Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

Four component [4+2] [4+2] [4+2] domino cycloaddition
MeO
O
NO 2
+ + +
N Ph
Ph
O
270 276
271
R
274b
R=Ph
REACTIVITY OF NITRONATES 597
CH2Cl2, 15 kbar
r.t., 16 h, 83%
Scheme 3.175
Ph
N
O
H
OMe
O
N
O
O H H
Ph
279a
+
Ph
O
N
H OMe
O
N
O
O H H H
Ph
279b
279a/279b ~ 2 : 1
molecule to give the [3 + 2]-cycloaddition product. According to NMR data, the
Þrst two reactions are regio- and stereoselective, and the latter step affords regioisomers
(272) and (273), one of which gives two stereoisomers.
Examples of three- and four-component sequences, each occurring at superhigh
pressure with various stereoselectivities, are given in the lower port of
Scheme 3.175. Methoxydiene (270) and nitrostyrene are necessary participants
of these sequences.
This method is expected to Þnd more use in the future.
3.4.5. Nitronates in Enantioselective [3 + 2]-Cycloaddition Reactions
This problem has already been touched upon in previous sections. However,
taking into account the importance of these reactions in modern organic synthesis,
it is worthwhile to consider the main data in a special section.
The enantioselectivity of [3 + 2]-cycloaddition reactions is determined by the
preference of a particular facial attack of the reagents containing chiral inductors.
Most of such reactions proceed in the absence of a catalyst and, consequently,
the inductor should be present in either the dipole or the dipolarophile.
For acyclic alkyl nitronates, only one process of this type has been documented
(17) (Scheme 3.176).
Starting nitro compound (280) is generated by the Michael reaction with enantiomerically
pure dihydrofuranone containing the ( + )-menthyl fragment. Stereoand
enantiomerically pure nitronates (281) can be prepared and isolated upon
alkylation of nitro substrate (280). One of nitronates (281a, R ′ = Et) is readily
involved in the [3 + 2]-cycloaddition reaction with ethyl acrylate to give
enantiomerically pure isoxazolidine (282a) in satisfactory yield. Although the
complete conÞguration of (282a) was not determined, it can be concluded that
Ph
Ph