Structural Identification Between Phthalazine-1,4-Diones and N-Aminophthalimides Via Vilsmeier Reaction

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Article Structural Identiﬁcation between Phthalazine-1,4-Diones and N-Aminophthalimides via Vilsmeier Reaction: Nitrogen Cyclization and Tautomerization Study

Cheng-Yen Chung 1,2, Ching-Chun Tseng 1,2, Sin-Min Li 3, Shuo-En Tsai 1,2, Hui-Yi Lin 1,* and Fung Fuh Wong 1,*

1 School of Pharmacy, China Medical University, No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan; [email protected] (C.-Y.C.); [email protected] (C.-C.T.); [email protected] (S.-E.T.) 2 The Ph.D. Program for Biotech Pharmaceutical Industry and School of Pharmacy, China Medical University, No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan 3 Institute of New Drug Development, China Medical University, No. 91 Hsueh-Shih Rd., Taichung 40402, Taiwan; [email protected] * Correspondence: [email protected] (H.-Y.L.); [email protected] or [email protected] (F.F.W.); Tel.: +886-422-053-366 (ext. 5112) (H.-Y.L.); +886-422-053-366 (ext. 5603) (F.F.W.); Fax: +886-422-078-083 (H.-Y.L. & F.F.W.)

Abstract: N-Aminophthalimides and phthalazine 1,4-diones were synthesized from isobenzofuran- 1,3-dione, isoindoline-1,3-dione, furo [3,4-b] pyrazine-5,7-dione, or 1H-pyrrolo [3,4-c] pyridine-1,3- dione with monohydrate hydrazine to carry out the 5-exo or 6-endo nitrogen cyclization under the different reaction conditions. Based on the control experimental results, 6-endo thermodynamic hydrohydrazination and kinetical 5-exo cyclization reactions were individually selective formation. Subsequently, Vilsmeier amidination derivatization was successfully developed to probe the struc- Citation: Chung, C.-Y.; Tseng, C.-C.; tural divergence between N-aminophthalimide 2 and phthalazine 1,4-dione 3. On the other hand, Li, S.-M.; Tsai, S.-E.; Lin, H.-Y.; Wong, F.F. Structural Identification between the best tautomerization of N-aminophthalimide to diazinone was also determined under acetic acid Phthalazine-1,4-Diones and mediated solution. N-Aminophthalimides via Vilsmeier Reaction: Nitrogen Cyclization and Keywords: vilsmeier reagent; phthalazine-1,4-diones; N-aminophthalimide; hydrazine Tautomerization Study. Molecules 2021, 26, 2907. https://doi.org/ 10.3390/molecules26102907 1. Introduction Academic Editor: Fangrui Zhong Nitrogen-containing heterocyclic compounds are widely applied to the biologically active pharmaceuticals, agrochemicals, and functional materials and become more and Received: 25 April 2021 more important [1–5]. Especially, heterocycle derivatives containing bridgehead amine Accepted: 10 May 2021 and hydrazine [6,7] such as N-aminophthalimides [8] and phthalazine 1,4-diones [9] have Published: 13 May 2021 received considerable attention. Therefore, the development of new efficient methods to synthesize N-heterocycles with structural diversity is one major interest of modern Publisher’s Note: MDPI stays neutral synthetic organic chemists [10–12]. with regard to jurisdictional claims in published maps and institutional affil- Heterocycles containing phthalazine 1,4-dione moiety have been reported to possess iations. different pharmacological properties including anti-inflammatory, cardiotonic vasorelaxant, anticonvulsant [13], antihypertensive [14], antibacterial [15], anti-cancer [16], and carbonic anhydrase enzyme activity [17]. On the other hand, phthalimide group was conceived as a nitrogen source [18], for the direct introduction of masked amino function via the classical Gabriel protocol [19,20] as well as for the protection of amino groups [21–23]. Copyright: © 2021 by the authors. N-aminophthalimides can be considered as phthalazine 1,4-dione tautomeric pairs. The Licensee MDPI, Basel, Switzerland. structural arrangement of hydrazine derivatives is the mainly associated with the intercon- This article is an open access article version of imine−enamine [24,25]. Herein, we selectively synthesize N-aminophthalimide distributed under the terms and conditions of the Creative Commons and phthalazine 1,4-dione derivatives in via the thermodynamic-kinetic control conditions. Attribution (CC BY) license (https:// They will provide as the precursors for constructing the pharmacological heterocyclic creativecommons.org/licenses/by/ compounds (PDE5 inhibitors) [26,27] or the chemiluminescent luminol derivatives [28]. 4.0/).

Molecules 2021, 26, 2907. https://doi.org/10.3390/molecules26102907 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 2907 2 of 15

Owing to the structural divergence between N-aminophthalimides 2 and phthalazine-1,4- diones 3, we explored the Vilsmeier amidination derivatization to identify them in this work [29–33]. Furthermore, we successfully developed the prototrophic tautomeric interconversion from N-aminophthalimides to phthalazine 1,4-diones under Brønsted–Lowry acidic condition [29–35].

2. Results and Discussion Initially, isobenzofuran-1,3-diones (1a–c), isoindoline-1,3-dione (1d), furo [3,4-b] pyrazine- 5,7-dione (1e), and 1H-pyrrolo[3,4-c] pyridine-1,3-dione (1f)[34–36] were purchased or prepared as the starting materials. Reacting compounds 1a–f with monohydrate hydrazine in ethanol solution at low (at 0 ◦C or –20 ◦C) or room temperature led directly to the 5-exo cyclization N-aminophthalimide products 2a–f (81–94%) without the accumulation of hydrohydrazination products 3a–f (Table1). On the other hand, the 6-exo cyclization of distal nitrogen instead of proximal one was exclusively observed at reflux for ~4 h, and the corresponding 6-endo phthalazine 1,4-dione products 3a–f were formed (83–91%, Table1). Fortunately, compounds 2a–f and 3a–f can be selectively prepared via kinetic and thermodynamic control reaction with hydrazine hydrate. From the fundamental perspective, these symmetric molecules of N-aminophthalimide products 2a–f provide themselves for investigation of tautomeric conversion controlling processes as well as convenient platforms for the structural identity. Additionally, phthalazine 1,4-diones 3a–f were used as the reference standards. Al- though compounds 2 and 3 were tautomeric pairs, they significantly possessed the different polarity, such as, Rf = 0.33 for N-aminophthalimide 2a and Rf = 0.41 for phthalazine 1,4-dione 3a (EA/MeOH = 9/1). All of the 2a–f and 3a–f compounds were fully characterized by spectroscopic methods. For example, compound 2a presented peaks at 3447 and −1 −1 3482 cm for stretching of the –NH2 group and at 1020 cm for stretching of the N–N group in FT-IR spectrum. For compound 3a, its IR absorption peaks were at 3461 cm−1 for stretching of the –NH–NH– group and at 1051 cm−1 for stretching of the N–N group. In 1H-NMR, compounds 2 and 3 presented similar chemical shift and coupling constants, resulting in difficult structural identification of each other. For example, the 1H-NMR spectra of compounds 2a and 3a were similar as shown in Figure1. This observation drove us to develop a novel identification method. On the other hand, N-aminophthalimides 2c and 2e reveal the existence of intramolecular hydrogen bonding phenomenon between carbonyl and amino group in 1H-NMR spectra. This phenomenon leads to the different chemical shift values between Ha and Ha’ in an aromatic ring (Figure1). However, compounds 3a–f were favorable for the free 1 base form in DMSO-d6 solvent. For example, the H-NMR spectrum of compound 3e was presented in Figure1. Furthermore, Compound 2c was dissolved in DMSO-d6 and heated at ~100 ◦C by NMR technique. The sample was monitored in 0, 10, 20, 30, and 60 min, the timed programming result was shown in Figure2, we found that the intramolecular hydrogen bonding phenomenon was very clearly stable. MoleculesMolecules 2021MoleculesMolecules, 262021, x FOR, 26 20212021, PEER 2907Molecules,, 2626,, xREVIEWx FORFOR 20212021 PEERPEER,, 2626,, xREVIEWxREVIEW FORFOR PEERPEER REVIEWREVIEW 3 of 15 33 ofof 1515 3 of 1533 ofof 1515 Molecules 2021Molecules , 26, x FOR 20212021 PEER Molecules,, 2626,, xxREVIEW FORFOR 2021 PEERPEER, 26, REVIEWREVIEWx FOR PEER REVIEW 3 of 15 33 ofof 1515 3 of 15 Molecules 2021Molecules Molecules , 26, x FOR 20212021 PEERMolecules ,, 2626,, REVIEWxx FORFOR 2021 PEERPEER,, 26,, REVIEWxxREVIEW FORFOR PEERPEER REVIEWREVIEW 3 of 15 33 ofof 1515 3 of 15 Molecules 2021Molecules ,, 26,, xx FORFOR 2021 PEERPEER , 26, x REVIEWREVIEW FOR PEER REVIEW 3 of 15 3 of 15

