From the culture medium of Pseudomonas soecies pyridine derivatives were isolated which contain the (methoxythio)carbonyl (-CO-S-OCH,) qroup thus far not described in literature. From the culture medium of Pseudomonas putida and of two related species' after treatment with CH2N2 pyridine derivatives have been isolated which contain the -CO-SOCH qroup (the monothio analoq of a peracid ester) thus far not described in the literature, uiz. 6-[(methoxythio)carbonyl]pyridine-2-monothiocarboxylic acid S-methyl ester (!), 6-[(methoxythio)carbonyl]pyridine-2-carboxylic acid methyl ester(g), and 2,6-di[(methoxythio)carbonyl]pyridine (2). 2 has an elemental composition (by exact mass measurement) of CgH9N03S2. The fraqmentation pattern very much resembles that of pyridine-2,6-di(monothiocarboxylic acid) di-S-methyl ester3 (4); in contrast to 4, however, [M - 'SOCH3]+ (m/z 180) and [M - 'COSOCH3]+ (m/z 152) (instead of [M - *SCH3]+ and [M - 'COSCH3]+ is observed. Below m/z 152 the mass spectra of 1 and $ - except for slipht differences in the relative intensities - are identical. This demonstrates the presence of a COSCH3 group in 2-position (loss of the C-6 substituent from 1 as well as from 4 leads to ions of identical structure which fraament by very characteristic rearranqement processes of the remaininq COSCH3 qrouo;3 see also below). The 'H-NMR spectrum (CD2C12) exhibits between 7.99 and 8.20 ppm a 3 H pattern typical for pyridine derivatives 2,6-disubstìtuted with non-identical qroups,4 and 3 H sinqlets at 2.46 (COSCH3) and 3.87 ppm (OCH3) which excludes a methyl sulfoxide (CH3-S=O) structure requirin9 a methyl siqnal between 2.6 and 2.8 ppm. 5 The presente of a thiocarbonyl qroup can be excluded by the UV spectrum (Xmax 3,6 ' 286 nm, 20% hexane in CHC13) as C=S is manifested by an absorption at Q 420 nm, while the oresence of a second carbonyl band at 1696 cm-' (in addition to one at 1675 cm -1 characteristic3 for the COSCH3 group) suqgests for the second functional qroup a thioester-like structure. The presente of an S-O-C unit is finally corroborated by stronq IR absorptions7 at 984 and 913 cm -1 accompanied by a less -1 intense oair at 741 and 729 cm . 2 and 3 which accompany 1 only in minute quantities were identified essentially from their mass spectra: 2 (M+ C9H9NO4S) shows - in the same way as 1 - loss of 'SOCH (m/z 164) and of 'COSOCH (m/z 136), but starting from the latter it-fragments typically for a COOCH3 (m/z 136 - 'OCH3; m/z 136 - CO2; mlz 136 - CO; +COOCH3) rather than for a COSCH3 group.3 2 (M+ C9H9N04S2) also loses 'SOCH (m/z 196) and 'COSOCH (m/z 168). Loss of the second COSOCH group then occurs by rearranqement processes analogous to those observed for the further deqradation of [M - 'COSCH3]+ from 2 as, e.g., loss of CO from m/z 168 followed by that of CH20 (m/z 110) or CH30' (m/z '09).3 Treatment of the culture medium with CH3CHN2 yields the corresponding ethyl esters. From this observation it follows that the free acids rather than esters are the genuine metabolites. Their biogenetic significance lies in their intermediacy in the biosynthesis of pyridine-2,6-di(monothiocarbocylic acid) from pyridine-2,6 dicarboxylic acid -COOH + -COSOH + -COSH as was learned from feeding experiments with labelled precursors. 8