Conformerism, enantiomorphism and double catemer motifs in para-substituted nostoclide analogues

Abstract

We have here elucidated the crystal structures of five nostoclide analogues. A common feature in all compounds is a substituent at the para-position of the benzylidene group. Compounds with either bromine (3) or hydroxyl (4) as para-substituent crystallizes with Z’ = 2 as result of conformerism. It was also observed that Z' > 1 in the compound with a para-dimethylamino substituent (1). However, its four crystallographically independent molecules are conformationally similar. They are not related by crystallographic symmetry due to the offset packing of their C–H … OC nonclassical hydrogen bonded double chains. This compound (1) has also crystallized in a chiral space group (P21) despite the lack of a stereocenter. Such enantiomorphism phenomenon is related to the presence of only one of the two mirror benzyl conformations with phenyl ring at the equatorial position opposite the lactone oxygen atom. The molecular mean plane of nostoclide analogues has been featured by high level of planarity, except in the brominated compound where two twisted conformations occurred due to rotations on the single bond axis into benzylidene group. The benzyl conformation has been the greatest difference between the two crystallographically independent molecules of the para-hydroxylated compound (4). The crystal packing of the compounds is marked by double catemer motif assembled through C–H … OC non-classical hydrogen bonds, although C–H … π interactions do play an important role in stabilizing the crystal packing of some compounds of the series.