. . Table 1. TheTableTable thermodynamic 1.1. TheTheTable thermodynamicthermodynamic -1.kinetic The thermodynamic control--kinetickinetic synthesis controlcontrol--kinetic of synthesis synthesisN- aminophthalimidescontrol ofof synthesis NN--aminophthalimidesaminophthalimides of 2aN-–-aminophthalimidesaminophthalimidesf and phthalazine 2a2a––ff andand phthalazine phthalazine1,4 2a2a-diones––ff andand phthalazine3aphthalazine 1,41,4–f--diones.diones 3a 3a 1,41,4––f-f-diones.. 3a3a––ff. TableTable 1. TheTable 1. thermodynamicThe 1. thermodynamic-kineticTheTable thermodynamic -1.kinetic The thermodynamic control--kinetic synthesis control control-kinetic of synthesis synthesis N- aminophthalimidescontrol ofof synthesisN--aminophthalimides-aminophthalimides of 2aN-–aminophthalimidesf and phthalazine 2a–2aff and–f and phthalazine 1,4 2a phthalazine-diones–f and phthalazine3a1,4–-f-.diones. 1,4-diones 3a 1,4–ff-..diones 3a–f. 3a–f. Table 1. TheTableTableTable thermodynamic 1. 1.1. The TheTheTable thermodynamic thermodynamicthermodynamic- -1.kinetic The thermodynamic control---kinetickinetickinetic synthesis control controlcontrol-kinetic of synthesis synthesis synthesisN--aminophthalimides aminophthalimidescontrol of ofof synthesis N NN---aminophthalimidesaminophthalimidesaminophthalimides of 2a 2aN–-–aminophthalimidesff andand phthalazinephthalazine 2a 2a2a–––fff and andand phthalazine phthalazine phthalazine 1,41,4 2a--diones–f and 3aphthalazine3a 1,4 1,41,4––-ff--diones.diones.diones 3a 3a 3a1,4–––ff-f. diones. 3a–f.

◦ a The reaction condition at –20 C within 4h. b The reaction condition at 0 ◦C within 4h. c Compound 2f and 3f were provided and prepared from our previous work [28].

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a The reactionMolecules condition2021, at26 ,– 290720 °C within 4h. b The reaction condition at 0 °C within 4h. c Compound 2f and 3f were provided 4 of 15 anda The prepared reaction fromcondition our previous at –20 °C workwithin [ 284h.]. b The reaction condition at 0 °C within 4h. c Compound 2f and 3f were provided and prepared from our previous work [28].

Figure 1. 1H NMR spectra of N-aminophthalimides 2a, 2c and 2e and phthalazines 1,4-dione 3a FigureandFigure 1.3e1. H1. 1 NMRH NMR spectra spectra of N of-aminophthalimides N-aminophthalimides2a, 2a2c, and2c and2e and2e and phthalazines phthalazines 1,4-dione 1,4-dione3a and3a 3e. and 3e.

Figure 2. TheFigure timed 2. programming The timed programming result of N-aminophthalimides result of N-aminophthalimides2c of 1H NMR 2c of spectra 1H NMR (a) 0spectra min, ( b(a)) 10 0 min, min, (c) 20 min, (d) 30 min, ((eFigureb)) 60 10 min. min, 2. The (c) timed20 min, programming (d) 30 min, (e result) 60 min of .N -aminophthalimides 2c of 1H NMR spectra (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 60 min. Vilsmeier amidinationVilsmeier methodology amidination was methodology essentially examined was essentially for the examinedapplicable forpro- the applicable tectedVilsmeier utilizationprotected amidination of primary utilization methodology amines. of primary The was usual essentially amines. method The examined was usual directly method for the treating was applicable directly primary pro- treating primary aminestected utilizationwith dimethylformamideamines of primary with dimethylformamide amines. (DMF) The and usual coupling (DMF) method agents and was including coupling directly POClagents treating3, P including2 O primary5, PCl5, POCl3,P2O5, amines with dimethylformamidePCl5, (COCl)2, PyBOP, (DMF) SOCl and2, acyl coupling chlorides, agents triﬂuoroacetic including POCl anhydride3, P2O5, (TFAA), PCl5, or sulfonyl chloride to give the corresponding amidine products [37–39]. To further probe the structural divergence, pyrazolopyridopyridazine diones 2f and N-aminopyrazolopyrrolopyridine-6,8- diones 3f were selected as model cases for the further control experiments [28]. At ﬁrst, we

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(COCl)2, PyBOP, SOCl2, acyl chlorides, trifluoroacetic anhydride (TFAA), or sulfonyl chlo- Molecules 2021, 26, 2907 5 of 15 Molecules 2021, 26, x FOR PEER REVIEWride to give the corresponding amidine products [37–39]. To further probe the5 of structural 15

