The Mediterranean sponge Reniera sarai is a rich source of new alkaloids. Until now the study has been limited to the less polar compounds. In this paper we report the first studies on the more polar UV absorbing alkaloid fraction, which resulted a mixture of threehomologs, sarains A-C. A resolutive X-ray study on an acetate derivative of sarain-A has descovered an unprecedented alkaloid skeletonexhibiting a central cage structure, characterized by a chargedpseudobase moiety, in the middle of two cyclic alkyl chains. © 1989.