Up till now, we have considered minor adjustments of functional groups on the periphery of the morphine skeleton or drastic simplification of the morphine skeleton.If we make the molecule more rigid so that it takes up fewer conformations, we might eliminate the conformations which are recognized by undesirable receptors, and thus restrict the molecule to the specific conformation which fits the desired receptor.In this way, we would hope to eliminate such side-effects as dependence and respiratory depres- sion.The best example of this tactic in the analgesic field is provided by a group of compounds known as the oripavines.