Kinetic evidence for the coexistence of zwitterionic (T) , neutral (T) and anionic (T) intermediates during rearrangement of S -(2-oxotetrahydrofuran-3-yl)- N -(4-methoxyphenyl) isothiuronium Bromide to 5-(2-Hydroxyethyl)-2-(4-methoxyphenylimino)-1,3- thiazolidin-4-one