H. Antaki, J. Am. Chem. Soc. 1958, 80, 3066–3068. DOI
H. Antaki, J. Org. Chem. 1962, 27, 1371–1374. DOI
In 1958, working at the Research Institute for Tropical Medicine, Cairo, H. B. F. Antaki published the first systematic ultraviolet absorption spectra of the pyrido[1,2-a]pyrimidine class in the Journal of the American Chemical Society. The paper was received 15 October 1957. He extended and confirmed this work in a second paper in the Journal of Organic Chemistry in 1962, which also reported a related fused heterocycle and continued a parallel antiparasitic screening programme — see below.
The 1958 paper reported the preparation of 3-acetyl- and 3-cyano-4H-pyrido[1,2-a]pyrimidin-4-ones through reaction of 2-aminopyridines with α-ethoxymethylenecarboxylic esters, followed by cyclisation under reduced pressure (J. Am. Chem. Soc., 1958, 80, 3066–3069; DOI: 10.1021/ja01545a041). Antaki's aminomethylenecyanoacetate esters — the uncyclised precursors to his 3-cyano pyridopyrimidines — were later cited in Bristol-Myers patents US 4,122,274 and US 4,209,620 as literature precedent for the corresponding acrylate starting materials used in their tetrazolyl pyrido[1,2-a]pyrimidin-4-one antiallergic agents.
The two papers together constitute a single argument in two stages. The 1951 structural correction had established what the compounds were. The 1958 and 1962 papers established why they had to be that structure — providing the electronic and mechanistic proof from spectroscopic evidence.
The first UV spectra in this record appear not in a published paper but in Antaki's 1950 doctoral thesis. Two hand-drawn curves (Fig. I, Fig. II) are compared directly and described as "nearly identical," used as confirming evidence that two differently-obtained bases shared the same ring system.
![Doctoral thesis, 1950: Fig. I and Fig. II, hand-drawn UV absorption curves for the two pyrido[1,2-a]pyrimidinone-type bases compared as 'nearly identical'](/thesis-uv-figs-1-2.jpg)
The link between the thesis and the published record is documented by Antaki himself, not inferred. In the 1962 paper (J. Org. Chem., 27, 1371), reporting on the same pyrido[1,2-a]pyrimidinone series, Antaki cites his own 1950 thesis by name as the source establishing that the compound's spectrum "displays two bands." The unpublished thesis data is, by his own footnote, the same evidence carried forward into the published record twelve years later.
The interpretation of why those bands appear where they do was built in stages, with sources named at each step:
Antaki states directly that the underlying zwitterionic-resonance interpretation itself was not his own:
"Such a view has previously been advanced solely on the basis of chemical and infrared spectral evidence."
— citing Snyder and Robison, 1952. His own stated contribution was independent ultraviolet corroboration of that view.
Antaki measured the UV spectra of a series of derivatives in absolute ethanol and identified two characteristic absorption bands that appear consistently across the entire class, including the 6,7-benzo-fused pyrimido[1,2-a]quinoline analogues:
He provided the first detailed mechanistic interpretation of these bands:
He then made a structural argument from the spectroscopic evidence:
"This may be considered as evidence for the major contribution of zwitterionic fully aromatic structures such as VIII to the resonance state of the molecule."
This is not a recording of spectral data. It is a structural argument — establishing from spectroscopic evidence the electronic character of an entire compound class. The absorption pattern was not random. It was systematic, mechanistically explained, and structurally significant.
The 1962 paper extends the same comparative chromophore analysis to a second, structurally related fused heterocycle: 5-methyl-s-triazolo[2,3-a]pyrimidin-7-one — in current nomenclature, a member of the 1,2,4-triazolo[1,5-a]pyrimidin-7-one class. Antaki prepared this compound by reacting 3-amino-1,2,4-triazole with ethyl β-aminocrotonate, and compared its UV spectrum directly against the pyrido[1,2-a]pyrimidinone series to isolate the effect of replacing a ring carbon with sulfur or nitrogen on the position and intensity of the characteristic long-wavelength band.
This was not the first synthesis of the ring itself. Antaki states directly that his compound is identical to material already prepared by a different route — condensation of 3-amino-1,2,4-triazole with ethyl acetoacetate — reported by C. Bülow and K. Haas, Ber. Dtsch. Chem. Ges. 42, 4638 (1909). Antaki's own footnote cites this as "(1907)"; the correct publication year for Berichte volume 42 is 1909, confirmed against the modern literature — likely a typographical error in the original 1962 paper. Antaki's contribution to this particular compound is therefore not priority of synthesis but UV characterisation: establishing where it sits, spectroscopically, within the same chromophore framework developed for the pyrido[1,2-a]pyrimidinone series.
