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Stable Bicyclic Functionalized Nitroxides: the Synthesis of Derivatives of Aza-Nortropinone–5-Methyl-3-Oxo-6,8-Diazabicyclo[3

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Stable Bicyclic Functionalized Nitroxides: the Synthesis of Derivatives of Aza-Nortropinone–5-Methyl-3-Oxo-6,8-Diazabicyclo[3 molecules Article Stable Bicyclic Functionalized Nitroxides: The Synthesis of Derivatives of Aza-nortropinone–5-Methyl-3-oxo-6,8- diazabicyclo[3.2.1]-6-octene 8-oxyls Larisa N. Grigor’eva, Alexsei Ya. Tikhonov *, Konstantin A. Lomanovich and Dmitrii G. Mazhukin * N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Academician Lavrentiev Ave. 9, 630090 Novosibirsk, Russia; [email protected] (L.N.G.); [email protected] (K.A.L.) * Correspondence: [email protected] (A.Y.T.); [email protected] (D.G.M.); Tel.: +7-383-330-8867 (A.Y.T.); +7-383-330-6852 (D.G.M.) Abstract: In recent decades, bicyclic nitroxyl radicals have caught chemists’ attention as selective catalysts for the oxidation of alcohols and amines and as additives and mediators in directed C-H oxidative transformations. In this regard, the design and development of synthetic approaches to new functional bicyclic nitroxides is a relevant and important issue. It has been reported that imidazo[1,2- b]isoxazoles formed during the condensation of acetylacetone with 2-hydroxyaminooximes having a secondary hydroxyamino group are recyclized under mild basic catalyzed conditions to 8-hydroxy- 5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octenes. The latter, containing a sterically hindered cyclic N-hydroxy group, upon oxidation with lead dioxide in acetone, virtually quantitatively form sta- ble nitroxyl bicyclic radicals of a new class, which are derivatives of both 2,2,6,6-tetramethyl-4- oxopiperidine-1-oxyl (TEMPON) and 3-imidazolines. Citation: Grigor’eva, L.N.; Tikhonov, A.Y.; Lomanovich, K.A.; Mazhukin, Keywords: bicyclic nitroxide; condensation; acetylacetone; base-catalyzed recyclization; 3-imidazoline D.G. Stable Bicyclic Functionalized nitroxide; TEMPON; 8-hydroxy-5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octene Nitroxides: The Synthesis of Derivatives of Aza-nortropinone–5- Methyl-3-oxo-6,8-diazabicyclo[3.2.1]- 6-octene 8-oxyls. Molecules 2021, 26, 3050. https://doi.org/10.3390/ 1. Introduction molecules26103050 Among various types of organic radicals, nitroxyl radicals (NRs) are the most stable, most robust, and best-studied class of paramagnetic species. In addition to numerous Academic Editor: Mario Waser publications on their synthesis and properties [1–9], more and more reports include numer- ous examples of their use. Thus, NRs are actively employed as oxidants and as trapping Received: 20 April 2021 reagents in synthetic chemistry [10], as ligands in metal complex catalysis [11], as agents in Accepted: 18 May 2021 nitroxide-mediated radical polymerization [12], as molecular spin probes [13], and spin Published: 20 May 2021 labels [14] and in many other applications [15,16]. In the last decade, new areas of the use of nitroxides have been actively developing. Publisher’s Note: MDPI stays neutral For instance, NRs can participate in UV—or visible-light—excited photosynthesis owing with regard to jurisdictional claims in to their ability to participate in light-induced hydrogen atom transfer processes [17–19]. published maps and institutional affil- A popular NR, 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), has been employed as a iations. mediator for obtaining N-centered radicals during electrosynthesis. For example, TEMPO- mediated electrolysis of anilides or ketoximes generates amidyl or iminoxy radicals that trigger subsequent cyclization via an interaction of the forming radical with an unsaturated part of the molecule thereby leading to the closure of a pyrroline-type structure or cyclic Copyright: © 2021 by the authors. nitrone [20]. With a sterically more hindered dispirocyclic NR under the conditions of Licensee MDPI, Basel, Switzerland. electrochemical synthesis from sodium azide, it is possible to generate an azidyl radi- This article is an open access article cal, which at low temperatures can react twice with alkenes thus implementing alkene distributed under the terms and diazidation [21]. Taking into account all these “old” and new abilities of stable NRs, it conditions of the Creative Commons seems worthwhile to direct our efforts toward the design and synthesis of new classes Attribution (CC BY) license (https:// of functional nitroxides that may find applications in various modern fields of materials creativecommons.org/licenses/by/ chemistry and biology. 