<5-(ADENIN-9-YL)-5-DEOXY-L-PENTOFURANOSYL>PHOSPHONATES - A NOVEL TYPE OF NUCLEOTIDE ANALOGS RELATED TO HPMPA. II. SYNTHESIS OF L-ribo and L-xylo CONFIGURATED DERIVATIVES BY RECYCLIZATION OF DIETHYL (5-RS)-<1,2-O-ISOPROPYLIDENE-5-O-METHANESULFONYL-D-PENTOFURANOS-5-C-YL>PHOSPHONATES ...