The regioselective syntheses of 3-alkyl(aryl)-2-(anthracen-9'-ylimino)-1,3-thiazolidin-4-ones (2) and 2-alkyl(aryl)imino-3-(anthracen-9'-yl)-1,3-thiazolidin-4-ones (3) from N-(anthracen-9-yl)-N'-alkyl(aryl)thioureas were accomplished effectively using methyl bromoacetate and bromoacetyl bromide, respectively. Detailed structural characteristics were confirmed mainly by NMR techniques. The mass spectrometric behavior of the resulting sets of compounds of known structures was shown to be characteristic for each set. Some interesting fragmentation pathways involving the transfer and rearrangements of various moieties were also revealed, as well as regioisomerization for particular substituent-specific fragmentations.

The synthesis of regioisomeric 3-alkyl(aryl)-2-(acridin-9'-yl)imino-1,3-thiazolidin-4-ones (8b-i) and 2-alkyl(aryl)imino-3-(acridin-9'-yl)-1,3-thiazolidin-4-ones (11a-i) was performed by the reaction of 3-(acridin-9-yl)-1-alkyl(aryl)thioureas 5a-i with methyl bromoacetate and bromoacetyl bromide, respectively, via the corresponding isothiourea hydrobromides with excellent regioselectivity. The structure, NMR spectra and mass spectrometric behavior of the resulting compounds are discussed.J. Heterocyclic Chem., 42, 907 (2005).Introduction.

The mass spectrometric behaviour under electron ionization of several 3,4-(alkyl/aryl)-disubstituted 1,2,4-oxadiazole-5(4H)-ones (1-13) and 1,2,4-thiadiazole-5(4H)-thiones (14-17), and that of 3-aryl-5-alkyl- or arylthio-1,2,4-thiadiazoles (18-24), was studied. These five-membered rings split similarly to the corresponding 1,2,4-thiadiazole-5(4H)-ones, although substitution has also a clear effect on the routes of fragmentation and the magnitude of secondary processes. In particular, the fragmentation of 1,2,4-oxadiazole-5(4H)-ones (1-6), which do not bear aromatic substituents, was, in addition to the ring ruptures, fairly complicated. The other compounds fragmented more systematically and relatively few unpredictable fragmentations occurred.

The fragmentation behaviour of ten 3,4-disubstituted 1,2,4-oxadiazole-5(4H)-thiones and seven 3,4-disubstituted 1,2,4-thiadiazole-5(4H)-ones studied here confirmed the earlier observations about the partial rearrangement of the former after ionization into the latter before further fragmentation. In the case of eight 3-(substituted phenyl)-4-(p-tolyl)-1,2,4-oxadiazole-5(4H)-thiones the fragmentations reflecting the above-mentioned molecular ion rearrangement show a clear correlation on the substituent sigma values. The electron-withdrawing substituents destabilize the molecular ion, so higher amounts of the rearranged ion [R(1)NCO](+.) are obtained. A good correlation of log[R(1)NCO](+) against sigma was obtained (r = 0.96). Only a satisfactory correlation prevailed for log([R(1)NCO](+)*/[R(1)NCS](+)*) against sigma(r = 0.87).

The reactions of ethyl 5‐oxotricyclo[4.3.1.13,8]undecane‐4‐carboxylate (2) with methyl‐substituted 2‐aminopyridines in polyphosphoric acid (PPA) gave two products, linearly‐condensed pyridopyrimidinones 4a‐c and 2‐pyridylcarboxamides 5a‐c, whereas the reactions with amino, hydroxy and nitro derivatives of 2‐aminopyridine furnished only linearly‐condensed pyridopyrimidinones (4g‐j). Use of a mixture of PPA and phosphorus oxychloride as solvent afforded both linearly‐ (4a‐c,e,f) and angularly‐condensed (6a–c,e,f) pyridopyrimidinones. In toluene, with p‐toluenesulfonic acid as catalyst, 2‐pyridylcarboxamides 5a‐f were obtained. In a mixture of PPA and phosphorus oxychloride at 80–120 °C, 5a‐f yielded angularly‐condensed pyridopyrimidinones 6a‐f. All the products exhibited characteristic features, as determined by NMR and electron ionization mass spectrometry and X‐ray crystallography.