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Free Radicals in Organic Synthesis
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Free Radicals in Organic Synthesis

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Chemistry".

Deadline for manuscript submissions: closed (10 May 2016) | Viewed by 52123

Special Issue Editor

Prof. Dr. Fawaz Aldabbagh

E-Mail Website
Guest Editor
Department of Pharmacy, School of Life Sciences, Pharmacy & Chemistry, Kingston University, Penrhyn Road, Kingston upon Thames KT1 2EE, UK
Interests: free radical organic and polymer chemistry; heterocyclic and medicinal chemistry
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Over the past twenty years, the use of free radicals in organic synthesis has become commonplace. In this Special issue, we welcome reports on the use of free radicals in the development of new synthetic methodology, target orientated synthesis, and as reaction intermediates. These reactive free radicals typically have fleeting lifetimes, however, new reports on the synthesis of stable free radicals are also welcome.  Stable free radicals, such as nitroxides, have found uses as spin traps, profluorescent probes, and importantly as mediators in controlled/living radical polymerizations. A synthetic transformation that has been revolutionized over the past five years is homolytic aromatic substitution (HAS) with traditional non-chain Group 14 organometal hydride-mediated and photochemical techniques superseded by more efficient and benign visible-light photocatalysis and base-mediated HAS. Therefore we welcome reports circumventing the use of toxic heavy metals and expensive transition metal catalysts, and others demonstrating the green credentials of free radicals in organic synthesis.

Prof. Dr. Fawaz Aldabbagh
Guest Editor

Manuscript Submission Information

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Keywords

  • cyclizations
  • green chemistry
  • heterocycles
  • homolytic substitutions
  • reaction mechanisms
  • persistent radicals
  • synthetic intermediates

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Published Papers (5 papers)

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Research

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1686 KiB  
Article
Rational Design and Synthesis of New, High Efficiency, Multipotent Schiff Base-1,2,4-triazole Antioxidants Bearing Butylated Hydroxytoluene Moieties
by Wageeh A Yehye, Noorsaadah Abdul Rahman, Omar Saad, Azhar Ariffin, Sharifah Bee Abd Hamid, Abeer A. Alhadi, Farkaad A. Kadir, Marzieh Yaeghoobi and Abdulsalam A. Matlob
Molecules 2016, 21(7), 847; https://doi.org/10.3390/molecules21070847 - 28 Jun 2016
Cited by 32 | Viewed by 9727
Abstract
A new series of multipotent antioxidants (MPAOs), namely Schiff base-1,2,4-triazoles attached to the oxygen-derived free radical scavenging moiety butylated hydroxytoluene (BHT) were designed and subsequently synthesized. The structure-activity relationship (SAR) of the designed antioxidants was established alongside the prediction of activity spectra for [...] Read more.
A new series of multipotent antioxidants (MPAOs), namely Schiff base-1,2,4-triazoles attached to the oxygen-derived free radical scavenging moiety butylated hydroxytoluene (BHT) were designed and subsequently synthesized. The structure-activity relationship (SAR) of the designed antioxidants was established alongside the prediction of activity spectra for substances (PASS). The antioxidant activities of the synthesized compounds 4–10 were tested by the DPPH bioassay. The synthesized compounds 4–10 inhibited stable DPPH free radicals at a level that is 10−4 M more than the well-known standard antioxidant BHT. Compounds 8–10 with para-substituents were less active than compounds 4 and 5 with trimethoxy substituents compared to those with a second BHT moiety (compounds 6 and 7). With an IC50 of 46.13 ± 0.31 µM, compound 6 exhibited the most promising in vitro inhibition at 89%. Therefore, novel MPAOs containing active triazole rings, thioethers, Schiff bases, and BHT moieties are suggested as potential antioxidants for inhibiting oxidative stress processes and scavenging free radicals, hence, this combination of functions is anticipated to play a vital role in repairing cellular damage, preventing various human diseases and in medical therapeutic applications. Full article
(This article belongs to the Special Issue Free Radicals in Organic Synthesis)
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5203 KiB  
Article
Synthesis and Evaluation of Ciprofloxacin-Nitroxide Conjugates as Anti-Biofilm Agents
by Anthony D. Verderosa, Sarah C. Mansour, César De la Fuente-Núñez, Robert E. W. Hancock and Kathryn E. Fairfull-Smith
Molecules 2016, 21(7), 841; https://doi.org/10.3390/molecules21070841 - 27 Jun 2016
Cited by 30 | Viewed by 8964
Abstract
As bacterial biofilms are often refractory to conventional antimicrobials, the need for alternative and/or novel strategies for the treatment of biofilm related infections has become of paramount importance. Herein, we report the synthesis of novel hybrid molecules comprised of two different hindered nitroxides [...] Read more.
As bacterial biofilms are often refractory to conventional antimicrobials, the need for alternative and/or novel strategies for the treatment of biofilm related infections has become of paramount importance. Herein, we report the synthesis of novel hybrid molecules comprised of two different hindered nitroxides linked to the piperazinyl secondary amine of ciprofloxacin via a tertiary amine linker achieved utilising reductive amination. The corresponding methoxyamine derivatives were prepared alongside their radical-containing counterparts as controls. Subsequent biological evaluation of the hybrid compounds on preformed P. aeruginosa flow cell biofilms divulged significant dispersal and eradication abilities for ciprofloxacin-nitroxide hybrid compound 10 (up to 95% eradication of mature biofilms at 40 μM). Importantly, these hybrids represent the first dual-action antimicrobial-nitroxide agents, which harness the dispersal properties of the nitroxide moiety to circumvent the well-known resistance of biofilms to treatment with antimicrobial agents. Full article
(This article belongs to the Special Issue Free Radicals in Organic Synthesis)
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1948 KiB  
Article
The Suppression of Columnar π-Stacking in 3-Adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl
by Christos P. Constantinides, Andrey A. Berezin, Georgia A. Zissimou, Maria Manoli, Gregory M. Leitus and Panayiotis A. Koutentis
Molecules 2016, 21(5), 636; https://doi.org/10.3390/molecules21050636 - 14 May 2016
Cited by 17 | Viewed by 5376
Abstract
3-Adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (4) crystallizes as chains of radicals where the spin bearing benzotriazinyl moieties are isolated from each other. Magnetic susceptibility studies in the 5–300 K temperature region indicate that radical 4 demonstrates typical paramagnetic behavior stemming from non-interacting S = ½ spins. Full article
(This article belongs to the Special Issue Free Radicals in Organic Synthesis)
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Review

