The stereochemistry of the branched C-ethoxycarbonyl-methy.lene derivatives 1a, 1b, 2a, and 2b was evaluated by analysis of their 1H- and 13C-NMR shift data, unequivocally supported by NOE-effects clearly discriminating diastereomers. The X-ray-analysis of isomer 2a confirmed the NMR-data and provided the picture of its detailed structure