(3R,3aS,5R,6aS)-5-methylhexahydrofuro[2,3-b]furan-3-yl {(1S,2R)-1-benzyl-3-[(3S,4aS,8aS)-3-[(tert-butylamino)carbonyl]octahydroisoquinolin-2(1H)-yl]-2-hydroxypropyl}carbamate AIDS-028271 AIDS028271 IsoquinCON Thf-Thf Urethane analog