1 to r 5 a 1 and a 2 have the meanings given in the claims and in the description particularly the preparation of 7 8 dihydro 5h pteridin 6 one derivatives and intermediates thereof application_number US 74921207 A description the present invention solves the problem outlined above by the method of synthesising compounds of formula i described hereinafter the invention thus relates to a process for preparing compounds of general formula i r 1 denotes a group selected from the group consisting of chlorine fluorine bromine methanesulphonyl ethanesulphonyl trifluoromethanesulphonyl para toluenesulphonyl ch 3 s u2550 o u2014 and phenyls u2550 o u2014 r 2 denotes hydrogen or c 1 c 3 alkyl r 3 denotes hydrogen or a group selected from the group consisting of optionally substituted c 1 c 12 alkyl c 2 c 12 alkenyl c 2 c 12 alkynyl and c 6 c 14 aryl or a group selected from the group consisting of optionally substituted and or bridged c 3 c 12 cycloalkyl c 3 c 12 cycloalkenyl c 7 c 12 polycycloalkyl c 7 c 12 polycycloalkenyl c 5 c 12 spirocycloalkyl and saturated or unsaturated c 3 c 12 heterocycloalkyl which contains 1 to 2 heteroatoms r 4 r 5 which may be identical or different denote hydrogen or optionally substituted c 1 c 6 alkyl or r 4 and r 5 together denote a 2 to 5 membered alkyl bridge which may contain 1 to 2 heteroatoms or r 4 and r 3 or r.