benzyl (2S,2'S,4R,9a'R)-4-(2-((R)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-3-(phenylselanyl)propanamido)benzamido)-2'-methyl-3',5-dioxo-2',3',4,5-tetrahydro-3H-spiro[furan-2,9'-imidazo[1,2-a]indole]-1'(9a'H)-carboxylate