Synthetic and spectroscopic study of 5‐amino‐2‐acyl‐1,2,4‐thiadiazolin‐3‐ones

5‐Amino‐2‐acyl‐1,2,4‐thiadiazolin‐3‐ones 2–1 can be synthesized from 5‐amino‐2 H ‐1,2,4‐thiadiazolin‐3‐one ( 1–1 ) via a selective acylation with an acid anhydride in pyridine. The 1 H nmr spectral characteristics of 5‐amino‐2‐acyl‐1,2,4‐thiadiazolin‐3‐ones 2–1 is in particular, compared with 5‐amino‐2 H ‐1,2,4‐thiadiazolin‐3‐one ( 1–1 ) and 5‐amino‐2‐alkyl‐1,2,4‐thiadiazolin‐3‐ones 1–2, 1–3 . The 5‐amino group of 2–1 appeared as two peaks in its 1 H nmr spectrum, which merged to a single peak at a higher temperature, while those of compound 1–1, 1–2 and 1–3 appear only as a single peak. The restricted rotation of the C(5)‐N(5) (at amino) bond of 5‐amino‐2‐acetyl‐1,2,4‐thiadiazolin‐3‐one (2a‐1) is about 14.5 Kcal/mol.