6H,11H-1,3,5,2,4-Trioxadisilocino[6'',7'':4',5']furo [2',3':4,5]oxazolo[3,2-a]pyrimidin-11-one, 6a,7a,13a,13b-tetrahydro- 7a-(.alpha.-hydroxy-.alpha.-methoxymethyl)- 2,2,4,4-tetrakis(methylethyl)-, stereoisomer NSC680837