One-Pot Synthesis of Triazolobenzodiazepines Through

Communicationmolecules One-Pot Synthesis of Triazolobenzodiazepines CommunicationThrough Decarboxylative [3 + 2] Cycloaddition of One-Pot Synthesis of Triazolobenzodiazepines ThroughNonstabilized Decarboxylative Azomethine [3 Ylides + 2] Cycloaddition and Cu-Free ofClick NonstabilizedReactions Azomethine Ylides and Cu-Free Click Reactions Xiaoming Ma 1,†, Xiaofeng Zhang 2, †, Weiqi Qiu 2, Wensheng Zhang 3, Bruce Wan 2, Jason Evans 2 and Wei Zhang 2,* Xiaoming Ma 1,†, Xiaofeng Zhang 2,†, Weiqi Qiu 2, Wensheng Zhang 3, Bruce Wan 2, Jason1 School Evans of Pharmaceutical2 and Wei Zhang Engineering2,* and Life Science, Changzhou University, Changzhou 213164, China; [email protected] 1 School of Pharmaceutical Engineering and Life Science, Changzhou University, Changzhou 213164, China; 2 Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston, MA [email protected] 02125, USA. [email protected] (X.Z.); [email protected] (W.Q.); [email protected] 2 Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston, MA 02125, (B.W.); [email protected] (J.E.) USA; [email protected] (X.Z.); [email protected] (W.Q.); [email protected] (B.W.); 3 [email protected] of Science, Jiaozuo (J.E.) Teachers’ College, 998 Shanyang Road, Jiaozuo 454100, China; 3 [email protected] of Science, Jiaozuo Teachers’ College, 998 Shanyang Road, Jiaozuo 454100, China; [email protected] ** Correspondence:Correspondence: [email protected]; [email protected]; Tel.: Tel.: +1-617-287-6147 +1-617-287-6147 † These These authors authors contribute contributedd equally equally to to this this work. work. Received: 16 January 2019; Accepted: 5 February 2019; Published: 8 February 2019 Received: 16 January 2019; Accepted: 5 February 2019; Published: 8 February 2019 Abstract: A one-pot synthesis of triazolobenzodiazepine-containing polycyclic compounds is Abstract:introduced.A The one-pot reaction synthesis process ofinvolves triazolobenzodiazepine-containing a decarboxylative three-component polycyclic [3 + 2] compounds cycloaddition is introduced.of nonstabilized The reactionazomethine process ylides, involves N-propargylation, a decarboxylative and three-componentintramolecular click [3 + reactions. 2] cycloaddition of nonstabilized azomethine ylides, N-propargylation, and intramolecular click reactions. Keywords: one-pot synthesis; decarboxylative [3 + 2] cycloaddition; nonstabilized azomethine Keywords:ylides; clickone-pot reaction synthesis; decarboxylative [3 + 2] cycloaddition; nonstabilized azomethine ylides; click reaction

1. Introduction 1. Introduction Triazolobenzodiazepines and related scaffolds are privileged heterocyclic systems for Triazolobenzodiazepines and related scaffolds are privileged heterocyclic systems for biologically biologically active molecules, such as benzodiazepine-bearing bretazenil [1], midazolam [2]; active molecules, such as benzodiazepine-bearing bretazenil [1], midazolam [2]; protease inhibitors [3], protease inhibitors [3], alprazolam [4], estazolam [5], and triazolam [6] (Figure 1). Due to their alprazolam [4], estazolam [5], and triazolam [6] (Figure1). Due to their medicinal signiﬁcance, the medicinal significance, the development of synthetic methods for triazolobenzodiazepine-bearing development of synthetic methods for triazolobenzodiazepine-bearing compounds continuously compounds continuously attracts the attention of organic and medicinal chemists [7–9]. attracts the attention of organic and medicinal chemists [7–9].