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3245 KiB  
Review
Radical Smiles Rearrangement: An Update
by Ingrid Allart-Simon, Stéphane Gérard and Janos Sapi
Molecules 2016, 21(7), 878; https://doi.org/10.3390/molecules21070878 - 6 Jul 2016
Cited by 76 | Viewed by 16612
Abstract
Over the decades the Smiles rearrangement and its variants have become essential synthetic tools in modern synthetic organic chemistry. In this mini-review we summarized some very recent results of the radical version of these rearrangements. The selected examples illustrate the synthetic power of [...] Read more.
Over the decades the Smiles rearrangement and its variants have become essential synthetic tools in modern synthetic organic chemistry. In this mini-review we summarized some very recent results of the radical version of these rearrangements. The selected examples illustrate the synthetic power of this approach, especially if it is incorporated into a domino process, for the preparation of polyfunctionalized complex molecules. Full article
(This article belongs to the Special Issue Free Radicals in Organic Synthesis)
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3768 KiB  
Review
Synthetic Strategies for 5- and 6-Membered Ring Azaheterocycles Facilitated by Iminyl Radicals
by John C. Walton
Molecules 2016, 21(5), 660; https://doi.org/10.3390/molecules21050660 - 18 May 2016
Cited by 55 | Viewed by 10007
Abstract
The totality of chemical space is so immense that only a small fraction can ever be explored. Computational searching has indicated that bioactivity is associated with a comparatively small number of ring-containing structures. Pyrrole, indole, pyridine, quinoline, quinazoline and related 6-membered ring-containing aza-arenes [...] Read more.
The totality of chemical space is so immense that only a small fraction can ever be explored. Computational searching has indicated that bioactivity is associated with a comparatively small number of ring-containing structures. Pyrrole, indole, pyridine, quinoline, quinazoline and related 6-membered ring-containing aza-arenes figure prominently. This review focuses on the search for fast, efficient and environmentally friendly preparative methods for these rings with specific emphasis on iminyl radical-mediated procedures. Oxime derivatives, particularly oxime esters and oxime ethers, are attractive precursors for these radicals. Their use is described in conventional thermolytic, microwave-assisted and UV-vis based preparative procedures. Photoredox-catalyzed protocols involving designer oxime ethers are also covered. Choice can be made amongst these synthetic strategies for a wide variety of 5- and 6-membered ring heterocycles including phenanthridine and related aza-arenes. Applications to selected natural products and bioactive molecules, including trispheridine, vasconine, luotonin A and rutaecarpine, are included. Full article
(This article belongs to the Special Issue Free Radicals in Organic Synthesis)
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