which, however, becomes flatter and flatter on passing from (1a) to (1e). As a result there is a progessive lessening of steric hindrance on the syn face which neatly parallels the observed increase in syn attack along the series (1a–e). Moreover the energy required to remove steric hindrance on passing from the boat-like to the half-planar conformation is definitively lower for (1c–e) than for (1b and