An original, effective approach to the synthesis of natural and synthetic 5Z,9Z-dienoic acids in high yields (61–67%) and with high selectivity (>98%) was developed. The approach is based on the use of the new intermolecular catalytic cross cyclomagnesiation of terminal aliphatic and oxygenated 1,2-dienes upon treatment with Grignard reagents in the presence of the Cp₂TiCl₂ catalyst. High activity of (5Z,9Z)-5,9-eicosadienoic acid as a human topoisomerase I inhibitor at concentrations above 0.1 μM was elucidated.