the stereochemistry of 5 was determined by
single-crystal X-ray diffraction analysis.3 Later, the absolute
structures of 2, 3, and 6 were established by Fukuyama and coworkers
through X-ray analysis of a derivative of 2, chemical
conversion of 2 into 3, and synthesis of the unnatural
diastereomer (6-epi-isomer) of 6.4 On the basis of these
structural studies, the assumed stereochemistry of 1 is shown in
Figure 1.