Figure 1. Biologically active triazolobenzodiazepine-related molecules. Figure 1. Biologically active triazolobenzodiazepine-related molecules. Highly efﬁcient and atom economic synthesis such as one-pot reactions and multicomponent reactionsHighly (MCRs) efficient have and gained atom increasing economic popularity synthesis in such the synthesizing as one-pot reactions of complex and molecules multicomponent including triazolobenzodiazepine-typereactions (MCRs) have gained compounds increasing [10 popularity–15]. For example, in the synthesizing the Martin group of complex reported molecules a cascade reductiveincluding triazolobenzodiazepine-type amination and intramolecular compounds [3 + 2] cycloaddition [10–15]. For reactionexample, sequence the Martin for group triazole-fused reported 1,4-benzodiazepines (Scheme1A) [ 10,11]. The Djuric group modiﬁed the van Leusen imidazole synthesisMolecules 2019 to, develop24, x; an intramolecular azide-alkyne cycloaddition for imidazole- www.mdpi.com/j and triazole-fusedournal/molecules benzodiazepine compounds (Scheme1B) [ 12]. The Kurth group reported a Lewis acid-catalyzed

Molecules 2019, 24, 601; doi:10.3390/molecules24030601 www.mdpi.com/journal/molecules MoleculesMolecules 2019 2019, ,24 24, ,x x 22 ofof 88 aa cascadecascade reductivereductive aminationamination andand intramolecularintramolecular [3[3 ++ 2]2] cycloadditioncycloaddition reactionreaction sequencesequence forfor triazole-fusedtriazole-fused 1,4-benzodiazepines1,4-benzodiazepines (Scheme(Scheme 1A)1A) [10,[10,11].11]. TheThe DjuricDjuric groupgroup modifiedmodified thethe vanvan LeusenLeusen imidazoleimidazole synthesissynthesis toto developdevelop anan intramolecularintramolecular azide-alkyneazide-alkyne cycloadditioncycloaddition forfor imidazole-imidazole- andand triazole-fusedtriazole-fusedMolecules 2019, 24 benzodiazepine,benzodiazepine 601 compoundscompounds (Schem(Schemee 1B)1B) [12].[12]. TheThe KurthKurth groupgroup reportedreported aa LewisLewis2 of 7 acid-catalyzedacid-catalyzed MCRMCR forfor imidazole-imidazole- andand triazole-fusedtriazole-fused benzodiazepinesbenzodiazepines throughthrough sequentialsequential [3[3 ++ 2]2] cycloadditioncycloaddition andand cycloadditiocycloadditionn reactionsreactions (Scheme(Scheme 1C)1C) [13].[13]. IntroducedIntroduced inin thisthis paperpaper isis aa newnew MCR for imidazole- and triazole-fused benzodiazepines through sequential [3 + 2] cycloaddition sequencesequence involvinginvolving decarboxylativedecarboxylative intermolecularintermolecular [3[3 ++ 2]2] cycloadditioncycloaddition ofof nonstabilizednonstabilized and cycloaddition reactions (Scheme1C) [ 13]. Introduced in this paper is a new sequence involving azomethineazomethine ylidesylides followedfollowed byby NN-propargylation-propargylation andand intramolecularintramolecular [3[3 ++ 2]2] cycloadditioncycloaddition forfor decarboxylative intermolecular [3 + 2] cycloaddition of nonstabilized azomethine ylides followed by triazolobenzodiazepinestriazolobenzodiazepines (Scheme (Scheme 1D).1D). N-propargylation and intramolecular [3 + 2] cycloaddition for triazolobenzodiazepines (Scheme1D).

SchemeScheme 1. 1. Atom AtomAtom economic economic economic synthesis synthesis of of triazolobenzodiazepines. triazolobenzodiazepines.

