Abstract3‐Benzoyl‐thiocarbazic acid O‐methyl ester7aand 3‐benzoyl‐dithiocarbazic acid methyl ester7bare methylated, in presence of alkali, at the SH group of1a, the ene‐thiol tautomer of1, to give the 3‐benzoyl‐isothiocarbazic acid O, S‐dimethyl ester8aand the 3‐benzoyl‐isodithiocarbazic acid dimethyl ester8bresp., which clearly differ from the N‐methylated compounds11aand12aor11band12bprepared from the two N‐methyl benzohydrazides9and10resp. (melting points, thin‐layer chromatography and NMR. spectra).The previously reported [1] ring closure of 3‐(ω‐chloroalkanoyl)‐thiocarbazic acid O‐alkyl esters and ‐dithiocarbazic acid alkyl esters can be interpretated as an intramolecular auto‐alkylation of CONHNHCSYR: the 3‐(chloroacetyl)‐compounds2areS‐alkylated(enethiol form) to the six‐membered thiadiazinones4, but the 3‐(3‐chloropropionyl)‐compounds3areN‐alkylatedto the pyrazolidinones6, the five‐membered ring being preferred in the latter case to the seven‐membered ring5which would be formed by S‐alkylation. Hence the position of alkylating attack depends on the size of the ring to be formed.As a consequence, starting from 3‐(4‐chlorobutyryl) compounds13, an alternative between the two N‐alkylation products14(five‐membered ring) and15(six‐membered ring) would be expected. On the contrary however, a combination of 1,3,4‐thiadiazole ring closure and chlorine elimination predominates, giving the 5‐(3‐hydroxypropyl)‐1,3,4‐thiadiazoles16. This reaction may proceedviathe butyrolacto