(5-(Adenin-9-yl)-5-deoxy-L-pentofuranosyl)phosphonates - A Novel Type of Nucleotide Analogues Related to HPMPA. Part 2. Synthesis of L-Ribo and L-Xylo Configurated Derivatives by Recyclization of Diethyl (5-RS)- (1,2-O-Isopropylidene-