prov:value
| - The present compounds (1) and their salts are prepared, for example, by the following reaction scheme: ##STR3## wherein Z is an atom or a group which can be eliminated, such as a halogen atom, a methane sulfonyloxy group, and a paratoluene sulfonyloxy group, and R1, R2, R3, R4, A, and the broken line have the same meaning as defined above.
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