Diastereotopy in some 3,4-dihydroisocoumarins and related compounds

Some complex situations of diastereotopy involving two methylene groups about a chiral centre are reported. In this regard, NMR data for protons at and vicinal to the chiral centre in eight different 3,4-dihydrooisocoumarins (1), as well as eleven related compounds (2) are presented. In the dihydroisocoumarins (1) both methylene groups adjacent to the chiral centre behave as diastereotopes, except if the proton at this carbon is replaced by hydroxyl or methoxyl; in which case the ring methylene protons at C-4 exhibit only a singlet. On the contrary, the open-chain compounds, (2) exhibit diastereotopy only for the methylene group directly bonded to the aromatic ring.