| 摘要 |
The invention provides a compound of formula I, <IMAGE> (I) wherein R is hydrogen, alkyl of 1-6 carbon atoms, or COR3 where R3 is hydrogen, alkyl of 1-5 carbon atoms, alkenyl of 2 to 5 carbon atoms, alkynyl of 2 to 5 carbon atoms, or cycloalkyl of 3 to 5 carbon atoms, R1 is hydrogen or alkyl of 1-6 carbon atoms, R2 is alkyl of 1-6 carbon atoms and R3 is as above defined.
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