The history of the Pyrido[1,2-a]pyrimidin-4-one

 

The history of the pyrido[1,2-a]pyrimidin-4-one. The forty years error and the correction byDr. Hekmat B Antaki (1923–1992)

The structure of the parent pyrido[1,2-a]pyrimidin-4-one ring system was settled by H. Antaki and V. Petrow in 1951, was characterized via ultraviolet spectroscopy from 1958–1962, and became the active core of globally marketed therapeutics.

The Question and the Era

This article reconstructs, from original documents, an obscure but highly consequential sequence in the history of a valuable chemical scaffold. The purpose is to preserve a precise account: what was corrected, how the correction was proved, how the work expanded into a general chemistry of condensed pyrimidines, and where that chemistry stands today.

The events belong to a period of structural chemistry now difficult to reconstruct without bias. Questions that are approached today by routinely combining NMR, mass spectrometry, crystallography, and computation had to be answered purely by chemical behavior, degradation, analogy, electronic reasoning, and independent synthesis. To evaluate the complexity of the problem fairly, one must look through the lens of the instruments and conceptual frameworks available to those who faced it at the time.

The achievement was finding the reasoning by which the structure could be understood within the constraints of the era. The question is how Antaki identified the decisive structural uncertainty and devised a way to resolve it under the conditions of 1950.

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The Road (1950)

The compound at the center of the question had been prepared early in the century, and by 1950 its structure had been approached and re-approached for nearly four decades. In his doctoral thesis, Contributions to the Chemistry of Heterocyclic Compounds (University of London, 1950), Hekmat Antaki set out that long history in full—giving each earlier worker their due.

A significant, overlooked aspect of this period was a deep-seated error in the existing literature regarding how pyridine derivatives condensed with aromatic acids. Early pioneers Reissert (1895) and Räth (1931) had confidently claimed that these reactions yielded 1,8-naphthyridine architectures (specifically, 2,3-benzo-4-hydroxy-1,8-naphthyridine). Räth had even attempted to validate this via an alkaline oxidation that yielded what he identified as "2-aminonicotinic acid," melting at 217°C—far below the true 310°C standard reported by Philips for the authentic compound.

Antaki's work confirmed that these products were actually 2,3-benzo-4-keto-1-aza-4-quinolizines. While oxidative degradation established the broader skeleton, it remained fundamentally blind to the finer question: the precise position of the carbonyl group within that skeleton.

As mapped in the original thesis records in IMG_5449_2.jpg, Antaki systematically charted the temperature-dependent pathways of 2-aminopyridine and ethyl acetoacetate to isolate the precise conditions under which intermediate compounds transitioned into the final base.

The Antaki Synthesis

Three-component condensation for hexahydroquinoline formation — first reported by H. Antaki, Research Institute for Tropical Medicine, Cairo, 1963

The Synthesis

In 1963, Hekmat Bechir Fathallah Antaki reported the first synthesis of ethyl 4-aryl-1,4,5,6,7,8-hexahydro-2-methyl-5-oxoquinoline-3-carboxylates via three-component condensation of cyclohexane-1,3-dione, aromatic aldehydes, and ethyl β-aminocrotonate in ethanol and glacial acetic acid under reflux for one hour.

The synthesis requires no transition metal catalysts, no inert atmosphere, and no specialised equipment. The three starting materials are commercially available and inexpensive. The product crystallises directly from the reaction mixture. Antaki reported a full series of aryl substituents at the 4-position — p-nitrophenyl, p-methoxyphenyl, 3,4-dimethoxyphenyl, p-dimethylaminophenyl, p-chlorophenyl, and o-nitrophenyl derivatives — with melting points and elemental analyses fully documented.

The hexahydroquinoline intermediates were further subjected to oxidative dehydrogenation by chromium trioxide in dilute acetic acid to yield the corresponding 5,6,7,8-tetrahydroquinolines, demonstrating controlled manipulation of scaffold oxidation state.

