Tin-oxygen compounds derived from asymmetric diols are shown to form molecular complexes which are composed of either two molecules of opposite configuration (meso complexes A and C) or of identical configuration (chiral complexes B and D). The diastereomeric relationship of these complexes is used for the optical enrichment of diols (1,3-propanediol, diethyl tartrate, 1-benzylglycerol, and 2,3-butanediol). The method involves conversion of diols to cyclic tin derivatives (1, 2, 6, and 7) and fractionating crystallization. Regeneration of the parent diols under mild, anhydrous conditions provides optically pure compounds in a single crystallization step. Extensive NMR data on the molecular complexes are given, and their high degree of orientation is demonstrated by their conversion to macrocyclic products (3aa, 3ab, 5aa, and 5ab) in high regio- and stereospecificity.