Patent ID: 7727956

Claim:
A compound of formula in which R 1 is C 1 -C 2 -alkyl, whereby C 1 -C 2 -alkyl is substituted with a substituent selected from the group consisting of trimethylsilyl, alkoxycarbonyl, C 3 -C 6 -cycloalkyl, alkoxy-substituted phenyl, 2-pyridyl and 3-pyridyl, or R 1 is C 4 -C 8 -alkyl, R 2 is hydrogen or C 1 -C 4 -alkyl, R 3 is C 1 -C 2 -alkyl, whereby C 1 -C 2 -alkyl is substituted with a substituent trimethylsilyl, or R 3 is C 4 -C 6 -alkyl, R 4 is hydrogen or methyl, or one of its salts.