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group) demonstrated that this cycloaddition is limited mainly to using maleimides. 104 Other
dienophiles (diethylfumarate, p-benzoquinone and dimethylacetylene dicarboxylate) generally
gave yields of the cycloadduct lower than 50%.
Scheme 3.28 Diels-Alder reaction of imino-oxazolidinones 155a and 155b.
R 1
O
O
N
N
Bn
O
155a, R 1 = -CO 2Me
155b, R 1 = i-Pr
O
R
N
O
2
1.3 equiv.
toluene, 1h, 90 o C
R 1
N
O
O
N
O R
N
2
O
O
R 1
O
O
O
N
H
N
Bn
O
N
R2 O
156a, R 1 = CO 2Me, R 2 = Ph, 73%
156b, R 1 = i-Pr, R 2 = Me, 95%
X-ray of 156a
The X-ray crystal structure of 156a reveals several interesting features. As a result of the endo-
cycloaddition occurring from the concave face of 155a, the product adopts a folded shape with
the N-phenylpyrrolidinone moiety projecting directly above the new diene system (Figure 3.2).
Moreover, the two double bonds of the 1,3-diene are twisted out of planarity by a 39.9° angle. In
addition, the α,β-unsaturated amide is also twisted in the opposite direction by 42.4°.
Figure 3.2 Stereoview of the X-ray crystal structure of 156a with diene and amide torsional
angles.
MeO 2C
O
O
O
N
H
N Bn
O
N
Ph
156a
O
torsional angle
of diene = 39.9 °
57
torsional angle
of α,β-unsaturated amide = 42.4 °