Of the three relatively stable conformations of the threo isomer (la, lb, IC) the contribution of rotamers la and IC in which the 4 large groups are adjacent to each other would not be expected to be high. The staggered interactions of rotamer lb are fewer, and the interaction between the Ale and CC13 groups could be further reduced as a result of intramolecular H bonding between the two OH groups. Therefore, we consider the struc-
