Abstract
Diastereotopy has been investigated in eight different derivatives of 3,4-dihydroisocoumarins by changing the substituent on the pyran ring chiral centre, C-3. A diastereotopic NMR effect is observed for the prochiral methylene protons on the benzopyran ring and the effect is greatly influenced by the π-electron system of the substituent. The effect is also observed for the substituents, however, the extent of diastereotopy depends upon their nature and the separation of methylene groups from the chiral centre.