divergence, pyrazolopyridopyridazine diones 2f and N-aminopyrazolopyrrolopyridine- 6,8-diones 3f were selected as model cases for the further control experiments [28]. At first, employedwe(COCl) employed2, PyBOP, Vilsmeier Vilsmeier SOCl reagent2, acyl reagent chlorides, (halomethyleniminium (halomethyleniminium trifluoroacetic anhydride salt) salt) [29 (TFAA),– [2339]– to33 ] compoundsor to sulfonyl compounds chlo-2f and 2f and3f (Scheme3fride (Scheme to 1give). The 1).the reactionsThecorresponding reactions were amidinewere individually individually products monitored [37 monitored–39]. byTo further TLC by method. TLCprobe method. the When structural compoundWhen com- 2fpounddivergence,was completely2f was pyrazolopyridopyridazine completely consumed consumed for 4 h diones atfor 65 4 h◦ 2fC, at and the65 °N correspondingC,-aminopyrazolopyrrolopyridine the corresponding acquired acquired amidination- amidi- productnation6,8-diones product4 was 3f were formed 4 wasselected and formed obtainedas model and cases inobtained 89% for yieldthe in further without89% yieldcontrol producing without experiments chlorinatedproducing [28]. At first, compoundchlorinated 5compound.we The employed structure 5. TheVilsmeier of structure compound reagent of compound (halomethyleniminium4 was fully 4 was characterized fully salt) chara [2 by9cterized–33 spectroscopic] to compounds by spectroscopic methods 2f and meth- and single-crystalods3f (Schemeand single 1). X-ray -Thecrystal reactions diffraction X-ray werediffraction study. individually Based study. onmonitored Based1H NMR on by 1H spectroscopicTLC NMR method. spectroscopic When characterization, com- character- pound 2f was completely consumed for 4 h at 65 °C, the corresponding acquired amidi- compoundization, compound4 possesses 4 possesses singlet signalsinglet of signal pyridine of pyridine ring proton ring proton Ha around Ha around 9.03 ppm, 9.03 ppm, and nation product 4 was formed and obtained in 89% yield without producing chlorinated significantand significant amidinyl amidinyl moiety moiety signals signals of iminium of iminium proton proton H around Hb around 7.70 ppm 7.70 and ppm two and peaks two compound 5. The structure of compound 4 was fully characterizedb by spectroscopic meth- of NMe around2 2.97 and 3.02 ppm (Figure3). These results showed the free primary peaksods and of2 singleNMe-crystal around X- ray2.97 diffraction and 3.02 study. ppm Based(Figure on 3).1H TheseNMR spectroscopic results showed character- the free pri- amine group of compound 2f was successfully converted into the amidinyl substituent. maryization, amine compound group 4of possesses compound singlet 2f signalwas successfully of pyridine ring converted proton H intoa around the amidinyl 9.03 ppm, substit- Onuent.and the significantOn other the hand,other amidinyl chlorinationhand, moiety chlorination signals of compound ofof iminium compound3f protonwas 3f accomplished wasHb around accomplished 7.70 without ppm withoutand amidination two amidi- productnationpeaks ofproduct4 NMeformation2 around4 formation by 2.97 Vilsmeier and by 3.02 Vilsmeier reagent ppm (Figure atreagent reflux 3). Theseat for reflux 4 results h, affording for showed 4 h, affording the the correspondingfree pri- the corre- productspondingmary amine5 withproduct group down-field of5 withcompound down proton 2f-field was signal protonsuccessfully Hc ofsignal pyridine converted Hc of ringpyridine into around the amidinyl ring 9.68 around substit- ppm 9.68 in good ppm yieldinuent. good (80%, On yield the Scheme other (80%, 1hand, Schemeand Figurechlorination 1 and3)[ Figure27 of]. compound Based 3) [27 on]. the3fBased was above accomplishedon the derivatization above withoutderivatization study, amidi- Vilsmeier study, reactionVilsmeiernation wasproduct reaction conceived 4 formation was as conceived the by significant Vilsmeier as the reagent derivatization significant at reflux derivatization agent for 4 toh, identifyaffording agent isomers the to corre-identify between iso- 2fmersspondingand between3f. product 2f and 5 with 3f. down-field proton signal Hc of pyridine ring around 9.68 ppm in good yield (80%, Scheme 1 and Figure 3) [27]. Based on the above derivatization study, Vilsmeier reaction was conceived as the significant derivatization agent to identify isomers between 2f and 3f.

SchemeScheme 1.1. The results of 7 7-aminopyrazolopyrrolopyridine-6,8-dione-aminopyrazolopyrrolopyridine-6,8-dione 2f2f andand pyra- pyrazolopyridopyri- dazinezolopyridopyridazineScheme dione 1. The3f resultstreated of dione with 7-aminopyrazolopyrrolopyridine Vilsmeier3f treated reagent. with Vilsmeier -reagent.6,8-dione 2f and pyrazolopyridopyridazine dione 3f treated with Vilsmeier reagent.

Figure 3. 1H NMR spectra of 7-amidination product 4 and dichloropyridazine 5.

For further investigation into the reactivity of Vilsmeier amidination derivatization, Vilsmeier reaction was carried out using different substrates including N-aminophthalimides 2a–e at 50 ◦C for 0.5 h. Various substituted reactants 2a–e were demonstrated to perform MoleculesMolecules 2021 2021, ,26 26, ,x x FOR FOR PEER PEER REVIEW REVIEW 66 of of 15 15

1 FigureFigure 3. 3. 1 1HH NMR NMR spectra spectra of of 7 7--amidinationamidination product product 44 and and dichloropyridazine dichloropyridazine 5.5.

For further investigation into the reactivity of Vilsmeier amidination derivatization, Molecules 2021, 26, 2907 ForFor furtherfurther investigationinvestigation intointo thethe reactivityreactivity ofof VilsmeierVilsmeier amidinationamidination derivatization,derivatization,6 of 15 VilsmeierVilsmeier reacti reactionon was was carried carried out out using using different different substrates substrates including including NN--aminoph-aminoph- thalimidesthalimides 2a2a––ee atat 5050 °°CC forfor 0.50.5 h.h. VariousVarious substitutedsubstituted reactantsreactants 2a2a––ee werewere demonstrateddemonstrated toto performperform thethe reactionsreactions smoothly,smoothly, regardlessregardless ofof whetherwhether electronelectron--donatingdonating oror electronelectron-- withdrawingwithdrawingthe reactions smoothly,substituents, substituents, regardless and and the the of corresponding corresponding whether electron-donating amidination amidination orproducts products electron-withdrawing 66−−1010 werewere af- af- fordedfordedsubstituents, inin 74−88%74−88% and yields theyields corresponding (Table(Table 2).2). AllAll amidination productsproducts 66– products–1010 werewere fully6fully−10 characharawerecterizedcterized afforded byby in spectro-spectro- 74−88% scopicscopicyields methods, (Tablemethods,2). Alland and products they they actually actually6–10 presented presentedwere fully singlet singlet characterized peak peak for for by the the spectroscopic significant significant amidinyl amidinyl methods, moi- moi- and they actually presented singlet peak for the signiﬁcant amidinyl11 moiety signals of iminium etyety signalssignals ofof iminiumiminium protonproton HH andand twotwo peakspeaks ofof NMeNMe22 inin 1HH--NMR.NMR. Subsequently,Subsequently, aa 1 seriesseriesproton ofof H phthalazinephthalazine and two peaks 1,4 1,4--dionesdiones of NMe 3a 3a2––eine were wereH-NMR. treated treated Subsequently, with with Vilsmeier Vilsmeier a seriesr reagenteagent of (POCl phthalazine(POCl33/DMF)/DMF) 1,4- at at series of phthalazine 1,4-diones 3a–e were treated with Vilsmeier reagent◦ (POCl◦ 3/DMF) at 6565diones °°CC oror3a 8080– e °°CCwere forfor 2 treated2––44 hh.. TheThe with chlorinationchlorination Vilsmeierreagent happenedhappened (POCl smoothlysmoothly3/DMF) toto at affordafford 65 C the orthe 80 desireddesiredC for prod-prod- 2–4 h. The chlorination happened smoothly to afford the desired products 11–15 in high yields uctsucts 1111––1515 inin highhigh yieldsyields (82−90%,(82−90%, TableTable 2),2), exceptexcept forfor 3d3d (31%).(31%). OwingOwing toto thethe electronelectron-- (82−90%, Table2), except for 3d (31%). Owing to the electron-rich property of nitrogen richrich propertyproperty ofof nitrogennitrogen atomsatoms onon thethe aromaticaromatic motifmotif ofof cocompoundmpound 3d3d,, thethe complicatedcomplicated atoms on the aromatic motif of compound 3d, the complicated aromatic substitution and aromaticaromatic substitutionsubstitution andand polylizationpolylization werewere proceeded.proceeded. AllAll chlorinatedchlorinated productsproducts 1111––1515 polylization were proceeded. All chlorinated products 11–15 were also fully characterized werewere alsoalso fullyfully characterizedcharacterized byby spectroscopicspectroscopic methods,methods, andand twotwo peakspeaks forfor thethe significantsignificant by spectroscopic methods, and two peaks13 for the signiﬁcant dione moieties were con-13 dionedione moietiesmoieties werewere convertedconverted intointo ––NN == 1133CC––ClCl singlesinglett signalsignal atat δδ 153153––157157 ppmppm inin 1313CC-- verted into –N = 13C–Cl singlet signal at δ 153–157 ppm in 13C-NMR spectrum. Therefore, NMRNMR spectrum. spectrum. Therefore, Therefore, Vilsmeier Vilsmeier reagent reagent (POCl (POCl33/DMF)/DMF) was was used used as as the the derivatization derivatization Vilsmeier reagent (POCl /DMF) was used as the derivatization reagent for the different reagentreagent for for thethe differentdifferent reactivereactive3 phenomenonphenomenon toto distinguishdistinguish N N--aminophthalimidesaminophthalimides 22 and and reactive phenomenon to distinguish N-aminophthalimides 2 and phthalazine-1,4-diones 3. phthalazinephthalazine--1,41,4--dionesdiones 33.. Table 2. Derivatization results of N-aminophthalimides 2a–e and phthalazine 1,4-diones 3a–e with Vilsmeier reagent. TableTable 2.2. DerivatizationDerivatization results results of of NN--aminophthalimidesaminophthalimides 2a 2a––ee and and phthalazine phthalazine 1,4 1,4--dionesdiones 3a 3a––ee with with Vilsmeier Vilsmeier reagent. reagent.

