PROCESS FOR THE PREPARATION OF (1S,4R)-OR(1R,4S)-4-(2-AMINO-6-CHLORO-9H-PURIN-9-YL)-2-CYCLOPENTENE-1-METHANOL

A novel process for the preparation of (1S,4R)- or (1R,4S)-4-(2-amino-6-chloro-9H-purin-9-yl)-2-cyclo- pentene-1-methanol of the formula: is described. This entails in a first stage acylating (~)- 2-azabicyclo[2.2.1]hept-5-en-3-one of the formula: to give a (~)-2-azabicyclo[2.2.1]hept-5-en-3-one derivative of the general formula: in which R1 denotes C1-4-alkyl, C1-4-alkoxy, aryl or aryloxy, the latter being reduced in a second stage to give a cyclopentene derivative of the general formula: in which R1 has the stated meaning. The cyclopentene derivative V is then converted in a third stage biotechnologically into a (1R,4S)- or (1S,4R)-1-amino-4- (hydroxymethyl)-2-cyclopentene of the formula: the latter being converted in a fourth stage with N-(2- amino-4,6-dichloro-5-pyrimidinyl)formamide of the formula: into a (1S,4R)- or (1R,4S)-4-[(2-amino-6-chloro-5- formamido-4-pyrimidinyl)amino]-2-cyclopentene-1-methanol of the formula: Finally, the compound of forumula IX or X is cyclized in a fifth stage in a known manner to form the final product of formula I or II.