4.0/). Molecules 2021, 26, 3050. https://doi.org/10.3390/molecules26103050 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 11 Molecules 2021, 26, 3050 stable NRs, it seems worthwhile to direct our efforts toward the design and synthesis2 of 11of new classes of functional nitroxides that may find applications in various modern fields of materials chemistry and biology. BicyclicBicyclic NRsNRs (BNRs) have have been been known known for for a long a long tim time,e, starting starting from from the 1960s the 1960s (Fig- (Figureure 1, structures1 , structures A–GA);– however,G); however, their theiractive active use as use catalysts as catalysts for the foroxidation the oxidation of alcohols, of alcohols,in particular, in particular, those containing those containing a sterically a sterically hindered hindered hydroxyl hydroxyl group, began group, relatively began rel- re- ativelycently [ recently22–26]. Other [22–26 functional]. Other functional groups—primary groups—primary amines [27], amines diols [ 27[28],], diolsand cyclic [28], and car- cyclicbamates carbamates [29]—as [29it ]—asturned it turnedout, are out, also are prone also proneto oxidative to oxidative transfo transformationsrmations in the in pres- the presenceence of BNR of BNR mediators, mediators, forming forming nitriles nitriles and and lactone lactone derivatives. derivatives. Moreover, Moreover, BNRsBNRs werewere recentlyrecently shownshown toto be be superior superior additives additives for for enantioselective enantioselective Ni-catalyzed Ni-catalyzed diarylation diarylation of of vinylarenesvinylarenes [ 30[30]] and and as as mediators mediators for for electrochemical electrochemicalα-cyanation α-cyanat ofion secondary of secondary piperidines, piperi- morpholine,dines, morpholine, and pyrrolidine and pyrrolidine [31]. [31]. FigureFigure 1. 1.Selected Selected examples examples of of known known BNRs: BNRs: (A (A) Norpseudopelleterine) NorpseudopelleterineN -oxylN-oxyl [32 [32];]; (B ()B trimethyliso-) trimethyl- quinuclidineisoquinuclidineN-oxyl N-oxyl [33 ];[33]; (C) ( nortropaneC) nortropaneN-oxyl N-oxyl [34 ];[34]; (D )(D dimethylnortropinone) dimethylnortropinoneN-oxyl N-oxyl [35 [35];]; (E) ( 9-E) azabicyclo[3.3.1]nonane9-azabicyclo[3.3.1]nonaneN -oxylN-oxyl (ABNO) (ABNO) [ 36[36];]; ((FF)) 2-azaadamantane2-azaadamantane NN--oxyloxyl (AZADO) (AZADO) [37]; [37 ];and and (G(G)) 9-azanoradamantane 9-azanoradamantaneN N-oxyl-oxyl (nor-AZADO) (nor-AZADO) [ 38[38].]. ( H(H)) The The BNRs BNRs obtained obtained in in this this work. work. InIn thethe presentpresent work,work, wewe reportreport thethe firstfirst synthesissynthesis ofof BNRsBNRs thatthat areare derivativesderivatives ofof 5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octene5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octene 8-oxyl, combining8-oxyl, 2,2,6,6-tetramethyl-4-combining oxopiperidine-1-oxyl2,2,6,6-tetramethyl-4- (TEMPON)oxopiperidine and-1 3-imidazoline-oxyl (TEMPON) NR moietiesand 3-imidazoline (Figure1, structure NR moietiesH). (FigureThis 1, paper structure is a tributeH). to Professor Leonid B. Volodarsky who was the founder of the schoolThis of stable paper NRs is a oftribute the imidazoline to Professor series Leonid and B. has Volodarsky inspired ourwho research was the by founder his brilliant of the ideasschool for of over stable 40 years.NRs of the imidazoline series and has inspired our research by his bril- liant ideas for over 40 years. 2. Results 2.1. The Synthesis of Bicyclic Nitroxides 2. Results Previously, it has been demonstrated that the reaction of acetylacetone with 2-hydrox- 2.1. The Synthesis of Bicyclic Nitroxides yamino oximes having a hydroxyamino group at the secondary carbon atom, 1a–d, pro- ceedsPreviously, through a number it has ofbeen intermediates demonstrated in tautomeric that the equilibriumreaction of withacetylacetone each other andwith finally2-hydroxyamino leads to derivatives oximes having of tetrahydroimidazo[1,2- a hydroxyamino groupb]isoxazoles at the secondary2a–d [39]. carbon Refluxing atom, of the1a– latterd, proceeds in aqueous through alkali a number is accompanied of intermediates by the loss in tautomeric of a water moleculeequilibrium followed with each by recyclization,other and fin whichally leads entails to thederivatives opening of thetetrahydroimidazo[1,2 five-membered heterocycle-b]isoxazoles and the2a– forma-d [39]. tionRefluxing of a six-membered of the latter tetrahydropyridinein aqueous alkali is derivativeaccompanied (cyclic by enaminoketonethe loss of a water3a– dmolecule) as the mainfollowed product by recyclization, and 3-acetyl-1-hydroxypyrrole which entails the4a opening–d as a minorof the product.five-membered We found heterocycle that this alkali-catalyzedand the formation reaction of a forsix compounds-membered 2atetrahydropyridine,c,d under much milder derivative conditions (cyclic (cooling enamino- at 0–5ketone◦C, 3–43a–d h)) yieldsas the amain completely product different and 3-acetyl result:-1- bicyclehydroxypyrrole5 becomes 4a the–d main as a product.minor prod- Its structureuct. We wasfound proved that onthis the alkali basis-catalyzed of spectral reaction and analytical for compounds data. For instance,2a,c,d under IR spectra
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