The present invention provides a process for the preparation of a compound of formula (VI), or a salt or derivative thereof, wherein R1 and R2 are independently C1-8 alkyl and * represents a stereocentre.本發明提供一種用於製備式(VI)化合物或其鹽或其衍生物的方法,其中R1及R2獨立地為C1-8烷基,且*表示立體中心。