1,3-Dipolar1,3-Dipolar cycloadditioncycloaddition ofof primaryprimary aminoamino esters,esters, aldehydes,aldehydes, andand and activatedactivated alkenesalkenes isis aa well-establishedwell-established three-componentthree-componentthree-component reaction reactionreaction [16– 21[1[1].6–21].6–21]. The azomethine TheThe azomethineazomethine ylides derived ylidesylides from derivedderived deprotonation fromfrom deprotonationof iminium ions of are iminium CO R-stabilized ions are CO ylides2R-stabilized A (Figure ylides2A) [ 22 A– (Figure30]. In recent2A) [22–30]. years, ourIn recent lab has years, reported our deprotonation of iminium2 ions are CO2R-stabilized ylides A (Figure 2A) [22–30]. In recent years, our lablaba series hashas reportedreported of azomethine aa seriesseries ylides ofof azomethine azomethine A-based [3 ylidesylides + 2]cycloadditions A-basedA-based [3[3 + + 2] 2] for cycloadditions cycloadditions diverse heterocyclic forfor diversediverse scaffolds heterocyclicheterocyclic [31–35], scaffoldsscaffoldsincluding [31–35],[31–35], one-pot [3includingincluding + 2] and clickone-potone-pot reactions [3[3 ++ 2] for2] and triazolobenzodiazepinesand clickclick reactionsreactions forfor triazolobenzodiazepines [triazolobenzodiazepines32]. Compared to the reactions [32].[32]. ComparedComparedof stabilized toto ylides thethe reactionsreactions A, cycloadditions ofof stabilizedstabilized of nonstabilized ylidesylides A,A, cycloadditionscycloadditions ylides B are less ofof explorednonstabilizednonstabilized (Figure ylidesylides2B) [ 36BB –areare42 ]. lessless We α exploredexploredhave recently (Figure(Figure reported 2B)2B) the[36–42].[36–42]. synthesis WeWe ofhavehave-trifluoromethyl recentlyrecently reportedreported pyrrolidines thethe synthesissynthesis through decarboxylativeofof αα-trifluoromethyl-trifluoromethyl [3 + 2] α pyrrolidinespyrrolidinescycloaddition throughthrough of nonstabilized decarboxylativedecarboxylative azomethine [3[3 ++ ylides 2]2] cyclcycl Boaddition derivedoaddition from ofof nonstabilizednonstabilized-amino acids azomethine [azomethine43]. Presented ylidesylides in this BB derivedderivedpaper is from afrom new αα application-amino-amino acidsacids of nonstabilized [43].[43]. PresentedPresented azomethine inin thisthis paperpaper ylides is inis a thea newnew one-pot applicationapplication [3 + 2] and ofof nonstabilized clicknonstabilized reactions azomethineazomethinefor triazolobenzodiazepines. ylidesylides in in thethe one-potone-pot [3[3 + + 2] 2] and and clickclick reactionsreactions forfor triazolobenzodiazepines.triazolobenzodiazepines.

Figure 2. Azomethine ylides from amino esters or amino acid. FigureFigure 2. 2. Azomethine Azomethine ylides ylides from from amino amino esters esters or or amino amino acid. acid. 2. Results and Discussions Reaction conditions for the synthesis of proline 4a through one-pot [3 + 2] cycloaddition were developed using 1:1.2:1 of 2-azidebenzaldehyde 1a, 2-aminoisobutyric acid 2a, and N-ethylmaleimide 3a in the presence of 0.3 equiv. of AcOH for decarboxylation [43] (Table1). After screening Molecules 2019, 24, x 3 of 8

2. Results and Discussions Reaction conditions for the synthesis of proline 4a through one-pot [3 + 2] cycloaddition were Moleculesdeveloped2019 , 24using, 601 1:1.2:1 of 2-azidebenzaldehyde 1a, 2-aminoisobutyric acid 2a, 3and of 7 N-ethylmaleimide 3a in the presence of 0.3 equiv. of AcOH for decarboxylation [43] (Table 1). After screening solvents including 2-methyltetrahydrofuran, toluene, EtOH and CH3CN as well as solvents including 2-methyltetrahydrofuran, toluene, EtOH and CH3CN as well as reaction time and reaction time and temperature, it was found that a reaction using CH3CN as a solvent at 110 °C for 6 temperature, it was found that a reaction using CH CN as a solvent at 110 C for 6 h afforded 4a in 93% h afforded 4a in 93% LC (liquid chromatography)3 yield with a dr (diastereomer)◦ of 6:1 (Table 1, entry LC (liquid chromatography) yield with a dr (diastereomer) of 6:1 (Table1, entry 6). The stereochemistry 6). The stereochemistry of 4a was determined according to the literature report [38]. of 4a was determined according to the literature report [38].

