Borate(1-), tetrafluoro-, hydrogen, compd. with (2R-(2.alpha.,3.beta.,3.alpha.beta.,9.alpha.beta.))-2-[(2,2-dimethyl-1-oxopropoxy)methyl]-2,3,3a,9a-tetrahydro-6-imino-6H-furo[2',3':4,5]oxazolo[3,2-a]pyrimidin-3-yl-2,2-dimethylpropanoate (1:1) NSC300277