Treatment of 5-arylidene derivatives of 2-phenylimino-4-thiazolidinones with Grignard reagents does not open the hetero-ring, the double bond of the lateral chain, the CC bond and the CN bond of Ic, enter into reaction, yielding colourless products, believed to have structure III. On the other hand, 5-benzylidene-3-phenyl-2-phenylimino-4-thiazolidinone (Id) on treatment with Ph Mg Br undergoes addition