Microwave Mediated Synthesis of Quinazolinone Natural products of  Peganum harmala for Medicinal Chemistry Applications 

By Amelie Duval

Introduction

In the pharmaceutical field, quinazolinones have become an important pharmacophoric scaffold due to their presence in natural compounds with a wide range of medicinal chemistry applications. Among these, Peharmaline A (1), a natural alkaloid with b-Caroline and tricyclic pyrroloquinazolinone cores (Figure 1) that exhibits various biological activities, including antimalarial, anticancer, anti-inflammatory, and antibacterial activities has garnered significant interest of synthetic and medicinal chemists in recent years. In addition, quinazolinones possess stability and change adaptability that makes it easy to prepare, which is prime for scientific investigations. Peharmaline A is found in Peganum harmala L., a species coming from the family of medicinally important Zygophyllaceae. In Chinese traditional medicine, P. harmala L. seed extract is widely used to treat malignancies of the digestive system and malaria. 

(±}-Peharmaline A (1} 

Figure 1: (±)-Peharmaline A precursors 

Objective of the Proposal 

We proposed to develop a novel approach to synthesize Peharmaline A utilizing the deoxyvasicinone and 5-methoxytryptamine, and extend the same to access a library of analogues of peharmaline to further study their biomedicinal potential. 

Methodology – for the Synthesis of (±)-Peharmaline A and Its Analogues 

According to our retrosynthetic plan shown in Scheme 1, we have proposed a total synthetic route to achieve (±)-Peharmaline A 1 in a three-step process: through Pictet-Spengler reaction with the use of 5- methoxytryptamine 3 and deoxyvasicinone methyl oxalate intermediate 2, which itself can be made by of the acylation reaction of deoxyvasicinone 4 with methyl oxalyl chloride 5. It required us to develop a short and effective synthesis for deoxyvasicinone itself through adapting and modifying the reported synthesis into a practical large–scale under microwave irradiation chemistry one-pot synthesis. Starting from commercially available isatoic anhydride 6 and pyrrolidinone 7, they would serve as the building blocks and will result in (±)-Peharmaline in just three steps. It would also provide a divergent approach towards the (±)-Peharmaline A analogues in a modular fashion. 

Scheme 1: Retrosynthetic plan of synthesizing (±)-Peharmaline A 

Results and Discussions 

Scheme 1 describes our efforts toward the development of a total synthetic approach to the synthesis of (±) -Peharmaline A. After a lot of careful experimentation with stoichiometry and heating (temperatures and time intervals), we finally optimized the synthesis of deoxyvasicinone to work on 1Og scale. It required thorough mixing of 1:1.25 equals of isatoic anhydride 5 and the pyrrolidinone 6 and heating of three minutes at medium power household MW (with one-minute intervals). Once the reaction was completed, it was allowed to cool to room temperature, and any remaining starting materials were removed under reduced pressure. The next step was acylation of the deoxyvasicinone 4. The acylation reaction conditions needed a lot of optimizations. We started with the available chemical in the lab i.e., ethyl oxalyl chloride 6 instead of methyl oxalyl chloride, which will lead to Methyl (±)-Peharmaline A, while we awaited the other chemical to arrive from the vendor. 

After a great deal of experimentation to optimize the temperature conditions, the equivalence of acylation reaction, and the solvent environment, we found out that 3 eq of ethyl oxalyl chloride and 2 eq of Et3N base in anhydrous DCM was needed to complete the reaction with deoxyvasicinone, resulting in  32.6 % of yield. Further, it was confirmed that the initially maintained O °C temperature needs to be increased after the complete addition (1 h) of ethyl oxalyl chloride to overnight refluxing to 48 h for the completion of the reaction to obtain a green clean solid product. The addition of both ethyl oxalyl chloride and Et3N base was separated into two portions, with the first half equivalent being added in at OoC initially, and the second half equivalent being added after 24 hours of refluxing in the same method before placing the reaction back to reflux for an additional 24 h. The acylated deoxyvasicinone 2 was obtained in enol form and it was confirmed by NMR and X-ray crystallography. 7 The methyl oxalyl chloride had arrived from the vendor and was tested with the same chemical and temperature environment, and experiments are still on-going to examine the identity and quality of the product. Additional tests are being conducted with deoxyvasicinone with benzoyl chloride and oxalyl chloride to determine if analogues can be developed to further the research with the final (±)-Peharmaline A and other quinazolinone products. 

Scheme 2: Synthesis approach of (±)-Peharmaline A analogue 

 

The next step in the sequence was the Pictet-Spengler reaction. We decided to establish it first with the commercially available tryptamine instead of the expensive 5-methoxy tryptamine (required to be synthesized in the lab later). Accordingly, we combined the tryptamine base 3.85b with acylated deoxyvasicinone 2 under refluxing conditions. After a lot of experimentation trials, our efforts towards the Pictet-Spengler reaction conditions were successful. It ensued in the presence of a catalytic amount of acid that could facilitate the reaction forward to the formation of (±)-Peharmaline A analogue when there was no water present in the sample. As we ran these optimization reactions on a mg scale, we obtained the product through preparative TLC thus far. We need to further optimize them on a larger scale and obtain the pure compound. 

 

Scheme 3: Pictet-Spengler reaction for the final assembly 

 

Figure 2 shows the changes in the 1H NMR peaks along the path of (±)-Peharmaline A analogue (8) (C) formation from the deoxyvasicinone 4 (A). 

Figure 2: Comparison of (±)-Peharmaline A analogue synthesis process A) 1H NMR of Deoxyvasicinone 4 in CDCl3 synthesized through MW irradiation chemistry. B) 1H NMR of enol form of acylated deoxyvasicinone 2 in CDCl3) 1H NMR of Peharmaline A analogue 8 in CDCl3 synthesized in TFMS acid/DCM solution. 

Conclusions and Future Directions

In summary, we have established the groundwork needed for the synthesis of peharmaline A and its analogues. Scheme 4 deliniates our approach of making a library of (±)-Peharmaline A analogues by employing different tryptamines and tyrosines–based primary arylethanamines in Pictet-Spengler reaction by applying our optimized reaction conditions (anh. DCM/TFMSA/reflux-24 h) towards appending the b-Carboline ring to the deoxyvasicinone ring. As can be seen, it is a modular and divergent approach to making (±)-Peharmaline A analogues via employing different acid chlorides as well, as seen in Scheme 5. We propose to carry it out when the grad student that I was associated with returns in the next winter and summer breaks. 

 

Scheme 4: Future study: Scoping Pictet-Spengler towards (±)-Peharmaline A analogues 

Scheme 5: Other modifications for Diversity Oriented Synthesis of Peharmaline A 

Acknowledgments

Support from the UMassD OUR is greatly appreciated. Many thanks to Fazmina Anver and Dr. Rasapalli for their teaching and mentoring in the lab. 

References

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