The pyrido[1,2-a]pyrimidin-4-one is a fused bicyclic ring system in which a pyridine ring and a pyrimidinone ring share a bridgehead nitrogen atom. It is described in the medicinal-chemistry literature as a privileged scaffold — a molecular framework that, decorated in different ways, yields compounds active against many unrelated biological targets. Today its derivatives appear in marketed medicines spanning psychiatry, allergy, and rare genetic disease.
The ring's early history was a story of error. The correct structure was not obvious, and for decades it was assigned wrongly. The contribution of Antaki and Petrow in 1951 was to settle it.
In 1911, Palazzo and Tamburini prepared the first compound in the series but assigned it the wrong structure — the 2-oxo isomer rather than the 4-oxo. The same incorrect assignment was repeated by Seide (1925), Crippa and Scevola (1937), and Khitrik (1939), and stood uncorrected for nearly forty years across four independent research groups.
In 1951, H. Antaki and V. Petrow resolved it. By an independent and unambiguous synthesis — reacting 2-bromopyridine with ethyl β-aminocrotonate — they demonstrated that the correct structure was the 4-oxo isomer. The canonical review of the ring system describes this as an "unequivocal synthesis" and records that the products of this chemistry were "first described by Antaki" (Hermecz and Mészáros, Advances in Heterocyclic Chemistry, Vol. 33, 1983).
The assignment was confirmed independently within a year by Adams and Pachter (J. Am. Chem. Soc. 1952, 74, 5491), who synthesised the parent 2-one and 4-one, recorded their ultraviolet spectra as reference standards, and concluded on that basis that the Antaki and Petrow product was the 4-one. It was relied upon by later workers and confirmed again by independent modern methods: Yale and colleagues at the Squibb Institute for Medical Research repeated the reactions and found their products "identical in m.p., ir, uv, and pmr spectra… as described by Antaki and Petrow," verifying the 4-one structures by X-ray crystallography and NMR.
The 1951 paper did more than correct a single structure. Antaki and Petrow introduced ethyl β-aminocrotonate as a reagent "markedly superior" to ethyl acetoacetate for this class of condensation, and used it to prepare a series of fused heterocyclic systems — including several they described as novel ring systems — by extending the reaction across a range of cyclic amidines. The structural correction of the pyrido[1,2-a]pyrimidinone was one result within a broader methodological advance.
Having established the structure, Antaki characterised the electronic identity of the class. His 1958 paper (J. Am. Chem. Soc. 80, 3066) provided the first systematic ultraviolet absorption spectra of the pyrido[1,2-a]pyrimidine series, identifying a constant intense band in the 330–390 nm region (reported as mμ) and explaining it as conjugative interaction with the β-amino-α,β-unsaturated chromophore. The 1962 paper (J. Org. Chem. 27, 1371) clarified the mechanistic origin of these bands.
This spectroscopic framework provided a basis for distinguishing the isomeric forms of the ring by ultraviolet absorption.
The ultraviolet work is treated in full on its own page: Ultraviolet Spectroscopy (1958) →
The pyrido[1,2-a]pyrimidin-4-one is not a laboratory curiosity. Hermecz and Mészáros record that more than a dozen of its derivatives have been introduced into human therapy. Confirmed marketed medicines built on this ring span strikingly different therapeutic areas:
| Medicine | Use | Form of the ring |
|---|---|---|
| Risperidone | Antipsychotic | Reduced (tetrahydro) |
| Paliperidone (Invega) | Antipsychotic; the 9-hydroxy metabolite of risperidone | Reduced (tetrahydro) |
| Pemirolast | Antiallergic; used ophthalmically for allergic conjunctivitis, with additional indications in some countries | Aromatic |
| Risdiplam (Evrysdi) | First approved oral therapy for spinal muscular atrophy | Aromatic |
| Rimazolium (PROBON®) | Non-narcotic analgesic; marketed in Hungary from 1975 | Reduced (tetrahydro) |
That a single core serves an antipsychotic, an antiallergic, an analgesic, and a treatment for a severe genetic neuromuscular disease — in both its aromatic and reduced forms — is what the literature means in calling it a privileged scaffold. Marketed medicines on this ring span half a century, from rimazolium (1975) to risdiplam (2020). Beyond them, a longer set of compounds on this ring (including seganserin, lusaperidone and barmastine) has reached clinical investigation.
