Illustrated Glossary of Organic Chemistry

Cram's rule: As stated by Donald Cram of UCLA in 1952, "In certain non-catalytic reactions that diastereomer will predominate, which could be formed by the approach of the entering group from the least hindered side when the rotational conformation of the C-C bond is such that the double bond is flanked by the two least bulky groups attached to the adjacent asymmetric center. Developed Cram as an early explanation for some cases of asymmetric induction, a process wherein one stereocenter influences the configuration of other stereocenters formed during a reaction.

In this demonstration of Cram's rule, phenylmagnesium bromide (PhMgBr; a nucleophile) adds to an aldehyde carbonyl group (an electrophile) from the least hindered face, giving as the major product the secondary alcohol shown above.