectivity of this reaction. Surprisingly, under acidic
conditions, the hydroxyselenylation pathway is blocked, and products
of a tethered silanoxyselenylation are exclusive.
An asymmetric direct doubly vinylogous Michael addition has been developed for the generation of sterically congested vicinal quaternary and tertiary stereocenters. This doubly vinylogous Michael addition of β,γ-unsaturated butenolides to 3-methyl-4-nitro-5-alkenyl isoxazoles, powered by a bifunctional squaramide, affords a broad range of densely functionalized enantioenriched γ,γ-disubstituted butenolides in high yields with excellent diastereo- and enantioselectivities in most cases. Moreover, the strategy highlights the first application of β,γ-unsaturated butenolides in an asymmetric 1,6-conjugate addition.
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