It is shown that the vinyl group of chlorophyll a derivatives (methylpheophorbide a, methylpyropheophorbide a, 13-amides of chlorin e 6 ) can be oxidized to the aldehyde under mild conditions by atmospheric oxygen in the presence of thiols as catalysts. The corresponding aldehyde can be obtained in 50-55% yields. The possible mechanism of the catalytic action of thiols in the oxidation of the vinyl group is proposed. Interest in the chemical transformations of chlorophylls and their derivatives is due not only to significant role of these compounds in nature, but also the possibility of their use as precursors for the synthesis of compounds, which find application in various fields (medicine, (1-3) catalysis, (4) design of polymeric nanomaterials for photoelectronics, (5-8) etc.) Oxidation of the vinyl group in chlorophyll a and its deriva- tives to the aldehyde with formation of the corresponding d-series derivatives is widely used in the chemistry of these compounds as an initial conversion step. The aldehyde group in the position 3 can be further involved in a variety of re- actions. The literature describes the chemical modification of the d-series derivatives using the reductive amination, (9,10) Wittig, (11) McMurray, (12,13) and Knoevenagel (11) reactions. Reactions of the aldehyde group of d-chlorins were used to form fluoroacyl (14) and ethenyl (15) substituents. Formylchlo- rins, obtained by oxidation of the vinyl group of chlorophyll a derivatives, can be used as aromatic aldehydes for forma- tion of any porphyrin macrocycle in the synthesis of various dimeric porphyrins. (16,17) Oxidation of the vinyl group of chlo- rophyll a derivatives are carried out in most cases by reduc- tive ozonolysis (11,18,19) or using osmium compounds, mainly, catalytically, when diol, formed in situ by the action of os- mium tetraoxide in catalytic amounts, is oxidized with sodium periodates (see reviews (18-20) and references therein). Wagner reaction (oxidation by KMnO 4 ), used in the conversion of chlorophyll a to chlorophyll d in order to prove the structure of the latter, (21) did not received application in the chemistry of chlorophylls and their derivatives because of low selectiv- ity. Recently it was reported that when the various thiols react with methylpyropheophorbide a in the presence of air, instead of the expected addition of thiol to vinyl group, the various oxidation processes take place, including oxidation of the vi- nyl group to an aldehyde. (22) Oxidation of the vinyl group, ac- cording to (22) , occurs in the presence of five-fold excess of thiol and four-fold excess of p-toluenesulfonic acid. A similar oxidation, we have also observed when tried to carry out the addition reaction with a number of thiols with participation of