Journal of Medicinal Chemistry 0 Copyright 1979 by the American Chemical Society
December 1979
Volume 22, Number 12
Communications to the Editor N-Acetimidoyl- and N-Formimidoylthienamycin Derivatives: Antipseudomonal P-Lactam Antibiotics
Table I. Solution and Solid-state Stabilitya
t . , . h.b at 23 "C
Sir:
no. -1 2 2(cryst)
Thienamycin (1)is a recently discovered p-lactam anti-
3
concn, mg/mL 60 60
lod
60
deionized H,O 0.5 2.5 17 5.0
1M PO,, pH7 0.5 0.5 0.5
solid,c month
l(80) 6(80)
6 (100) 5(100)
Based o n ",OH extinguished UV absorbance (ref 2). t , , = time at which 90% of initial UV absorbance Kept over Drierite a t room temperature; remains. Buffered with 0.1 amount remaining at given time. equiv of NaHCO,. [I
1, R = H 2, R = CH=NH 3, R = C(CH,)=NH
biotic of unique structure and exceptional breadth of antibacterial activity. Of particular interest is its exceptional potency against Pseudomonas s p p . and its 0-lactamase stability.' The novel carbapenem nucleus and aminoethylthio side chain of thienamycin, however, contribute to its chemical instability. For example, in comparing the stability of thienamycin to benzylpenicillin, comparable stability exists only between pH 2 and 5. Near pH 7, the most stable pH for thienamycin, benzylpenicillin is more stable. Also, the stability of thienamycin is inversely related to its concentration. This latter characteristic makes thienamycin less attractive for use as a parenterally administered drug. It has been postulated that the concentration-dependent instability of thienamycin is due to the intermolecular aminolysis of the azetidinone by the cysteamine side chain.2 Derivatization of the amino group of thienamycin to a less nucleophilic species seems an attractive route to more stable thienamycin analogues. Since the naturally occurring N-acetyl derivative3 has greatly diminished antipseudomonal activity, retention of antipseudomonal activity appeared to require a basic functionality. I t was therefore considered likely that conversion of the amine to a stronger base would, by virtue of its existence to a greater extent in a protonated form, result in a compound with increased stability in concentrated solution as well as high antipseudomonal activity. We also hoped to obtain a crystalline derivative which would facilitate purification. We now report that the N-acetimidoyl(3)and particularly the N-formimidoyl (2) derivatives fulfilled these expectations. The amidines are readily prepared by the reaction of thienamycin with an imidate ester in aqueous solution at pH 8.2. The reaction mixtures were adjusted to pH 7 and chromatographed at 4 OC on a column of Dowex 50 X 4 (Na+ cycle, 200-400 mesh) resin eluted with deionized water. The amidines are retarded relative to thienamycin
and are obtained as amorphous powders on lyophilization of the appropriate fractions. Thus, the reaction of 1 with methyl formimidate gave 2 (45% yield): UV (HzO) A, 298 nm (E 8040,9970 NH20H ext); IR (Nujol mull) 1767 cm-' (P-lactam); NMR (D20, 100 MHz) 6 1.30 (d, J = 6 Hz,CH,), 2.7-3.6 (m, 4,CHz,SCH2),3.40 (dd, J = 3 and 6 Hz, H-6), 3.5-3.7 (m, 2, NCHz), 4.1-4.4 (m, 2, CHOH, H-5), 7.81 (9, 1, CHNH). Compound 2 was crystallized from water-ethanol, giving a monohydrate: UV (HzO)A, 299 nm (E 9670, 98% NH20H ext); [ c Y ] ~ ~+86.8" D (c 0.05, 0.1 M PO4,pH 7). Anal. (C12H17N3S04) C, H, N, S. The reaction of 1 with ethyl acetimidate yielded 3 (57%): UV (HzO) ,A, 298 nm (E 8,350, 99% NHzOH ext); IR (Nujol mull) 1774 cm-' (P-lactam);NMR similar to 2 except 6 2.24 (s,3, CH,G=NH) replacing 6 7.81; [CY]%D +74.6' (c 0.05,O.l M PO4,pH 7). The purity of the lyophilized amidines as measured by hydroxylamine extinguished UV absorbance compares favorably with that of thienamycin (e 7900,95% NHzOH ext). Additionally high-performance LC4 analyses show 2 to be 97% and 3 >99% single components. All lyophilized samples of 2 contain 2-3% of 1, indicating that some hydrolysis of the formamidine occurs during isolation. Crystalline 2 contained
