Unambiguous syntheses of four unreported monomethylated derivatives of 3-phenyldihydro-1,2,4-triazin-6(1H)-ones, namely, the 1-methyl (2), 2-methyl (3), 4-methyl (4) and the imidic O-methyl derivative (5), are described. Regioselectivity was achieved for the synthesis of (2) by addition of ethyl glycinate to the 1,3-dipolar nitrile imine derived from N- methylbenzohydrazonoyl bromide hydrobromide (8). The key step for the synthesis of (3) was addition of benzyl 3-methylcarbazate (14) to ethyl N-[ chloro (phenyl) methylene ] glycinate (15b). The 4-methyl compound (4) was prepared by cycloaddition of ethyl N-( thiobenzoyl ) sarcosinate (21) with hydrazine hydrate, and the O-methyl compound (5) was prepared by reaction of sodium methoxide with 6-chloro-3-phenyl-4,5-dihydro-1,2,4-triazine (23).
Novel syntheses of 4,5-dihydro-1,2,4-triazin-6(1H)-ones were developed by use of imidoyl chlorides. The overall yield of 4,5-dihydro-1,2,4-triazin-6(1H)-ones prepared from amino acid imidates and hydrazines was improved by the development of a much more efficient synthesis of the imidates. The new 1,2-, 1,4- and 5,5-dimethyl dihydro-1,2,4-triazin-6(1H)-ones have been synthesized by cyclocondensation/cycloaddition pathways. Base-catalysed methylation of 3-phenyl-4,5-dihydro-1,2,4-triazin-6(1H)-one (15) gave the 1- methylated derivative (8); under similar conditions 2-methyl-3-phenyl-2,5-dihydro-1,2,4-triazin-6(1H)-one (9) afforded the 1,2-dimethyl derivative (7).