Table 1. Three-component decarboxylative [3 + 2] cycloaddition a. Table 1. Three-component decarboxylative [3 + 2] cycloaddition a. O CHO Et N3 AcOH (0.3 eq) Et N NH O N O + + H2N CO2H solvent N3 O T, t 1a 2a 3a 4a

◦ b c Entry ntrySolvent Solvent T T (°C) ( C) t (h)t (h) 4a (%)4a (%) b dr (%) dr (%) c 1 12-Me THF 2-Me THF80 80 44 tracetrace — — 2 2MePh MePh110 110 44 tracetrace — — 3 3EtOH EtOH80 80 44 8282 5:1 5:1 4 4EtOH EtOH110 110 66 9393 6:1 6:1 5 5CH3CN CH 3CN110 110 44 9292 6:1 6:1 6 CH CN 110 6 93 6:1 6 CH3CN 3 110 6 93 6:1 7 CH3CN 125 12 88 6:1 7 CH3CN 125 12 88 6:1 a Reactiona Reaction conditions: conditions: 1:1.2:1 1:1.2:1 1a1a:2a:2a:3a:3a forfor [3 [3 + + 2] 2] cycloaddition. b bDetected Detected by by LC-MS. LC-MS.c Determined c Determined by 1H by NMR. 1H NMR.

DecarboxylativeDecarboxylative [3[3 + 2]+ cycloaddition2] cycloaddition product product4a was 4a then was used then for used the development for the development of conditions of forconditions the N-propargylation for the N-propargylation and sequential clickand reaction sequential for the click synthesis reaction of triazolobenzodiazepine for the synthesis 6aof. triazolobenzodiazepine 6a. In the presence of K2CO3, 4a reacted with propargyl bromide in CH3CN In the presence of K2CO3, 4a reacted with propargyl bromide in CH3CN at 80 ◦C for 2 h to give 5a in 94% LCat 80 yield °C (Tablefor 2 h2 ,to entries give 5a 2–5). in Without94% LC separation,yield (Table the 2, reactionentries 2–5). mixture Without was usedseparation, for intramolecular the reaction mixture was used for intramolecular click reaction at 100 °C under the catalysis of Cu salts (Table 2, click reaction at 100 ◦C under the catalysis of Cu salts (Table2, entries 2–4). The CuI-catalyzed click reactionentries 2–4). gave 6aThein CuI-catalyzed 89% LC yield, whichclick reaction is better thangave the 6a reactionsin 89% LC catalyzed yield, withwhich CuCl is better or CuBr. than In ourthe previousreactions work,catalyzed the intramolecularwith CuCl or CuBr. click In reaction our prev wasious accomplished work, the intramolecular under microwave click reaction heating andwas Cu-freeaccomplished conditions under [32 microwave]. In this work, heatingN-propargylation and Cu-free conditions compound [32].5a generated In this work, under N-propargylation the microwave compound 5a generated under the microwave heating was continuously heated at 150 °C for 1 h to heating was continuously heated at 150 ◦C for 1 h to give 6a in 88% LC yield without CuI catalyst Moleculesgive 6a 2019in 88%, 24, LCx yield without CuI catalyst (Table 2, entry 6). A Cu-free control reaction of 5a under4 of 8 (Table2, entry 6). A Cu-free control reaction of 5a under conventional heating at 100 ◦C for 3 h only gaveconventional 5% of 6a heating(Table2 ,at entry 100 °C 5). for 3 h only gave 5% of 6a (Table 2, entry 5). Table 2. One-pot N-propargylation and click reaction a. Table 2. One-pot N-propargylation and click reaction a. O O O Et Et N NH Br Et N N click N N N 3 K2CO3 N3 cat. (0.5 eq) O O T , t O T , t 1 1 2 2 N N N 4a 5a 6a ◦ b ◦ b EntryEntry Solvent Solvent T1 (°C) T1 ( C)t1 (h) t1 (h) 5a (%)5a (%) b Cat. Cat.T2 (°C) T2 ( C)t2 (h) t 2 (h) 6a (%)6a (%) b 1 1EtOH EtOH 80 80 2 2trace trace 2 2CH CH3CN3CN 80 80 2 2 94 94CuCl CuCl 100 1003 3 35 35 3 3CH CH3CN3CN 80 80 2 2 94 94CuBr CuBr 100 1003 3 60 60 4 4CH CH3CN3CN 80 80 2 2 94 94 CuI CuI100 100 3 3 89 89 5 CH3CN 80 2 94 — 100 3 5 5 c CH3CN 80 2 94 — 100 3 5 6 CH3CN 110 0.5 93 — 150 1 88 (dr 6:1) 6 c CH3CN 110 0.5 93 — 150 1 88 (dr 6:1) a ReactionReaction conditions:conditions: K 2KCO2CO3 (2.53 (2.5 equiv.) equiv.) and and propargyl propargyl bromide bromide (5.0 equiv.) (5.0 underequiv.) conventional under conventional or microwave or microwaveheating. b Detected heating. by LC-MS.b Detectedc Microwave by LC-MS. heating c Microwave for both N -propargylationheating for both and N click-propargylation reactions. and click reactions. After establishing the three-component [3 + 2] cycloaddition, N-propargylation, and sequential clickAfter reactions establishing for 6a shown the three-component in Tables1 and2, [3 we + then2] cycloaddition, aimed to combine N-propargylation these three reactions, and sequential in one pot.click Afterreactions modiﬁcation for 6a shown of the in conditions Tables 1 and shown 2, we in then Tables aimed1 and to2 ,combine the best these conditions three forreactions the one-pot in one pot. After modification of the conditions shown in Tables 1 and 2, the best conditions for the one-pot synthesis was to conduct the decarboxylative [3 + 2] cycloaddition in MeCN under conventional heating at 110 °C for 6 h, then to perform the N-propargylation and spontaneous Cu-free click reaction under microwave heating at 150 °C for 1 h to give 6a in 76% LC yield (Table 3, entry 3). A control reaction using CuI as a catalyst for the click reaction didn’t give a better yield (Table 3, entry 4).

