Direct diastereoselective addition of l-menthol to activated 1,2,4-triazin-5(4H)-one

For the first time in a triazine series it has been found that addition of a chiral O-nucleophile. l-menthol, to the C-6-unsubstituted atom of 3-pheny l-1,2,4-triazin-5(4H)-one 1 activated by aliphatic acid anhydrides proceeds diastereoselectively to form a mixture of 1-acyl-6-[(1 'R,3'R,4'S)-menthyl -3')]-3-phenyl-(6S)-1,6-dihydro-1,2,4-triazin-5(4H)-ones 2 and 1-acyl-6-[(1 'R,3'R, 4'S)-menthyl-3')]-3-phenyl-(6R)-1,6-dihydro-1,2,4-triazin-5(4H)-one s 3 in which the diastereomers 2 predominate. The diastereoselectivity of t he process improves as the size of the N-1-acyl substituent increases. (C) 2001 Published by Elsevier Science Ltd.