(4AR,6R,7R,7AS)-6-(6-AMINO-8-BROMO-9H-PURIN-9-YL)-2-[(4,5-DIMETHOXY-2-NITROBENZYL)OXY]TETRAHYDRO-4H-FURO[3,2-D][1,3,2]DIOXAPHOSPHININ-7-OL 2-OXIDE