or c 4 c 14 aryl or osir 9 r 10 r 11 r where r 9 r 10 and r 11 are preferably each independently c 1 c 12 alkyl or c 4 c 14 aryl r 1 and r 2 are particularly preferably each independently hydrogen tert butoxy trityloxy tert butyldimethylsilyloxy tert butyldiphenylsilyloxy trimethylsilyloxy triethylsilyloxy triisopropylsilyloxy neopentoxy or 1 adamantoxy for the purposes of the invention preference is in each case given to those compounds of the formula i in which r 1 and r 2 are identical r 3 and r 4 are preferably each independently r 12 or 13 or nr 14 r 15 where r 12 r 13 r 14 and r 15 are each independently c 1 c 12 alkyl or c 4 c 14 aryl or nr 14 r 15 together is a cyclic amino radical having 4 to 12 carbon atoms for example pyrrolidinyl or piperidinyl or r 3 and r 4 together are u2014 o u2014 r 16 u2014 o u2014 where r 16 is ethylene 1 2 phenylene 1 3 phenylene 1 2 cyclohexylene 1 1 u2032 ferrocenylene di or tetra c 1 c 8 alkyl substituted 1 1 u2032 diphenyl 2 2 u2032 methylene diyl 1 2 ferrocenylene 2 2 u2032 1 1 u2032 binaphthylene or 2 2 u2032 1.