Frequently, in the case of extracts from natural sources, but also in the case of synthetic products, researchers face the problem of isolating some structures, which are inseparable chromatographically. For these cases, one of the solutions is their selective chemical separation, according to the model “reactive-inert” or their derivatization into separable reaction products. The purpose of this study was to demonstrate the usefulness of unconventional chemical methods, especially of sensitized photooxidation in solving this problem. Thus, ent-trahiloban-19-oic (1) and ent-kaur-16-en-19-oic (2) acids can be obtained in sufficient quantities from sunflower (Helianthus annuus L.) waste in the form of a mixture of ~1: 2, but isolating them in pure form is quite difficult [1,2]. For this reason, chemical separation of compounds 1 and 2 was attempted, applying sensitized photooxidation. In these conditions, only ent-kaur-16-en-19-oic (2) reacts, giving corresponding hydroperoxide 3, in 61% yield (Scheme). Compound 3, by a cascade of reaction was converted in alcohol 4 and its acetate 5, in high yields. In such a way, the ratio on compound 1 and 2 after photooxidation reaction, according to GC-MS analysis, changed into ~2:1. Reagents and conditions: i. CH2N2, Et2O, 1 h, r.t., 99%; ii. O2, TPP, h, DCM, 7 h, 5°C, 61%; iii. Thiourea, MeOH, 5 h, r.t., 83%; iv. Ac2O, Py, 24 h, r.t., 93%. Scheme The same transformations were done in a series of methyl esters 6 and 7 in comparable yields. Looking to the results, it much be concluded that sensitized photooxidation may be a useful tool for isolation of chromatographically inseparable compounds.