: N-[[2-[[(Diphenylmethoxy)carbonyl]amino]-6-oxo-1,6-dihydro-9H-purin-9-yl]acetyl]-N-[2-[[(9H-fluoren-9-ylmethoxy)carbonyl]amino]ethyl]glycine N-[2-[[[(9H-Fluoren-9-yl)methoxy]carbonyl]amino]ethyl]-N-[2-[2-[[(benzhydryloxy)carbonyl]amino]-6-oxo-1,6-dihydro-9H-purin-9-yl]acetyl]glycine