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Figure 3.1 Stereoviews of the energy minimized models of 154a and 155a.
O
HN
O
Bn
2
NH
2
1
1
CO2Me 154a
MeO 2C
The newly obtained bicyclic triene 155a was also modeled in order to visualize its three-
dimensional structure. As illustrated in Figure 3.1 the fused bicyclic structure is relatively planar
due to the presence of multiple sp 2 -hybridized atoms and the bottom face is blocked by the
benzyl substituent. Therefore, cycloaddition reaction with the diene is expected to occur
selectively with the top face (the facial assignment is relative since all compounds are racemic).
To test this hypothesis, triene 155a was reacted with N-phenylmaleimide (1.3 equiv.). Reaction
occurred in less then 1 h at 90 °C to afford the cycloadduct 156a in 73% yield as a single
diastereomer (Scheme 3.28). The relative stereochemistry of 156a was assigned by X-ray
crystallography, confirming that the cycloaddition occurred with endo selectivity and the diene
approached from the opposite face of the benzyl group (Appendix B). Excess dienophile (2
equiv) still gave exclusive formation of 156a, indicating a reduction in the rate of the second
cycloaddition. Similarly, cycloaddition of 155b with N-methylmaleimide resulted in formation of
156b in 95% yield as a single diastereomer. Additional experimentation by Dr. Yan (Brummond
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2
O
O
2
N
1
N
Bn
1
O
155a