The synthesis of 2,4-dichloro-5-(1-o-carboranylmethyl)-6-methylpyrimidine is described. Synthetically, this novel o-carboranyl pyrimidine is approached from ethyl 2-acetyl-4-pentynoate through alkylation of ethyl acetoacetate with propargyl bromide and subsequent condensation with thiourea to yield 6-methyl-5-(2-propynyl)-2-thio-4(1H,3H-pyrimidinone. Hydrolysis of the 2-thione and chlorination with POCl3 gives 2,4-dichloro-6-methyl-5-(2-propynyl)pyrimidine. Gentle refluxing of this product with B10H14/CH3CN in toluene yields the target, 2,4-dichloro-5-(1-o-carboranylmethyl)-6-methylpyrimidine, a potential synthon for a variety of 2,4-substituted 5-(1-o-carboranylmethyl)-6-methylpyrimidines and/or the corresponding nido-undecaborates. © 1991, American Chemical Society. All rights reserved.