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To explore the interconversion reactivity of the tautomerization, the solvent scope was first examined by using 7-aminopyrazolopyrrolopyridine-6,8-dione 2f. Compound 2f was screened and refluxed in the various solvents including CH2Cl2, THF, EtOH, MeCN, toluene, dioxane, and DMSO for 24 h. However, the reactions in CH2Cl2, EtOH, toluene recovered the starting material 2f without conversion happening (Table 3 entries 1–3). The use of polar THF, MeCN, dioxane, and DMSO led to lower interconversion ratios of 2f/3f from 93/7 to 88/12 (Table 3, entries 4–7). Subsequently, Brønsted–Lowry acids including acetic acid (AcOH), methanesulfonic acid (TsOH), methanesulfonic chloride (TsCl), and trifluoroacetic acid (TFA) were studied for the interconversion reaction at reflux for 4 h (Entries 8–11, Table 3). Several experimental observations are worthy to discuss: we firstly found that the conversion ratios were improved under acidic condition (Entries 8–11, Ta- ble 3). Secondly, under the strong acid such as trifluoroacetic acid (pKa = 0.30), p-tol- uenesulfonic acid (TsOH, pKa = −1.9), and methanesulfonic chloride (TsCl), the low conversion ratio and decomposed products were observed (Entries 8–10, Table 3).

Table 3. Derivatization results of N-aminophthalimides 2a–e and phthalazine 1,4-diones 3a–e with Vilsmeier reagent.

Entry S.M. Solvent Reaction Time (h) Product Ratio of 2f/3f a 1 2f CH2Cl2 24 3f non-conversion 2 2f Toluene 24 3f non-conversion 3 2f EtOH 24 3f non-conversion 4 2f THF 24 3f 88/12 5 2f CH3CN 24 3f 93/7 6 2f Dioxane 24 3f 91/9 7 2f DMSO 24 3f 89/11 TsOH 8 2f 4 3f 61/39 (pKa = –1.9) 9 2f TsCl 4 3f 56/44 TFA 10 2f 4 3f 54/46 (pKa = 0.30) AcOH 11 2f 4 3f 6/94 (pKa = 4.76) a The ratio was identified by 1H-NMR.

For further investigations, the timed programming of the thermodynamic conversion of compound 2f was carried out under acetic acid (AcOH) solution and shown in Figure 4. The reaction mixture was sampled at 1.5, 2.3, 3.5, and 5 h and detected by the 1H-NMR spectroscopic method. This result showed that compound 2f was gradually converted to

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To explore the interconversion reactivity of the tautomerization, the solvent scope was first examined by using 7-aminopyrazolopyrrolopyridine-6,8-dione 2f. Compound 2f To explore the interconversion reactivity of the tautomerization, the solvent scope was screened and refluxed in the various solvents including CH2Cl2, THF, EtOH, MeCN, was first examined by using 7-aminopyrazolopyrrolopyridine-6,8-dione 2f. Compound 2f toluene, dioxane, and DMSO for 24 h. However, the reactions in CH2Cl2, EtOH, toluene recoveredwas screened the starting and refluxed material in the2f variouswithout solvents conversion including happening CH2Cl2, THF, (Table EtOH,3 entries MeCN, 1–3). toluene, dioxane, and DMSO for 24 h. However, the reactions in CH2Cl2, EtOH, toluene The use of polar THF, MeCN, dioxane, and DMSO led to lower interconversion ratios recovered the starting material 2f without conversion happening (Table 3 entries 1–3). The of 2fuse/3f offrom polar 93/7 THF, to MeCN, 88/12 dioxane, (Table3 and, entries DMSO 4–7). led to Subsequently, lower interconversion Brønsted–Lowry ratios of 2f/3f acids includingfrom 93/7 acetic to 88/12 acid (Table (AcOH), 3, entries methanesulfonic 4–7). Subsequently, acid (TsOH), Brønsted methanesulfonic–Lowry acids including chloride (TsCl),acetic and acid trifluoroacetic (AcOH), methanesulfonic acid (TFA) were acid studied (TsOH), for methanesulfonic the interconversion chloride reaction (TsCl), atand reflux for 4trifluoroacetic h (Entries 8–11, acid Table(TFA)3 ).were Several studied experimental for the interconversion observations reaction are worthyat reflux tofor discuss: 4 h we firstly(Entries found 8–11, thatTable the 3). conversionSeveral experimental ratios were observations improved are under worthy acidic to discuss: condition we firstly (Entries 8–11,found Table that3). the Secondly, conversion under ratios the were strong improved acid such under as acidic trifluoroacetic condition (Entries acid (pKa 8–11, =Ta- 0.30), p-toluenesulfonicble 3). Secondly, acid under (TsOH, the strong pKa = acid−1.9), such and as methanesulfonic trifluoroacetic acid chloride (pKa = (TsCl),0.30), p- thetol- low conversionuenesulfonic ratio acid and (TsOH, decomposed pKa = −1 products.9), and methanesulfonic were observed chloride (Entries (TsCl), 8–10, Tablethe low3). conversion ratio and decomposed products were observed (Entries 8–10, Table 3). Table 3. Derivatization results of N-aminophthalimides 2a–e and phthalazine 1,4-diones 3a–e with Table 3. Derivatization results of N-aminophthalimides 2a–e and phthalazine 1,4-diones 3a–e with VilsmeierVilsmeier reagent. reagent.

