| 摘要 |
A compound of the formula : <CHEM> wherein R<1> is alkyl, ar(lower)alkyl which may have suitable substituent(s), or heterocyclic(lower)alkyl, R<2> is hydrogen or an acyl group, and R<3> is carboxy(lower)alkyl, protected carboxy(lower)alkyl, carboxyaryl or protected carboxyaryl, and a pharmaceutically acceptable salt thereof, processes for their preparation and pharmaceutical compositions comprising them or a pharmaceutically acceptable salt thereof in admixture with pharmaceutically acceptable carriers. |