Table 3. Conditions for the one-pot synthesis of 6a a.

CHO O N3 Et H N CO H Br N 2 2 AcOH cat. N + 2a μw, T , t 1a CH3CN K2CO3 2 2 O Et 癈 μ 110 , 6 h w, T1, t1 N N O N O N

3a 6a

Entry T1 (°C) t1 (h) Cat. T2 (°C) t2 (h) 6a (%) b 1 110 0.5 — 150 1 75 2 150 0.5 — 150 1 51 76 (dr 6:1) 3 150 1 — 150 1

4 110 0.5 CuI 110 1 70

a Reaction conditions: 1:1.2:1 1a:2a:3a, K2CO3 (2.5 equiv.), propargyl bromide (5 equiv.). b Detected by LC-MS, 6:1 dr.

Int. J. Mol. Sci. 2018, 19, 3891 3 of 16

had a comparable effect to that of GdCl3 administration. We published quite dramatic changes in gene expression of proteins considered to be involved in hepatic iron metabolism and in hepatic gene expression of acute-phase cytokines, and demonstrated that Kupffer cells are involved in this process [22]. Zymosan is known to trigger an inflammatory response through the activation of Kupffer cells in the liver while the precise effects of GdCl3 on Kupffer cells are not still defined. The current study aims to further extend the understanding of the changes in gene expression of the main CXC- and CC-chemokines as well as of adhesion molecules induced by the uptake of corpuscular matter by the liver macrophages. Moreover, it may be responsible for the recruitment of neutrophil granulocytes causing a possible interaction of the Kupffer cells and granulocytes. To further expand our findings and to rule out any changes caused by endotoxin contamination in our rat model we used C3H/HeJ mice. These mice have a missense mutation in the third exon of toll-like receptor 4 (TLR-4), yielding a non-functional TLR-4; they are hypo-responsive to the effects of lipopolysaccharide and are resistant to lethal endotoxin-induced shock as compared with normal mice. One of the main objectives of this work, however, is to introduce a caveat for the interpretation of data obtained by injecting different materials intraperitoneally without taking into account the changes in expression of certain genes induced in the liver.