Entry S.M. Solvent Reaction Time (h) Product Ratio of 2f/3f a Entry S.M. Solvent Reaction Time (h) Product Ratio of 2f/3f a 1 2f CH2Cl2 24 3f non-conversion 1 2 2f2f TolueneCH2 Cl2 24 24 3f 3f non-conversionnon-conversion 2 3 2f2f EtOHToluene 24 24 3f 3f non-conversionnon-conversion 3 4 2f2f THFEtOH 24 24 3f 3f 88/12non-conversion 4 5 2f2f CH3CNTHF 24 24 3f 3f 93/7 88/12 5 6 2f2f DioxaneCH3 CN24 24 3f 3f 91/9 93/7 6 7 2f2f DMSODioxane 24 24 3f 3f 89/11 91/9 7 2f TsOHDMSO 24 3f 89/11 8 2f 4 3f 61/39 (pKa =TsOH –1.9) 8 2f 4 3f 61/39 9 2f (pKaTsCl = –1.9) 4 3f 56/44 9 2f TFATsCl 4 3f 56/44 10 2f 4 3f 54/46 (pKa = 0.30)TFA 10 2f 4 3f 54/46 AcOH(pKa = 0.30) 11 2f 4 3f 6/94 (pKa =AcOH 4.76) 11 2f 4 3f 6/94 a The ratio was identified(pKa by = 1H 4.76)-NMR. a The ratio was identified by 1H-NMR. For further investigations, the timed programming of the thermodynamic conversion ofFor compound further investigations, 2f was carried out the under timed acetic programming acid (AcOH) of solution the thermodynamic and shown in conversionFigure of compound4. The reaction2f was mixture carried was out sampled under at acetic 1.5, 2.3, acid 3.5, (AcOH) and 5 h solutionand detected and by shown the 1H in-NMR Figure 4. The reactionspectroscopic mixture method. was This sampled result showed at 1.5, 2.3,that 3.5,compound and 5 h2f and was detectedgradually byconverted the 1H-NMR to spectroscopic method. This result showed that compound 2f was gradually converted to the thermodynamic stable product 3f (Figure4). Finally, transformation reaction was equilibrated at reflux for more than 5 h, and the conversion ratio was obtained proximately 6/94 (2f/3f, Entry 11 of Table4 and Figure4). Based on the above experimental result, acetic acid was conceived as the best acidic solvent with 6/94 conversion ratio. Fortunately, 2f can be successfully and smoothly transformed to more thermodynamically stable product 3f by refluxing in acidic medium [40,41]. To further demonstrate the reliable of conversion procedure, N-aminophthalimides 2a–e were also used as starting materials at reflux for 8–9 h. Fortunately, compounds 2a–e can be smoothly transformed to give the corresponding thermodynamically pyrazolopyridopyridazine diones 3a–e under acetic acid solvent, with the ratio of 2a–e/3a–e from 6/94 to 1/99 (Table4). Molecules 2021, 26, x FOR PEER REVIEW 8 of 15

MoleculesMolecules 2021 2021,, 26,,, x26x FORFOR,, xx FORFOR PEERPEER PEERPEER REVIEWREVIEW REVIEWREVIEW 8 of8 15 of 15

the thermodynamic stable product 3f (Figure 4). Finally, transformation reaction was equilibratedthethe thermodynamic thermodynamic at reflux for stablemore stable productthan product 5 h, 3f and 3f(Figure (Figurethe conversion 4). 4). Finally, Finally, ratio transformation transformation was obtained reaction proxi- reaction was was matelyequilibrated equilibrated6/94 (2f/3f at, Entry refluxat reflux 11 for offor more Table more than 4 thanand 5 h,Figure5 andh, and the4). the Basedconversion conversion on the ratio above ratio was wasexperimental obtained obtained proxi- proxi- Molecules 2021, 26, x FOR PEER REVIEW matelymately 6/94 6/94 (2f /(3f2f,/ 3fEntry, Entry 11 11of Tableof Table 4 and 4 and Figure Figure 4). 4).Based Based on8 of on the15 the above above experimental experimental result,mately aceticmately 6/94 acid 6/94 ( was2f (/2f3f conceived/, 3fEntry, Entry 11 as11of theofTable Tablebest 4 acidicand 4 and Figure solvent Figure 4). with 4).Based Based 6/94 on conversion onthe the above above ratio. experimental experimental For- tunately,result,result, 2f acetic can acetic be acid successfullyacid was was conceived conceived and assmoot theas the hlybest best transformed acidic acidic solvent solvent to with more with 6/94 thermodynamically 6/94 conversion conversion ratio. ratio. For- For- stabletunately, producttunately, 2f 3f can2f by can refluxingbe besuccessfully successfully in acidic and mediumand smoot smoothly [40hly ,4transformed1 ].transformed To further to demonstratemoreto more thermodynamically thermodynamically the relia- the thermodynamicstablestable product product stable 3f by3f product byrefluxing refluxing 3f (Figure in acidicin 4). acidic Finally, medium medium transformation [40 [,4410].,4 1To reaction]. Tofurther further was demonstrate demonstrate the the relia- relia- bleequilibrated ofstable conversionstable atproduct reflux product procedure,for 3f more by3f byrefluxing than refluxing N 5- aminophthalimidesh, and in acidicinthe acidic conversion medium medium ratio 2a [4 –0was e[,44 were01 ,4obtained]. 1To]. also Tofurther proxi- furtherused demonstrate as demonstrate starting mate- the the relia- relia- rialsmately bleat 6/94 ble refluxof ( conversion2fof/ 3fconversion ,for Entry 8– 911 procedure,h. of procedure,Fortunately,Table 4 and N -Figure aminophthalimidesN -compoundsaminophthalimides 4). Based on 2a the–e above 2acan –2ae beexperimental –weree smoothly were also also used transformedused as startingas starting to mate- mate- giveresult, rialsthe aceticrials corresponding at acid refluxat wasreflux conceived for for 8 thermodynamically– 98 as–h.9 the Fortunately,h. best Fortunately, acidic solvent compounds pyrazolopyridopyridazine compounds with 6/94 conversion 2a –2ae –cane ratio.can be beFor-smoothly smoothlydiones 3atransformed –transformede under to to tunately, 2f can be successfully and smoothly transformed to more thermodynamically aceticgive acidgive the solvent, the corresponding corresponding with the thermodynamicallyratio thermodynamically of 2a–e/3a–e from pyrazolopyridopyridazine pyrazolopyridopyridazine 6/94 to 1/99 (Table 4). diones diones 3a– 3ae under–e under stable product 3f by refluxing in acidic medium [40,41]. To further demonstrate the relia- aceticacetic acid acid solvent, solvent, with with the the ratio ratio of 2aof – 2ae/–3ae/–3ae –frome from 6/94 6/94 to 1/99to 1/99 (Table (Table 4). 4). Molecules 2021, 26, 2907 ble of conversion procedure, N-aminophthalimides 2a–e were also used as starting mate- 8 of 15 rials at reflux for 8–9 h. Fortunately, compounds 2a–e can be smoothly transformed to give the corresponding thermodynamically pyrazolopyridopyridazine diones 3a–e under acetic acid solvent, with the ratio of 2a–e/3a–e from 6/94 to 1/99 (Table 4).