Rimazolium (PROBON®), a non-narcotic analgesic marketed in Hungary, was developed at the Hungarian pharmaceutical company CHINOIN by a research group that included István Hermecz and Zoltán Mészáros — the same chemists whose 1983 review of the ring system records that its structure was "first described by Antaki."
Ring identities confirmed against primary sources: risperidone and paliperidone (Janssen patent US 4,804,663); pemirolast (CAS 69372-19-6); risdiplam (IUPAC systematic name, PubChem CID 118513932); rimazolium (PROBON®, CAS 35615-72-6, ATC N02BG02).
The ring system was structurally established by Antaki and Petrow in 1951. The reduction, the substitution, the drug design, and the pharmacology of each medicine are the work of the companies that developed them. What the medicines share is the core ring.
The same ring system later reappeared in crop protection. DuPont Crop Protection developed mesoionic derivatives of the pyrido[1,2-a]pyrimidinone framework into the commercial insecticides triflumezopyrim (marketed as Pyraxalt, for rice planthoppers) and dicloromezotiaz (for lepidopteran pests). In describing the class, the DuPont team noted that mesoionic pyrido[1,2-a]pyrimidinones had been "known for several decades" before systematic biological investigation began (Accounts of Chemical Research, 2017).
The ring whose structure Antaki and Petrow established in 1951 thus underlies marketed products in two separate industries — pharmaceuticals and agrochemicals.
Antaki's work on this ring is cited throughout the standard reference literature. The canonical review of the ring system is that of Hermecz and Mészáros (Advances in Heterocyclic Chemistry, Vol. 33, 1983), which credits him as structural corrector, priority figure, and spectroscopic analyst, naming him in the running text. The ring's chemistry is treated again in Comprehensive Heterocyclic Chemistry II (1996), drawing on his 1951, 1958 and 1962 papers across its treatment of structure, spectroscopy and synthesis. His 1958 paper is cited as the primary reference (Reference 1a) for the reagent ethyl ethoxymethyleneacetoacetate in Wiley's Encyclopedia of Reagents for Organic Synthesis (e-EROS).
The pyrido[1,2-a]pyrimidin-4-one has a documented history of more than a century: a long-standing structural error, its correction, independent confirmation, and a continuing afterlife in medicine and agriculture.
| Year | Event |
|---|---|
| 1911 | Palazzo and Tamburini prepare the first compound in the series, assigning the incorrect 2-oxo structure |
| 1925–1939 | The incorrect assignment is repeated by Seide (1925), Crippa and Scevola (1937), and Khitrik (1939) — four research groups, the error standing for nearly forty years |
| 1951 | Antaki and Petrow establish the correct 4-oxo structure by unambiguous synthesis |
| 1952 | Adams and Pachter confirm the assignment independently, by ultraviolet reference spectra |
| 1958–1962 | Antaki provides the first systematic ultraviolet characterisation of the series |
| 1968–1973 | The structure is confirmed by independent groups — Shur and Israelstam (1968); Kato and colleagues at Tohoku University (1972), who reproduced Antaki's synthesis and upheld the 4-one assignment against a contrary proposal; and Yale and colleagues at the Squibb Institute (1972–73), using X-ray crystallography and NMR |
| 1975 | Rimazolium (PROBON®), an analgesic built on the ring, is marketed in Hungary — the first of a series of medicines |
| 1983 | Hermecz and Mészáros publish the canonical review of the ring system, recording its structure as "first described by Antaki" |
| 1984 onward | Risperidone, paliperidone, pemirolast and, in 2020, risdiplam — marketed medicines across psychiatry, allergy and rare genetic disease — carry the ring |
| 2017–2024 | The same ring appears in crop protection: the insecticides triflumezopyrim and dicloromezotiaz |
| 2026 | The ring's chemistry remains an active field of research |
Antaki, H.; Petrow, V. J. Chem. Soc. 1951, 551–556. DOI
Antaki, H. J. Am. Chem. Soc. 1958, 80, 3066–3068. DOI
Antaki, H. J. Org. Chem. 1962, 27, 1371–1374. DOI
The structural correction and its development are also set out, in their authors' own words, in the patent and review record: In Their Own Words →