2. Results

2.1. Immunofluorescence Analysis of the Rat Liver Following Intraperitoneal Injection of GdCl3 or Zymosan Immunofluorescence double staining showed increased numbers of myeloperoxidase positive + (MPO ) cells (recruited granulocytes) as early as 3 h (hours) after GdCl3 administration (Figure1B) compared to untreated animals (Figure1A) while the number of MPO + cells decreased at 24 h after + Molecules 2019, 24treatment, 601 (Figure1C). MPO cells were mainly located near the portal vessel and in close vicinity to 4 of 7 the liver macrophages. The same liver sections showed a progressive reduction of ED-1 positivity after GdCl3 administration, mainly near the portal vessel (Figure1) while ED-2 positivity remained unchanged (Figure2). Double immunofluorescence staining showed few ED-1 + and MPO+, and ED-2+ synthesis wasand to MPO conduct+ cells inthe close decarboxylative contact to each other near [3 + the 2] portal cycloaddition area (Figures1 and in2 MeCN). under conventional heating at 110 ◦CDouble for 6 immunofluorescenceh, then to perform staining the N of-propargylation liver sections after Zymosan and spontaneous treatment with Cu-free antibodies click reaction against ED-1, ED-2 and MPO showed an increased number of MPO+ cells at 3 h (Figure3B,D), under microwave heating at 150 ◦C for 1 h to give 6a in 76% LC yield (Table3, entry 3). A control which were located near the portal vessels but also through the liver parenchyma, compared to reaction usingcontrols CuI as (Figure a catalyst3A). Double for theimmunofluorescence click reaction staining didn’t showed give a few better MPO yield+/ED-1 (Table+ as well3, entry as 4). MPO+/ED-2+ cells near the portal area (Figure3A–D). Table 3. Conditions for the one-pot synthesis of 6a a.

◦ ◦ b EntryFigure 1. Double T1 immunofluorescence( C) t1 (h) staining of Cat. rat liver sections T2 ( (GdClC)3 treatment) t2 (h) with antibodies6a (%) directed against ED-1 (green) and MPO (red) followed by fluorescence immunodetection. Liver sections from1 different time 110 points of study 0.5 are shown: control — (A); 3 h 150 (B) and 24 h (C). 1 Results shown are 75 representative2 pictures 150 of six animals 0.5 and six slides — per time point. 150 (Original magnification 1 200 ). 51 × 3 150 1 — 150 1 76 (dr 6:1) 4 110 0.5 CuI 110 1 70

a b Reaction conditions: 1:1.2:1 1a:2a:3a, K2CO3 (2.5 equiv.), propargyl bromide (5 equiv.). Detected by LC-MS, 6:1 dr.

Under the optimized conditions for the one-pot synthesis [44], 13 analogues of triazolobenzodiazepines 6a–m were synthesized using different sets of azidobenzaldehydes 1 (R1 = H, 2 3 4 CF3, Br, Cl, NO2), amino acids 2 (R = H, Me; R = Me, Ph, i-Pr), and maleimides 3 (R = Me, Et, Ph, Bn, 4-Br-Ph) (Table4). The reactions of ﬁve different maleimides with 2-aminoisobutyric acids and 2-azidebenzaldehyde gave 6a–e in 55–65% isolated yields. The substitution groups on the benzaldehydes had some inﬂuence on the product yield. For example, the azidobenzaldehydes bearing electron-withdrawing groups, such as Br and CF3, gave 6f and 6g in lower yields (59% and 35%), while Molecules 2019, 24, x 5 of 8 the azidobenzaldehyde with the strong electron-withdrawing group NO2 gave no product of 6m. The 1 4 reactionsmaleimides of glycine (R4 = Me, and Et) leucine gave with6h–l in azidobenzaldehydes 44–55% yields. The stereochemistry (R = H, Br, Cl) of andproduct maleimides 6 was (R = Me, Et)established gave 6h–l duringin 44–55% the step yields. of the The decarboxylative stereochemistry [3 + of2] cycloaddition, product 6 was which established was determined during the step of theaccording decarboxylative to the literature [3 + 2] report cycloaddition, [38]. which was determined according to the literature report [38].

Table 4. 4. One-potOne-pot synthesis synthesis of triazolobenzodiazepines of triazolobenzodiazepines 6 a. 6 a.

a Reaction conditions, see [44]. ab ReactionIsolated yield. conditions, see [44]. b Isolated yield.

