| http://www.w3.org/ns/prov#value | - The present invention provides a compound of formula ##STR2## or a pharmaceutically acceptable salt thereof where A1 is selected from alkenylene of one to twelve carbon atoms and cycloalkenylene of three to eight carbon atoms, A2 is a valence bond or is selected from alkenylene of one to twelve carbon atoms and cycloalkenylene of three to eight carbon atoms, and R1 is selected from (a) hydrogen, (
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