Novel acid compounds

Penicillanic derivatives of the formula ##SPC4##

• wherein W is a group of the formula ##EQU16## wherein U is --OE, E is select-d from the group consisting of hydrogen, a non-toxic, pharmaceutically acceptable salt-forming cation and a conventional, non-toxic, pharmaceutically acceptable ester-forming group which is known to improve the absorption characteristics of penicillanic acid derivatives after oral administration to warm-blooded animals and Y is a group of the formula ##EQU17## wherein X and R are the same or different and each is selected from the group consisting of oxygen and sulphur, and Z.sup.1 and Z.sup.2 are the same or different and each is selected from the group consistng of (lower)-alkoxy, (lower)alkylthio, optionally substituted phenoxy, optionally substituted benzyloxy, optionally substituted amino, optionally substituted (lower) alkyl, optionally substituted phenyl, hydroxy and OM, said substitutents being selected from the group consisting of halogen, nitro, cyano, lower alkyl, and lower alkoxy M is a non-toxic, pharmaceutically acceptable salt-forming cation, or Z.sup.1 and Z.sup.2 together with the phosphorous atom form an optionally unsaturated, 5 to 7 membered phosphorous and carbon heterocycle ring optionally substituted with at most four lower alkyl groups.