2-Methoxyhexahydrocyclopenta[d][1,3,2]dioxaphosphinine AIDS-133862 AIDS133862 NSC630691 {3-.Alpha.-Methoxy-trans-2,4-dioxa-3-phosphabicyclo[4.3.0]} nonane mixture with 3-.beta.-Methoxy-trans-2,4-dioxa-3- {phosphabicyclo[4.3.0]nonane}