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