ternary ammonium salt), or a 3-N-carbamoyl protected derivative thereof, with a compound of formula (III) ##STR4## (wherein R, R.sub.1, R.sub.2 andR.sub.3 are as defined above; and X is a leaving group, e.g. a halogen atom such as chlorine, bromine or iodine or an acyloxy group, e.g. a hydrocarbylsulphonyloxy group such as mesyloxy or tosyloxy, or a haloalkanoyloxy group such as a dichloroacetoxyg