As a test for this precedented route to 5, we prepared 2b with completely stereospecific exo- deuterium labeling (Scheme I). 9 Cleavage of the Cl-C5 bond requires that the deuteriums of 2b end up in the aliphatic CH2's of 5. However, any intervention by 1 would make all CH2 groups equivalent and would thus produce aliphatic (A) and vinylic (V) deuteriums in 5. As shown in Table 11, sensitized photolysis of 2b gives both 5-A and 5-V. The four aliphatic ...