1 1 FigureFigure 4. 4. (a()a H) 1 NMRH NMR spectrum spectrum of the beginning of the beginning of the reaction of ( theH NMR reaction of compound (1H NMR 2f). (b of–d compound) 2f). ( b–d) Reaction at reflux for 1.5, 2.3, and 3.5 h (1H NMR of compounds 2f and 3f; the ratios of 2f/3f = 1 1 1 Figure 4. (a) H NMR spectrum of the beginning1 of the reaction ( H NMR of compound 2f). (b–d) Reaction~72/28, 48/52, at and reﬂux 36/64).1 for (1e 1.5,) Reaction 2.3, andat reflux 3.5 for h 5 ( hH ( H NMR NMR of compounds compounds 3f; the2f ratiosand1 1 of3f ; the ratios of 2f/3f = FigureFigure 4. ( a4.) (1Ha) 1NMRH NMR spectrum spectrum of the of1 thebeginning beginning of the of thereaction reaction (1H ( 1NMRH NMR of compound of compound 2f). 2f (b).– (db) –d) Reaction2f/3f =Figure ~6/94)Figure at. reflux4. ( 4.a) ( foraH) NMR H1.5, NMR 2.3 spectrum, andspectrum 3.5 ofh ( theofH the beginningNMR beginning of compounds of theof1 the reaction reaction 2f and ( H 3f( NMRH; the NMR ratios of compoundof ofcompound 2f/3f = 2f ). 2f (b).– (db)– d) ~72/28, 48/52, and 36/64). (e) Reaction at reﬂux1 1 for 5 h ( H NMR of compounds 3f; the ratios of ~72/28,Reaction 48/52,Reaction andat reflux at 36/64). reflux for ( fore1.5,) Reaction 1.5, 2.3 ,,2.3 and,, andat 3.5 reflux 3.5h ( 1hH for( 1NMRH 5 NMR h (of1H compoundsof NMR compounds of compounds 2f and2f and 3f;; 3f3fthethe;;; thethe the ratiosratios ratiosratiosratios ofof 2f ofofof/ 3f2f /=3f = 2f/3f = ~6/94). 1 1 2fTable/3f ~72/28,4.= The~6/94)~72/28, conversion 48/52,. 48/52, and results and 36/64). between 36/64). (e) N (Reaction-eaminophthalimide) Reaction at reflux at reflux 2a for–e and for5 h pyrazolopyridopyridazine 5( 1hH ( 1NMRH NMR of compoundsof compounds 3f;; 3fthethe;; the the ratiosratios ratiosratios ofof ofof dione 2f3a–/3fe2f. /=3f ~6/94) = ~6/94).. .. TableTable 4. 4.The The conversion conversion results results between betweenN N-aminophthalimide-aminophthalimide2a 2a––eeand and pyrazolopyridopyridazine pyrazolopyridopyridazine dionedioneTable3a 3a–Table–ee. .4. The 4. The conversion conversion results results between between N-aminophthalimide N-aminophthalimide 2a– 2ae and–e and pyrazolopyridopyridazine pyrazolopyridopyridazine dionedione 3a– 3ae.. –e..

Entry S.M. Reaction Time (h) Product 2a–e/3a–e a

1 8 1/99

a EntryEntry S.M. S.M.Reaction Reaction Time (h) Time (h)Product Product 2a 2a–e/3a–e–e/3a–e a 2 EntryEntry S.M.S.M. 9 ReactionReaction Time Time (h) (h) ProductProduct3/97 2a–2ae/3a–e/3a–e a– e aa

1 8 1/99 11 1 8 88 1/991/991/99

MoleculesMolecules 2021 2021, 26, x26 FOR, x FOR PEER PEER REVIEW REVIEW 9 of9 15 of 15 MoleculesMolecules 2021 2021, 26,, x26 FOR, x FOR PEER PEER REVIEW 2REVIEW 9 3/97 9 of9 15 of 15

22 2 9 99 3/973/973/97

33 3 10 1010 6/946/946/94 3 3 10 10 6/946/94

44 4 11 1111 5/955/955/95

55 5 3 33 4/964/964/96

a a 1 1 a Thea The ratio ratio of 2of/3 2was/3 was determined determined by crudeby crude 1H -1NMR.H-NMR. a The ratioTheThe of ratio2/ ratio3 wasof 2 of/ determined3 2was/3 was determined determined by crude 1byH-NMR. crudeby crude H- NMR.H-NMR.

3. Experimental3. Experimental Section Section 3.1.3.1. General General Procedure Procedure AllAll reagents reagents were were purchased purchased commercially. commercially. All All reactions reactions were were carried carried out out under under ar- ar- gongon or nitrogenor nitrogen atmosphere atmosphere and and monitored monitored by byTLC. TLC. Flash Flash column column chromatography chromatography was was carriedcarried out out on onsilica silica gel gel (230 (230–400–400 mesh). mesh). Analytical Analytical thin thin-layer-layer chromatography chromatography (TLC) (TLC) was was performedperformed using using pre pre-coated-coated plates plates (silica (silica gel gel 60 60F-254) F-254) purchased purchased from from Merck Merck Inc. Inc. Flash Flash columncolumn chromatography chromatography puri purificationfication was was carried carried out out by bygradient gradient elution elution using using n-hexane n-hexane 1 1 in ethylin ethyl acetate acetate (EtOAc) (EtOAc) unless unless otherwise otherwise stated. stated. 1H 1NMRH NMR spectra spectra were were recorded recorded at 400at 400 or or 500500 MHz MHz and and 13C 13 NMRC NMR spectra spectra were were record recorded edat 100at 100 or 125or 125 MHz, MHz, respectively, respectively, in CDClin CDCl3, 3, 500500 MHz MHz and and 13C 13 NMRC NMR spectra spectra were were record recorded edat 100at 100 or 125or 125 MHz, MHz, respectively, respectively, in CDClin CDCl3, 3, DMSODMSO-d6,- ord6, Dor2 OD 2solvent.O solvent. The The standard standard abbreviations abbreviations s, d, s, t,d, q, t, andq, and m referm refer to theto the singlet, singlet, DMSODMSO-d6,- ord6, Dor2 OD 2solvent.O solvent. The The standard standard abbreviations abbreviations s, d, s, t,d, q, t, andq, and m referm refer to theto the singlet, singlet, doublet,doublet, triplet, triplet, quartet, quartet, and and multiplet, multiplet, respectively. respectively. Coupling Coupling constant constant (J), ( wheneverJ), whenever dis- dis- cernible,cernible, ha sha beens been reported reported in Hz.in Hz. Infrared Infrared spectra spectra (IR) (IR) were were recorded recorded in neatin neat solutions solutions or or solids;solids; and and mass mass spectra spectra were were recorded recorded using using electron electron impact impact or orelectrospray electrospray ionization ionization –1 –1 techniques.techniques. The The reported reported wavenumbers wavenumbers are are referenced referenced to theto the polystyrene polystyrene 1601 1601 cm cm–1 ab-–1 absorption.sorption. ESI ESI-MS-MS ana analyseslyses were were performed performed on onan anApplied Applied Biosystems Biosystems API API 300 300 mass mass spec- spec- trometer.trometer. High High-resolution-resolution mass mass spectra spectra were were obtained obtained from from a JEOL a JEOL JMS JMS-HX110-HX110 mass mass spectrometer.spectrometer.

