Helmchens Postulates  are the theoretical models used to predict the elution order and extent of separation of diastereomers (including those formed from CDAs) that are adsorbed onto a surface. Although Helmchens postulates are specific for amides on silica gel using liquid chromatography, the postulates provide fundamental guidelines for other molecules. Helmchen's Postulates are:
#Conformations are the same in solution and when adsorbed.
#Diastereomers bind to surfaces (silica gel in normal phase chromatography) mainly with hydrogen bonding.
#Significant resolution of diastereomers is only expected when molecules can adsorb to silica through two contact points (two hydrogen bonds). This interaction can be perturbed by substituents. 
#Diastereomers with bulky substituents on the alpha carbon (R2) and on the nitrogen (R1) can shield the hydrogen bonding with the surface, thus the molecule will be eluted before similar molecules with smaller substituents.
Helmchen's postulates have been proven to be applicable to other functional groups such as: carbamates, esters, and epoxides.