NOVELTY - Making entecavir comprises converting protected (1R,3S,4R)-4-hydroxymethyl-5-methylene-cyclopentane-1,3-diol compounds (V) to entecavir.    USE - The process is useful for making entecavir, which is used as antiviral drug. No biological data given.    ADVANTAGE - The process: economically provides high grade entecavir products suitable for human consumption; provides entecavir in high overall yield; allows use of general synthetic routes for protecting all groups; utilizes crystallization to obtain high quality intermediates, thus convenient, efficient, environmentally friendly, time saving even at large scale; and utilizes six overall synthetic steps, whereas eight steps are used in conventional process.    DETAILED DESCRIPTION - Making entecavir comprises converting protected (1R,3S,4R)-4-hydroxymethyl-5-methylene-cyclopentane-1,3-diol compounds of formula (V) to entecavir.    either PG = protecting group; or    2PG = 6 or 7 member cyclyl (optionally substituted).    INDEPENDENT CLAIMS are also included for:    (1) preparing substituted cyclic compounds of formula (VII) comprising oxidizing substituted cyclopentanol compounds of formula (VIII) to provide an intermediate substituted formic acid compounds of formula of formula (II-a), and converting (II-a) to (VII);    (2) protected formic acid (1R,3R,4S)-4-hydroxy-3-hydroxymethyl-2-methylene-cyclopentyl ester compounds of formula (I-b); and    (3) protected (3aS,4R,5S,6aR)-5-hydroxy-4-(hydroxymethyl)hexahydro-2H-cyclopenta(b)furan-2-one compounds of formula (I), protected (3aS,4R,5S,6aR)-4-(hydroxymethyl)hexahydro-2H-cyclopenta(b)furan-2,5-diol compounds of formula (II) and protected (1R,3S,4R)-4-hydroxymethyl-5-methylene-cyclopentane-1,3-diol compounds of formula (V-b).    either R4, R5 = hydroxy protecting group; or    R4R5 = 6 or 7 membered cyclyl (optionally substituted); either    2PG1 = 6 or 7 membered cyclyl (optionally substituted); or    PG1 = hydroxy protecting group including triphenylmethyl (optionally substituted), tert-butyldimethylsilyl, alkoxymethyl or arylalkoxymethyl; and    n = 1-3.