3.2.3.2. Standard Standard Procedure Procedure for forSynthesis Synthesis of N of- AminophthalimidesN-Aminophthalimides 2a– 2af –ff StandardStandard procedure procedure for for synthesis synthesis of Nof- aminophthalimidesN-aminophthalimides 2a –2af. –Thef. The reliable reliable proce- proce- duredure involved involved the the treatment treatment of of isobenzofuran isobenzofuran-1,3-1,3-diones-diones (1a (–1ac),– c isoindoline), isoindoline-1,3-1,3-dione-dione (1d(),1d furo[3,4), furo[3,4-b] -pyrazineb] pyrazine-5,7-5,7-dione-dione (1e (),1e or), or1H 1-pyrrolo[3,4H-pyrrolo[3,4-c] -pyridinec] pyridine-1,3-1,3-dione-dione (1f ,( 1f1.0,, 1.0 equiv.equiv.) with) with monohydr monohydrateate hydrazine hydrazine (~5.0 (~5.0 equiv.) equiv.) in EtOH/Hin EtOH/H2O 2(2.0O (2.0 mL/2.0 mL/2.0 mL) mL) at 0at ° C0 °C equiv.equiv.) with) with monohydr monohydrateate hydrazine hydrazine (~5.0 (~5.0 equiv.) equiv.) in EtOH/Hin EtOH/H2O 2(2.0O (2.0 mL/2.0 mL/2.0 mL) mL) at 0at ° C0 °C or –or20 – 20°C °toC roomto room temperature temperature within within 4 h. 4 Whenh. When the the reaction reaction was was completed, completed, the the reaction reaction mixturemixture was was added added water water (10 (10 mL) mL) for for precipitation. precipitation. The The precipitate precipitate was was filtered, filtered, washed washed withwith cold cold water water (10 (10 mL) mL) and and n-hexane/EA n-hexane/EA (1/2, (1/2, 15 15mL) mL) to giveto give the the corresponding corresponding crude crude N-aNminophthalimides-aminophthalimides 2a –2af. –Thef.. The crude crude desired desired products products 2a –2af were–f were recrystallized recrystallized in ace-in acetone/THFtone/THF (1/4) (1/4) solution solution to obtainto obtain the the pure pure N- aminophthalimidesN-aminophthalimides 2a –2af in–f 81in –8194–%94 yields% yields. .. 13 13 TheThe low low solubility solubility of theof the compounds compounds 2a –2af made–f made the the 13C 13-NMRC-NMR characterization characterization of quater-of quater- narynary and and carbonyl carbonyl carbons carbons of theseof these substrates substrates unclear unclear [28 [].28 ]. N-AminophthalimideN-Aminophthalimide (2a ()2a [4)2 []:4 2White]: White solid; solid; 92% 92% yield; yield; mp mp 202 202–205–205 °C; °C; 1H 1NMRH NMR (DMSO (DMSO-d6,- d6, N-AminophthalimideN-Aminophthalimide (2a ()2a [4)2 []:4 2White]: White solid; solid; 92% 92% yield; yield; mp mp 202 202–205–205 °C; °C; 1H 1NMRH NMR (DMSO (DMSO-d6,- d6, 13 13 400400 MHz) MHz) δ 7.83 δ 7.83 (dd, (dd, J = J5.8, = 5.8, 3.4 3.4 Hz, Hz, 2H, 2H, Ar HAr),H 8.05), 8.05 (dd, (dd, J = J5.8, = 5.8, 3.4 3.4 Hz, Hz, 2H, 2H, Ar HAr);H 13);C 13 NMRC NMR (DMSO(DMSO-d6,- d1006, 100 MHz): MHz): δ 123.28 δ 123.28 (2 ×(2 C), × C), 130.53 130.53 (2 ×(2 C), × C), 134.81 134.81 (2 ×(2 C), × C), 167.36 167.36 (2 ×(2 C); × C); FT -FTIR- IR (DMSO(DMSO-d6,- d1006, 100 MHz): MHz): δ 123.28 δ 123.28 (2 ×(2 C), × C), 130.53 130.53 (2 ×(2 C), × C), 134.81 134.81 (2 ×(2 C), × C), 167.36 167.36 (2 ×(2 C); × C); FT -FTIR- IR (KBr):(KBr): 3482, 3482, 3341, 3341, 3262, 3262, 3093, 3093, 1778, 1778, 1719, 1719, 1605, 1605, 1468, 1468, 1409, 1409, 1291, 1291, 1197, 1197, 1091, 1091, 1075, 1075, 1020, 1020, 918, 918, –1 –1 + + 800,800, 718, 718, 526 526 cm cm–1; EIMS–1;; EIMSEIMS m/z m/z: 162:: 162162 (M (M+(M, 100),+,, 100),100), 105 105105 (12), (12),(12), 104 104104 (74), (74),(74), 77 (12),7777 (12),(12), 76 (26),7676 (26),(26), 50 (11)5050 (11)(11); HRMS;; HRMSHRMS (EI)(EI) m/z m/z: [M]: [M]+ Calcd+ Calcd for for C8 HC68NH26ON22:O 162.0429;2: 162.0429; Found: Found: 162.0425. 162.0425. (EI)(EI) m/z m/z: [M]: [M]+ Calcd+ Calcd for for C8 HC68NH26ON22:O 162.0429;2: 162.0429; Found: Found: 162.0425. 162.0425. N-AminoN-Amino-4,5-4,5-difluoro-difluoropthalimidepthalimide (2b ()2b: white): white solid; solid; 92% 92% yield; yield; mp mp 288 288–290–290 °C; °C; 1H 1NMRH NMR (D 2(DO,2 O, N-AminoN-Amino-4,5--4,5difluoro-difluoropthalimidepthalimide (2b ()2b: white): white solid; solid; 92% 92% yield; yield; mp mp 288 288–290–290 °C; °C; 1H 1NMRH NMR (D 2(DO,2 O, 500500 MHz): MHz): δ 7.40 δ 7.40 (t, J(t, = J9.38 = 9.38 Hz, Hz, 2H, 2H, Ar HAr);H 13);C 13 NMRC NMR (D 2(DO,2 O,125 125 MHz): MHz): δ 116.43 δ 116.43 (dd, (dd, J = J12.98, = 12.98, 500500 MHz): MHz): δ 7.40 δ 7.40 (t, J(t, = 9.38J = 9.38 Hz, Hz, 2H, 2H, Ar HAr);H 13);C 13 NMRC NMR (D 2(DO,2 125O, 125 MHz): MHz): δ 116.43 δ 116.43 (dd, (dd, J = 12.98,J = 12.98, 6.096.09 Hz, Hz, 2 × 2 C), × C), 134.91 134.91 (2 ×(2 C), × C), 148.62 148.62 (d, (d,J = J14.79 = 14.79 Hz), Hz), 150.61 150.61 (d, (d,J = J14.78 = 14.78 Hz), Hz), 175.06 175.06 (2 ×(2 × C).C). FT -FTIR- IR(KBr): (KBr): 3358, 3358, 3290, 3290, 3072, 3072, 2593, 2593, 1658, 1658, 1597, 1597, 1495, 1495, 1372, 1372, 1182, 1182, 1094, 1094, 1029, 1029, 910, 910, 788, 788,

Molecules 2021, 26, 2907 9 of 15

3. Experimental Section 3.1. General Procedure All reagents were purchased commercially. All reactions were carried out under argon or nitrogen atmosphere and monitored by TLC. Flash column chromatography was carried out on silica gel (230–400 mesh). Analytical thin-layer chromatography (TLC) was performed using pre-coated plates (silica gel 60 F-254) purchased from Merck Inc. Flash column chromatography puriﬁcation was carried out by gradient elution using n-hexane in ethyl acetate (EtOAc) unless otherwise stated. 1H NMR spectra were recorded at 400 or 13 500 MHz and C NMR spectra were recorded at 100 or 125 MHz, respectively, in CDCl3, DMSO-d6, or D2O solvent. The standard abbreviations s, d, t, q, and m refer to the singlet, doublet, triplet, quartet, and multiplet, respectively. Coupling constant (J), whenever discernible, has been reported in Hz. Infrared spectra (IR) were recorded in neat solutions or solids; and mass spectra were recorded using electron impact or electrospray ionization techniques. The reported wavenumbers are referenced to the polystyrene 1601 cm−1 absorption. ESI-MS analyses were performed on an Applied Biosystems API 300 mass spectrometer. High-resolution mass spectra were obtained from a JEOL JMS-HX110 mass spectrometer.

