Abstract: The subject invention involves novel acyl isothiocyanate resins of formula (I) and processes for making them: (a) starting with a phenyl carboxy resin, treating the resin with any reagent that converts the carboxy to an acyl halide, followed by treatment with tetranikylammonium thiocyanate or an alkali metal salt thereof, to provide the acyl isothiocyanate resin. The subject invention also involves processes for making guanidine compounds and related cyclized compounds using an acyl isothiocyanate resin, comprising the following steps: (b) reacting the acyl isothiocyanate resin with a primary amine; (c) reacting the product from Step (b) with a sulfur activating agent and ammonia or a primary or secondary amine; (d) treating the product from Step (c) with a strong base or medium strength acid to cleave product from the resin, providing the guanidine compound or/and the related cyclized compound.