Quinazolines spectra

The 2- and 4-pyrimidinamines show two N—H stretching bands at 3400 and 3500 cm due to symmetric and unsymmetric vibrations imines absorb at 3300 cm . Interpretation of quinazoline spectra is also available <70JSP(36)310>. 

The UV spectrum of quinazoline is less simple bands at 220, 267 and 311 all represent -rr - -rr transitions and a characteristic inflexion of low intensity at 330 nm represents the TT transition (63PMH(2)l). Quinazoline as cation (17) is 3,4-hydrated covalently, so... 

The hydrochloride of (3) holds water rather tenaciously, and the infrared spectrum indicates that the water is covalently bound. Mild oxidation of the cation (3) gives 4-hydroxyquinazoline in high yield and ring-chain tautomerism is excluded on the grounds that quinazo-line does not give a positive aldehyde test in acid solution, 2-Methyl-quinazoline also has an anomalous cationic spectrum and a high basic strength (see Table I), but 2,4-dimethylquinazoline is normal in both these respects, which supports the view that abnormal cation formation entails attack on an unsubstituted 4-position. ... 

The stabilizing influence in the hydrated cation is the amidinium resonance. If a solution of the cation is neutralized, a short-lived hydrated neutral molecule (4) (half-life 9 sec at pH 10) is obtained with an ultraviolet spectrum similar to that of the hydrated cation but shifted to longer wavelengths (5 m/ ). Supporting evidence can be derived from the anhydrous nature of the cation of 4-nitroiso-quinoline (pK 1.35), in which the nitro group has a similar electronic influence to that of the ring nitrogen atom N-I in quinazoline and where amidinium resonance is not possible. 

As yet no quinazoline has been discovered which has any appreciable amount (say, 1%) of hydrated species in the neutral molecule,but several quinazolines were shown to contain a mixture of anhydrous and hydrated species in the cations. Anhydrous neutral molecules and anhydrous cations have an ultraviolet absorption spectrum of the general type C (Fig. 3) and hydrated cations, the type... 

The hydrated cation of quinazoline in dilute acid solution becomes dehydrated when the acidity of the solution is progressively increased. At Ho —4.3, the solution consists predominantly of the anhydrous cation with some anhydrous dication ( 7%). The ultraviolet spectrum of the anhydrous cation is similar to that of the neutral molecule (there is a small bathochromic shift) and it is also similar to that of quinazoline in anhydrous dichloroacetic acid. When the acid strength is further increased to Ho —9.4, the quinazoline dication is formed (pKa —5.5). 

Fig. 4. Ultraviolet spectrum of quinazoline 3-oxidc. Solid line, neutral molecule dotted line, cation,...

Quinazoline-2,4-dione (108) has been assigned the dioxo formulation on the basis of its infrared spectrum. A dioxo structure has also been demonstrated for the derivative 108a by ultraviolet spectral comparisons and infrared data. ... 

Anhydrous quinazoline hydrochloride absorbs one molecule of water readily, and. the product is difficult to dehydrate completely even in a high vacuum at 60°. Infrared spectral data suggest that this water is covalently bound because of (o) the absence of several bands in the spectrum of the hydrate which are present in the spectrum of the anhydrous hydrochloride and (6) the presence of extra bands at 1474 and 1240 cm that have been attributed to C— H and O— H bending vibrations of the — CHOH group. 

The anomalous behavior of quinazoline was first discovered by Albert et who made the surprising observation that 4-methyl-quinazoline 2.5) was a weaker base than quinazoline (pA 3.5). Mason then observed that the ultraviolet spectrum of the quinazoline cation was abnormal but that the spectrum of 4-methylquin-azoline was normal (see Fig. 2). These anomalies led to the suggestion that water adds covalently to the cation of quinazoline to give 12 (R = H). The occurrence and position of hydration were confirmed by a detailed study of the ultraviolet and infrared spectra of the anhydrous and hydrated hydrochlorides and by mild oxidation of the cation to 4(3 )-quinazolinone. Using the rapid-reaction technique (the continuous-flow method), the spectrum of the unstable... 

Most substituents (Q, Me, OMe) in the 2-position have only a small effect, if any, on the hydration of the quinazoline cation they are similar in this respect to substituents in the 5-, 6-, and 8-positions (see above). Although hydration in the 2-aminoquinazoline cation was at first considered absent,a closer examination of the entire spectra of both species indicated that the cation spectrum may be that of a mixture. Hydration in the cation has now been confirmed by the rapid-reaction technique (the stopped-flow method) which showed that the unstable hydrated neutral species had a half-life of 4.0 sec at 20° and pH 9.60. The 2-hydroxyquinazoline cation has not been studied, but... 

The cation of 4,4 -biquinazolinyl and its 2,2 -dimethyl derivative readily add water across the 3,4- and 3, 4 -double bonds, but the cation of 2,2 -biquinazolinyl is not hydrated. Hydration in the 4,4 -isomers has been attributed to restricted rotation about the 4,4 -bond, a steric effect which is relieved by hydration. The ultraviolet spectrum of 2,2 -biquinazolinyl (neutral species and cation) shows that there is considerable conjugation between the quinazoline groups. Covalent hydration is absent from the latter compound because it would otherwise destroy the extended conjugation present. 

The 2-methylthio-3-substituted quinazolin-4(3H)-one 3 was obtained by dissolving 4 in 2% alcoholic sodium hydroxide solution and methylating with dimethylsulfate whilst stirring at room temperature (yield 88%, m.p 124-126 C). The IR spectrum of 3 showed the disappearance of the amino (NH) and thioxo (CS) stretching signals of the starting materials. It showed a peak for carbonyl (CO) stretching at 1680 cm The NMR spectriun of com-... 

It should be also noted that, in a very recent publication, Liu and coworkers were successful in applying microwave radiation within their domino approach towards the synthesis of pyrrolo[2,l-fc]quinazoline alkaloids such as deoxyvasicinone, 8-hy-droxydeoxyvasicinone, mackinazolinone and isaindigotone, which exhibit a promising broad spectrum of biological activities. In the case of isaindigotone, the authors were able to extend their strategy to a three-component procedure, which comprises the domino conversion of anthranilic acids and Boc-protected amino acids into the tricyclic core skeleton [43]. 

A quinazoline-2,4-dithione complex [Pd(LH)3(PPh3)] has been isolated from the reaction of the free ligand LH (51) with [Pd(PPh3)4], Unidentate co-ordination of the ligand via a sulphur atom was confirmed from the i.r. spectrum. Oxidative addition with iodine is reported. ... 

Two new quinazoline alkaloids of unusual structure were obtained from the seed husks of Zanthoxylum arborescens.12 Infrared, H n.m.r., and mass spectroscopy showed that the major alkaloid was a quinazolone containing a phenethyl substituent. The 13C n.m.r. spectrum indicated that an iV-methyl rather than an O- methyl group was present, but the spectroscopic studies did not distinguish between structures (19 R = H) and (20 R = H). The alkaloid was shown to be the iV-phenethyl derivative (20 R = H) by synthesis (Scheme 4), and a minor... 

In common with many quinazolines, compound (23) crystallizes as a monohydrate. Its H spectrum in CDC13, however, reveals H-4 as a singlet (10.44 ppm) precluding 3,4 addition typical of covalent hydration of quinazolines. The H spectrum in D2S04 gives an H-4 signal 3.32 ppm upheld consistent with 3,4-hydration in aqueous solution <83JHC447>. 

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