The 1,2,4-triazolo[1,5-a]pyrimidin-7-one class has since become an active scaffold in modern medicinal and agrochemical chemistry, with applications spanning anticancer, antimicrobial, antiparasitic, antiviral, and CNS research, and at least one marketed herbicide and one antibacterial natural product built on the ring. A 2024 review of synthetic routes to this class (Zedan, Shoman, Abdel-Aziz & Abdel-Rahman, Results in Chemistry 2024, 12, 101903) does not cite Antaki's 1962 paper among its 152 references — consistent with the compound's own priority resting with Bülow & Haas rather than with Antaki, and with this paper's contribution lying in spectroscopy rather than synthesis.
Alongside the UV and synthetic work, both papers document a live antischistosomal screening programme running in parallel at the Research Institute for Tropical Medicine. The 1958 paper reports that certain pyrido[1,2-a]pyrimidine esters produced a "hepatic shift on oral administration to Gerbils experimentally infected with Schistosoma mansoni" — in vivo activity data in a live animal model of the disease, not merely a synthetic or spectroscopic exercise. The 1962 paper explicitly continues this programme, noting that "the synthesis of some basic derivatives was considered" as a direct continuation of "previous work on the schistosomicidal activity in the pyrido[1,2-a]pyrimidine series."
This places the UV characterisation work in its proper context: it was not pursued as pure physical-organic chemistry, but as part of an active antiparasitic drug discovery effort, in which reliable spectroscopic structure confirmation was a practical necessity for a chemist working without routine access to NMR.
Before Antaki's papers, the electronic structure and correct tautomeric forms of pyrido[1,2-a]pyrimidines were still ambiguous. Structural assignments in the literature were sometimes conflicting, and there was limited understanding of how the fused ring system and substituents influenced the π-electron distribution.
Antaki's systematic approach — synthesising a series of derivatives, recording full UV curves with extinction coefficients, and linking the spectra to specific chromophores and resonance forms — gave chemists a reliable spectroscopic fingerprint. This was especially important because these compounds were being explored for biological activity, including the antischistosomal screening described above. His UV interpretation provided a quick, non-destructive way to confirm structure and conjugation pattern when NMR spectroscopy was not yet routine or widely accessible.
Both assignments were independently confirmed by Shur and Israelstam, writing in the Journal of Organic Chemistry in 1968, who cited the 1951, 1958, and 1962 papers collectively as the foundation for their own UV interpretation.
Hermecz and Mészáros adopted Antaki's UV assignments as the standard reference in their 1983 canonical review in Advances in Heterocyclic Chemistry (Vol. 33) — the definitive authority on this compound class for three decades.
The 1958 paper is listed as Reference 1(a) in the entry for Ethyl Ethoxymethyleneacetoacetate in Wiley's Encyclopedia of Reagents for Organic Synthesis (e-EROS, 2001) — the standard synthetic chemistry reference in the discipline. The first reference in the entry for the reagent most associated with this compound class is his 1958 paper.
Even with modern high-field NMR as the primary tool for structure elucidation, Antaki's UV contributions remain practically useful in everyday laboratory work on these scaffolds. The table below summarises how his work is still applied.
| Application | How Antaki's UV data is used | Why it remains valuable |
|---|---|---|
| Quick class confirmation | Check for the diagnostic intense band at 330–390 nm | Confirms the molecule possesses the characteristic extended conjugation of the pyrido[1,2-a]pyrimidine core |
| Chromophore integrity verification | Compare λmax and intensity with Antaki's reported values | Detects whether the expected β-amino-α,β-unsaturated system and pyridone-imine conjugation are present |
| Conjugation and electronic effects | Monitor shifts in the long-wavelength band upon substitution | Reveals how new substituents or modifications affect the push-pull electronic system |
| Tautomer and resonance insight | Presence and intensity of the 330–390 nm band | Supports contribution of zwitterionic aromatic forms — information not directly given by NMR |
| Structure confirmation support | Cross-check UV data alongside NMR and HRMS | Provides orthogonal evidence, especially useful for ambiguous regiochemistry or new derivatives |
| Medicinal chemistry screening | Rapid evaluation of electronic properties | Helpful when studying compounds for optical, fluorescent, or biological applications |
Modern laboratory use documented by researchers working on pyrido[1,2-a]pyrimidine and pyrimido[1,2-a]quinoline derivatives, 2025.
The 1951 structural correction and the 1958–1962 UV characterisation are not separate contributions. They are two stages of one argument.
The 1951 correction established that the field had been working from the wrong structure for forty years. The 1958 paper provided the mechanistic and electronic proof of why the correct structure was necessary — transforming an experimental assertion into a demonstrable chemical argument. Without the correct structure, the UV interpretation would rest on a false foundation. Without the UV interpretation, the structural correction remained an experimental result without a mechanistic explanation.
Together they provided the reliable foundation for the pyrido[1,2-a]pyrimidine series. That foundation enabled the subsequent chemical development and pharmaceutical applications that followed over the next seven decades.