The present invention relates to a compound of the formula (I), pharmaceutically acceptable salts, hydrolysable esters, isomers and hydrates thereof, and hydrates of the said esters or salts, wherein R
1
, R
2
, R
3
and
are as defined in the description. The present invention further relates to a process for preparing the compounds, to pharmaceutical compositions comprising the compounds, and to the use of the compounds in the manufacture of a medicament for the treatment and/or prophylaxis of infectious diseases.