There is provided a process for the preparation of a compound of formula III, comprising the step of oxidising the compound of formula 2b wherein n is 2； R 2a and R 2b are independently H or F； R 3 is 1-methylcyclopentylmethyl or 1-fluorocyclopentylmethyl； R 3' is CH 3 or F； R 4 is C 1 -C 6 alkyl, C 1 -C 6 haloalkyl or C 3 -C 6 cycloalkyl, wherein C 3 -C 6 cycloalkyl is optionally substituted with methyl, CF 3 or one or two fluoro. Compounds and processes used in the preparation of a compound of formula III are also provided.