Original Reaction Conditions (1963)

Parameter	Detail
Reactants	Cyclohexane-1,3-dione · Aromatic aldehyde · Ethyl β-aminocrotonate
Solvent	Ethanol and glacial acetic acid
Temperature	Reflux
Reaction time	1 hour
Workup	Product crystallises directly from the reaction mixture
Catalyst	None — no transition metal catalysts required
Atmosphere	Ambient — no inert atmosphere required
Aryl substituents reported	p-nitrophenyl, p-methoxyphenyl, 3,4-dimethoxyphenyl, p-dimethylaminophenyl, p-chlorophenyl, o-nitrophenyl
Oxidation step	Chromium trioxide in dilute acetic acid — converts hexahydroquinolines to tetrahydroquinolines

The significance of the 1963 paper extends beyond the synthesis of specific compounds. By systematically varying the aryl substituent at the 4-position across six electronically distinct groups, Antaki established the 4-aryl position as a modifiable site whose electronic character directly influences the scaffold's properties. The hexahydroquinoline framework is now recognised as a privileged scaffold in medicinal chemistry: a molecular platform that accommodates systematic structural variation at key positions to yield compounds active across multiple therapeutic targets. The six variants documented in 1963 were not an inventory of products. They were the first demonstration that this position could be tuned — the foundational evidence that the scaffold was a tool, not merely a molecule.

This synthesis has been formally classified as the Antaki synthesis — alongside the Hantzsch and Stankevich reactions — as one of three foundational multicomponent methods for hexahydroquinoline formation:

"These approaches established the mechanistic foundation for multicomponent HHQ formation and provided a framework for structural diversification in modern synthesis." — Oduselu et al., Frontiers in Chemistry, 2026

The Chemistry of Heterocyclic Compounds, the 1963 and 1965 papers in Acheson's Vol. 9 (1973), the standard reference for the cyclohexane-1,3-dione / ammonium acetate condensation; the 1963 paper again in Thummel's "Carbocyclic Annelated Pyridines" (Part 5, 1984). Comprehensive Heterocyclic Chemistry (Pergamon) — G. Jones cites the 1963 paper as the sole reference for the cyclohexane-1,3-dione variant of the Hantzsch synthesis (1st ed., 1984).

The 1963 paper is cited as reference 324 in Acheson, R.M. (ed.), Chemistry of Heterocyclic Compounds, Vol. 9 (Wiley, 1973) — the canonical Wiley heterocyclic chemistry reference series.

Bossert & Vater — Medicinal Research Reviews, 1989 DOI: 10.1002/med.2610090304

"hexahydroquinoline derivatives aroused our interest."

Friedrich Bossert and Wulf Vater, inventors of nifedipine. Reference 7 in that paper is Antaki, J. Chem. Soc., 1963. He appears again as reference 13, when the authors place his work in the historical synthetic lineage alongside the classic Hantzsch and Knoevenagel reactions. This review has been cited thousands of times.

Pharmaceutical Confirmation

Independent patent families citing Antaki's 1963 synthesis as prior art include filings by ICI/AstraZeneca (1991), Gilead Sciences (2014), and Shin Nippon Biomedical Laboratories (2013), spanning cardiovascular, urological, and transporter pharmacology across more than two decades.

Antimalarial Relevance

The hexahydroquinoline scaffold has attracted renewed pharmaceutical interest in antimalarial drug discovery as resistance to artemisinin-based therapies grows. Antaki directed his programme explicitly toward antiparasitic applications: "In continuation of previous work on the schistosomicidal activity in the pyrido[1,2-a]pyrimidine series, the synthesis of some basic derivatives was considered." — H. Antaki, J. Org. Chem., 1962 He was working on diseases of the poor, with tools accessible to resource-limited laboratories, six decades before the current antimalarial interest in this scaffold.

Primary Source

Antaki, H. The Synthesis of Ethyl 4-Aryl-5,6,7,8-tetrahydro-5-oxoquinoline-3-carboxylates and their Derivatives. J. Chem. Soc. 1963, 4877–4879. DOI: 10.1039/jr9630004877

hekmatantaki.org

Just some photos South America 2011

The Monkey Orchid

The Dracula Orchid

Getting into USA!

A few pleasant days in Mexico with R, a colleague and friend for many years, attending an Exhibition.

An amazing experience compared to the Frankfurt version, the show only starts at 14:00 and lasts for a few hours, against the long hours of Frankfurt.


The exhibition went so quickly between the few meetings we had and watching all this dancing on every stand. It is just amazing to see people sitting discussing business and suddenly a stand starts a nice music and you get everyone on their feet dancing for few minutes. The same chap shouting and negotiating prices is simply transformed...beautiful exico.

A pleasant adios night out dining right after the show in a beautiful Mexican restaurant with local music and dance invited by a friend G. More Tequila and he became very emotional:

-Charles, you are my brother! ................No...............more, you are not any more Charles and I am G, you are G and I am Charles!
And as to prove his point, he took out his exhibition badge that we still had on our lapels, put it on me, and took mine for himself... He became Charles and I am G.