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

462 NITRONATES
isomeric Þve-membered cyclic nitronate (24j). The signiÞcance of this approach
in the methodology of the synthesis of six-membered cyclic nitronates is unclear.
3.2.2.2.2. Synthesis of Six-membered Cyclic Nitronates by the [4 + 2]-cycloaddition
Reaction The [4 + 2]-cycloaddition reaction of conjugated nitroalkenes (42)
with oleÞns (43) is the most powerful and widely used method for the synthesis
of six-membered cyclic nitronates (35) (Scheme 3.38).
There are several advantages of this method over other approaches to the
synthesis of nitronates (35). First, direct precursors (42) and (43) are readily
available. Second, this process is simple and versatile. Finally, in many cases
high selectivity of the method allows the synthesis of diastereometrically and
enantiometrically pure nitronates (35).
The reaction presented in Scheme 3.38 is involved in the novel [4 + 2][3 + 2]
tandem strategy for the use of nitroalkenes (42) in target organic synthesis (for
details, see Section 3.4.4). Taking into account the great contribution of Prof.
S. Denmark the in investigation of different aspects of this strategy, the latter
can be classiÞed as Denmark’s approach. The principal aspects of [4 + 2]-cycloaddition
of nitro oleÞns were summarized in two Þne reviews (99, 100).
Although the reaction under consideration can be performed in the absence of
Lewis acids (LA), the presence of the latter leads to a substantial increase in the
reaction rate (101) (e.g., see Scheme 3.39).
SnCl4, Ti(OPr i )2Cl2, and methyl acetylenedicarboxylate (MAD) (bis(2,6-tertbutyl-4-methylphenoxy)-methyl-aluminum)
are most often used as LA. Meanwhile,
nonconventional procedures, for example, the reaction in water (102 and
references therein), can be used to perform cycloadditions of nitroalkenes.
NO 2
R'
R'
R
+
LA
R NO2 R'' O
N
O
42 43 35
Scheme 3.38
R''
O
N
O
H
H
H
cymene/177°C; 1 h
or
SnCl4/toluene/−78°C; 15 min
Scheme 3.39
>90%