6,7-Dimethoxy-16,4a-dihydro-1H,2H,3H,13H,4aH-1,3-oxazaperhydroino[3'',2''-6',5']phenanthridino[4',3'-2,1]benzo[4,5-d]1,3-dioxolane AIDS-108187 AIDS108187 Nitidine cyclic analog