The proposed mechanism for the synthesis of product 6a is outlined in Scheme 2. The condensation of 2-azidebenzaldehyde 1a and 2-aminoisobutyric acid 2a give oxazolidin-5-one I. It then underwent decarboxylation to form the nonstabilized azomethine ylide II for [3 + 2] cycloaddition with 3a to form 4a. Formation of 5a through propargylation followed by continuous heating for intramolecular click reaction affords product 6a. There are several reports in literature which demonstrated that intramolecular click reactions in one-pot synthesis could be achieved under Cu-free conditions [10,15,32,45,46].

Scheme 2. Mechanism for one-pot synthesis of 6a.

3. Summary A one-pot synthesis of fused-triazolobenzodiazepines was developed using readily available amino acids, maleimides, and 2-azidebenzaldehydes for decarboxylative [3 + 2] cycloaddition of nonstabilized azomethine ylides, followed by N-propargylation and a Cu-free intramolecular click

Molecules 2019, 24, x 5 of 8

maleimides (R4 = Me, Et) gave 6h–l in 44–55% yields. The stereochemistry of product 6 was established during the step of the decarboxylative [3 + 2] cycloaddition, which was determined according to the literature report [38].

Table 4. One-pot synthesis of triazolobenzodiazepines 6 a.

a b Molecules Reaction2019, 24, conditions, 601 see [44]. Isolated yield. 5 of 7

The proposed mechanism for the synthesis of product 6a is outlined in Scheme 2. The condensationThe proposed of 2-azidebenzaldehyde mechanism for the synthesis 1a and of2-aminoisobutyric product 6a is outlined acid in2a Schemegive oxazolidin-5-one2. The condensation I. It ofthen 2-azidebenzaldehyde underwent decarboxylation1a and 2-aminoisobutyric to form the acidnonstabilized2a give oxazolidin-5-one azomethine ylideI. It thenII for underwent [3 + 2] decarboxylationcycloaddition with to form 3a to the form nonstabilized 4a. Formation azomethine of 5a through ylide II propargylationfor [3 + 2] cycloaddition followed withby continuous3a to form 4aheating. Formation for intramolecular of 5a through click propargylation reaction affords followed product by continuous 6a. There are heating several for intramolecularreports in literature click reactionwhich demonstrated affords product that6a. Thereintramolecular are several reportsclick re inactions literature in whichone-pot demonstrated synthesis could that intramolecular be achieved clickunder reactions Cu-free inconditions one-pot synthesis[10,15,32,45,46]. could be achieved under Cu-free conditions [10,15,32,45,46].

Scheme 2. Mechanism for one-pot synthesis of 6a6a..

3. Summary 3. Summary A one-pot synthesis of fused-triazolobenzodiazepines was developed using readily available amino A one-pot synthesis of fused-triazolobenzodiazepines was developed using readily available acids, maleimides, and 2-azidebenzaldehydes for decarboxylative [3 + 2] cycloaddition of nonstabilized amino acids, maleimides, and 2-azidebenzaldehydes for decarboxylative [3 + 2] cycloaddition of azomethine ylides, followed by N-propargylation and a Cu-free intramolecular click reaction. This is a nonstabilized azomethine ylides, followed by N-propargylation and a Cu-free intramolecular click highly efficient and operational simple reaction process for fused-triazolobenzodiazepines, and only

CO2 and H2O were generated as byproducts.

Supplementary Materials: The following are available online. 1H-NMR, 13C-NMR, and 19F-NMR spectra of final products. Author Contributions: X.M. and X.F. developed above reactions; W.Q., W.Z., and B.W. expanded the substrates scope; X.F. and W.Z. conceived the project; W.Z. supervised the project and revised the manuscript. Acknowledgments: X.M. acknowledges the sponsorship of Jiangsu oversea Visiting Scholar Program for University Prominent Youth & Middle-aged Teachers and Presidents. We also acknowledge John Mark Awad and Guoshu Xie’s involvement in this project. Conflicts of Interest: The authors declare no conflict of interest.

References and Notes

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Sample Availability: Samples of the compounds are available from the authors.

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