Otmar, Miroslav: [5-(adenin-9-YL)-5-deoxy-L-pentofuranosyl]phosphonates - a novel type of nucleotide analogs related to HPMPA. 2. Synthesis of L-ribo and L-xylo configurated derivatives by recyclization of diethyl (5-RS)-[1,2-O-isopropyl|idene-5-O-methanesulfonyl-D-pentofuranos-5-C-YL]phosphonates under acidic conditions