Microwave-Assisted Natural Product Chemistry

Appukkuttan, Prasad; Eycken, Erik V. Van der

doi:10.1007/128_051

Cited by 20 publications

(4 citation statements)

References 179 publications

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“…Regardless of the relatively large body of published work in this area, − the exact reasons why microwave irradiation is able to enhance chemical processes are still unknown. Since the early days of microwave synthesis, the observed rate-accelerations and sometimes altered product distributions compared to conventionally heated experiments have led to speculation on the existence of so-called “specific-” or “nonthermal” microwave effects. , Such effects have been claimed when the outcome of a synthesis performed under microwave conditions was different from the conventionally heated counterpart at the same measured reaction temperature .…”

Section: Introductionmentioning

confidence: 99%

Nonthermal Microwave Effects Revisited: On the Importance of Internal Temperature Monitoring and Agitation in Microwave Chemistry

2007

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The concept of nonthermal microwave effects has received considerable attention in recent years and is the subject of intense debate in the scientific community. Nonthermal microwave effects have been postulated to result from a direct stabilizing interaction of the electric field with specific (polar) molecules in the reaction medium that is not related to a macroscopic temperature effect. In order to probe the existence of nonthermal microwave effects, four synthetic transformations (Diels-Alder cycloaddition, alkylation of triphenylphosphine and 1,2,4-triazole, direct amide bond formation) were reevaluated under both microwave dielectric heating and conventional thermal heating. In all four cases, previous studies have claimed the existence of nonthermal microwave effects in these reactions. Experimentally, significant differences in conversion and/or product distribution comparing the conventionally and microwave-heated experiments performed at the same measured reaction temperature were found. The current reevaluation of these reactions was performed in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using a multiple fiber-optic probe system. Using this technology, the importance of efficient stirring and internal temperature measurement in microwave-heated reactions was made evident. Inefficient agitation leads to temperature gradients within the reaction mixture due to field inhomogeneities in the microwave cavity. Using external infrared temperature sensors in some cases results in significant inaccuracies in the temperature measurement. Applying the fiber-optic probe temperature monitoring device, a critical reevaluation of all four reactions has provided no evidence for the existence of nonthermal microwave effects. Ensuring efficient agitation of the reaction mixture via magnetic stirring, no significant differences in terms of conversion and selectivity between experiments performed under microwave or oil bath conditions at the same internally measured reaction temperatures were experienced. The observed effects were purely thermal and not related to the microwave field.

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Section: Introductionmentioning

confidence: 99%

Nonthermal Microwave Effects Revisited: On the Importance of Internal Temperature Monitoring and Agitation in Microwave Chemistry

2007

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“…[12] The other related natural product luotonin A (9) and its congeners (9)(10)(11) were isolated by Nomura et al [13] along with vasicinone (3), deoxyvasicinone (4), vasicine (1), and deoxyvasicine (2) from the aerial parts of Peganum Nigellastrum Bunge, a Chinese medicinal plant which is a rich source of bioactive alkaloids and used for the treatment of rheumatism, inflammation, abscesses, and other maladies. [14] This coexistence of suggests a plausible biosynthesis for luotonins from vasicinone and anthranilic acid. [13] The basic fraction of Peganum Nigellastrum has been shown to exhibit antitumor activity.…”

Section: Introductionmentioning

confidence: 97%

Total and Diverted Total Synthesis of Pyrrolo‐Quinazolinone Alkaloids and Their Analogues

Rasapalli,

Huang,

Sammeta

et al. 2023

ChemistrySelect

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A short and expeditious total and diverted total synthesis of luotonin, vasicinone, and their analogues has been achieved from the key tricyclic quinazolinone intermediate which was accessed from simple substituted anthranilamide obtained from the coupling of β‐alanate with isatoic anhydride followed by Dieckmann condensation chemistry of the resulting diester. The tricyclic ketone exhibited interesting chemical properties, e. g. keto‐enol tautomerism. Friedlander condensation and Fischer‐Indolization were employed for further annulations to access polycyclic alkaloids and their analogues.

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“…Microwave-assisted synthesis has revolutionized many processes in recent years as a valuable alternative to the use of conductive heating for accelerating transformations in synthetic organic chemistry [1], colloidal science [2], natural product chemistry [3], medicinal chemistry [4–6], solid-phase peptide synthesis [7] and in the biosciences [8]. Despite the many advantages of this heating method, and the introduction of a wide range of instrumentation [1], the scale up of microwave-mediated reactions still poses a number of challenges, in particular as a result of a lack of uniform heating [9].…”

Section: Introductionmentioning

confidence: 99%

One-step synthesis of pyridines and dihydropyridines in a continuous flow microwave reactor

Bagley¹,

Fusillo²,

Jenkins³

et al. 2013

Beilstein J. Org. Chem.

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SummaryThe Bohlmann–Rahtz pyridine synthesis and the Hantzsch dihydropyridine synthesis can be carried out in a microwave flow reactor or using a conductive heating flow platform for the continuous processing of material. In the Bohlmann–Rahtz reaction, the use of a Brønsted acid catalyst allows Michael addition and cyclodehydration to be carried out in a single step without isolation of intermediates to give the corresponding trisubstituted pyridine as a single regioisomer in good yield. Furthermore, 3-substituted propargyl aldehydes undergo Hantzsch dihydropyridine synthesis in preference to Bohlmann–Rahtz reaction in a very high yielding process that is readily transferred to continuous flow processing.

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Microwave-Assisted Natural Product Chemistry

Cited by 20 publications

References 179 publications

Nonthermal Microwave Effects Revisited: On the Importance of Internal Temperature Monitoring and Agitation in Microwave Chemistry

Nonthermal Microwave Effects Revisited: On the Importance of Internal Temperature Monitoring and Agitation in Microwave Chemistry

Total and Diverted Total Synthesis of Pyrrolo‐Quinazolinone Alkaloids and Their Analogues

One-step synthesis of pyridines and dihydropyridines in a continuous flow microwave reactor

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