NSC696552 [6aR-(6a.alpha.,7a.alpha.,13a.alpha.,13b.beta.)]-6a,7a,13a, 13b-Tetrahydro-7a-(hydroxymethyl)-2,2,4,4-tetrakis(1-methyl ethyl)-6H,11H-1,3,5,2,4-trioxadisilocino-[6,7:4',5']furo [2',3':4,5]oxazolo[3,2-a]pyrimidin-11-one