Recent communications from these laboratories have described the isolation and structure elucidation of the olivanic acid derivatives MM 13902 (1), MM 4550 (2) and MM 17880 (3). Together with thienamycin (4) and PS-5(5) these metabolites comprise a family of naturally occurring antibiotics containing the 7-oxo-1-azabicyclo[3.2.0]hept-2-ene ring system (6). We now report the structures of four new members of this family, MM 22380 (7), MM 22381 (8), MM 22382 (9) and MM 22383 (10), isolated from Streptomyces olivaceus. The antibiotics were all isolated as freezedried mono-sodium salts (11)-(14), each of which showed a β-lactam carbonyl absorption at 1750 cm-1 in the i.r. (KBr) spectrum. The u.v. spectra of MM 22380 and MM 22381 (Table 1) contained the characteristic long-wavelength chromophore of other olivanic acids as typified by MM 17880 (3), whilst the additional lowerwavelength absorption observed in the spectra of MM 22382 and MM 22383 was similar to that present in MM 13902 (1). The 1H n.m.r. spectra (D2O) of the sodium salts of MM 22382 (13) and MM 22383 (14) revealed the presence of a trans, disubstituted double bond (J=14 Hz) which was absent in MM 22380 and MM 22381. Alkylation of each of the sodium salts with p-nitrobenzyl bromide in dimethylformamide afforded the p-nitrobenzyl esters [(15), m.p. 166-168℃], [(16), m.p. 163-165℃], [(17), m.p. 175-177℃] and [(18), m.p. 191-193℃]. Field-desorption mass spectral studies indicated that the p-nitrobenzyl esters of MM 22382 (17) and MM 22383 (18) both possessed a molecular weight of 447, whilst the corresponding esters of MM 22380 (15) and MM 22381 (16) each contained two additional hydrogen atoms (m/e, 449). These results, combined with analytical data on the p-nitrobenzyl esters, allowed a molecular formula of C13H18N2O5S to be assigned to both MM 22380 and MM 22381, and C13H16N2O5S to MM 22382 and MM 22383. It was apparent from the above evidence that the four new compounds consisted of two pairs of isomers, one pair related to MM 13902 (1) having the E-2-acetamidoethenylthio side-chain, and the other pair related to MM 17880 (3) with the corresponding saturated side-chain. Furthermore, from the molecular composition of the new metabolites it appeared that the sulphate moiety of MM 13902 and MM 17880 was absent and replaced by a hydroxyl-function. Further confirmation of this came from the i.r. spectra (KBr) of the p-nitrobenzyl esters (15)-(18), all of which lacked the characteristic sulphate absorption (1220-1270 cm-1), and the corresponding 1H n.m.r. spectra [(CD3)2NCDO] which all exhibited an exchangeable hydroxyl proton. The gross structure of MM 22380 and MM 22381 was thus established as 3-(2-acetamidoethylthio)-6-(1-hydroxyethyl)-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid and that of MM 22382 and MM 22383 as 3-[(E)-2-acetamidoethenylthio]-6-(1-hydroxyethyl)-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid. The configuration of the β-lactam hydrogen atoms in MM 13902 (1), MM 4550 (2) and MM 17880 (3) was determined to be cis by virtue of the observed coupling constants (J5,6=5.5-6.0 Hz) in the n.m.r. spectra. The smaller coupling (J5,6=3 Hz) in thienamycin and its derivatives is characteristic of a trans β-lactam configuration. It was apparent from the 1H n.m.r. spectra of the sodium salts (11)-(14) (D2O) and the corresponding esters (15)-(18) [(CD3)2NCDO] that the β-lactam protons in MM 22380 and MM 22382 possessed the larger coupling (J5,6=5.5-6.0 Hz), whereas the smaller coupling (J5,6=3 Hz) was observed in MM 22381 and MM 22383. MM 22380 (7) and MM 22382 (9) therefore contain a cis-oriented β-lactam and have the 5R, 6R-configuration, and MM 22381 (8) and MM 22383 (10) each possess a trans β-lactam with the 5R, 6S-configuration. X-Ray analysis of the p-nitrobenzyl ester of MM 22383 (18) confirmed the gross structure and established the configuration at C-8 to be S, the same as MM 13902, MM 4550 and MM 17880 but opposite to that of thienamycin. MM 22380, MM 22381 and MM 22382 would therefore also be expected to possess the 8S configuration and this was subsequently confirmed by chemical means (to be reported in a future paper). MM 22380 (7) and MM 22382 (9) are therefore desulphonated analogues of MM 17880 (3) and MM 13902 (1), respectively; MM 22381 (8) and MM 22383 (10) in contrast both possess a trans β-lactam as in thienamycin (4) but differ from the latter by virtue of the opposite configuration at C-8. The epithienamycins have similar gross structures to the olivanic acid derivatives (7)-(10) but their stereochemistry has not been described in detail. Compounds (7)-(10) all exhibit a broad spectrum of antibacterial activity and also possess β-lactamase inhibitory properties.