3.2. Standard Procedure for Synthesis of N-Aminophthalimides 2a–f Standard procedure for synthesis of N-aminophthalimides 2a–f. The reliable procedure involved the treatment of isobenzofuran-1,3-diones (1a–c), isoindoline-1,3-dione (1d), furo[3,4-b] pyrazine-5,7-dione (1e), or 1H-pyrrolo[3,4-c] pyridine-1,3-dione (1f, 1.0 equiv.) with monohydrate hydrazine (~5.0 equiv.) in EtOH/H2O (2.0 mL/2.0 mL) at 0 ◦C or –20 ◦C to room temperature within 4 h. When the reaction was completed, the reaction mixture was added water (10 mL) for precipitation. The precipitate was ﬁltered, washed with cold water (10 mL) and n-hexane/EA (1/2, 15 mL) to give the corresponding crude N-aminophthalimides 2a–f. The crude desired products 2a–f were recrystallized in acetone/THF (1/4) solution to obtain the pure N-aminophthalimides 2a–f in 81–94% yields. The low solubility of the compounds 2a–f made the 13C-NMR characterization of quaternary and carbonyl carbons of these substrates unclear [28]. ◦ 1 N-Aminophthalimide (2a)[42]: White solid; 92% yield; mp 202–205 C; H NMR (DMSO-d6, 400 MHz) δ 7.83 (dd, J = 5.8, 3.4 Hz, 2H, ArH), 8.05 (dd, J = 5.8, 3.4 Hz, 2H, ArH); 13C NMR (DMSO-d6, 100 MHz): δ 123.28 (2 × C), 130.53 (2 × C), 134.81 (2 × C), 167.36 (2 × C); FT-IR (KBr): 3482, 3341, 3262, 3093, 1778, 1719, 1605, 1468, 1409, 1291, 1197, 1091, 1075, 1020, 918, 800, 718, 526 cm−1; EIMS m/z: 162 (M+, 100), 105 (12), 104 (74), 77 (12), 76 (26), 50 (11); + HRMS (EI) m/z: [M] Calcd for C8H6N2O2: 162.0429; Found: 162.0425. ◦ 1 N-Amino-4,5-diﬂuoropthalimide (2b): white solid; 92% yield; mp 288–290 C; H NMR (D2O, 13 500 MHz): δ 7.40 (t, J = 9.38 Hz, 2H, ArH); C NMR (D2O, 125 MHz): δ 116.43 (dd, J = 12.98, 6.09 Hz, 2 × C), 134.91 (2 × C), 148.62 (d, J = 14.79 Hz), 150.61 (d, J = 14.78 Hz), 175.06 (2 × C). FT-IR (KBr): 3358, 3290, 3072, 2593, 1658, 1597, 1495, 1372, 1182, 1094, 1029, 910, 788, 743 cm−1; EIMS m/z: 198 (M+, 100), 141 (24), 140 (83), 113 (14), 112 (33); HRMS (EI) m/z: + [M] Calcd for C8H4F2N2O2: 198.0241; Found: 198.0233. N-Amino-4,5-dichlorophthalimide (2c): White solid; 81% yield; mp 302–305 ◦C; 1H NMR 13 (DMSO-d6, 400 MHz) δ 7.72 (s, 1H, ArH), 7.84 (s, 1H, ArH); C NMR (DMSO-d6, 100 MHz) δ 130.36, 131.12, 132.08, 132.25, 132.57, 141.90, 164.74, 169.22; FT-IR (KBr): 3466, 3321, 3220, 3027, 2646, 1653, 1619, 1521, 1393, 1356, 1308, 1106, 1022, 930, 809, 782, 613 cm−1; EIMS m/z: 232 (M+ + 2, 64), 230 (M+, 100), 174 (46), 173 (12), 172 (72), 146 (13), 144 (19), 109 (19), 74 + (17); HRMS m/z: [M] calcd for C8H4Cl2N2O2: 229.9650; found: 229.9653. N-Amino-2,3-pyrazinedicarboxylicphthalimide (2d)[42]: Brown solid; 82% yield; mp 220– ◦ 1 13 222 C; H NMR (D2O, 400 MHz) δ 8.59 (s, 2H, ArH); C NMR (D2O, 100 MHz) δ 143.28 (2 × C), 149.18 (2 × C), 172.29 (2 × C); FT-IR (KBr): 3430, 3282, 3164, 2936, 2750, 2636, 1679, 1621, 1353, 1161, 1107, 975, 825 cm−1. EIMS m/z: 164 (M+, 36), 150 (25), 124 (40), 106 (76), Molecules 2021, 26, 2907 10 of 15

+ 80 (100), 79 (30), 78 (56), 53 (69), 52 (95), 51 (52). HRMS m/z: [M] calcd for C6H4N4O2: 164.0334; found: 164.0338. Naphthalene-2,3-dicarboxylic hydrazide (2e): Yellow solid; 94% yield; mp 298–300 ◦C; 1H NMR (DMSO-d6, 400 MHz) δ 7.51–7.56 (m, 2H, ArH), 7.92–7.96 (m, 2H, ArH), 8.11 (s, 1H, 13 ArH), 8.16 (s, 1H, ArH); C NMR (DMSO-d6, 100 MHz) δ 127.12, 127.52, 128.32, 128.48, 128.68, 128.87, 132.17, 132.32, 133.31, 137.68, 168.21, 171.85; FT-IR (KBr): 3466, 3429, 3304, 3190, 3054, 2653, 1673, 1636, 1606, 1390, 1339, 1177, 1140, 1099, 964, 809, 758, 481 cm−1; EIMS m/z: 213 (14), 212 (M+, 100), 155 (10), 154 (45), 127 (14), 126 (39); HRMS m/z: [M]+calcd for C12H8N2O2: 212.0586; found: 212.0578. 7-Amino-1,3-diphenylpyrazolo[3,4-b]pyrrolo[3,4-d]pyridine-6,8(3H,7H)-dione (2f)[28]: White ◦ 1 solid; 83% yield; mp 217–219 C; H NMR (DMSO-d6, 400 MHz) δ 4.55 (br, 2H, NH2), 7.42 (t, J = 7.46 Hz, 1H, ArH), 7.49–7.55 (m, 3H, ArH), 7.60–7.65 (m, 4H, ArH), 8.25 (d, J = 8.08 Hz, 2H, ArH), 8.82 (s, 1H, ArH).

3.3. Standard Procedure for Synthesis of Phthalazine 1,4-Diones 3a–f The reliable procedure involved the treatment of isobenzofuran-1,3-diones (1a–c), isoindoline-1,3-dione (1d), furo[3,4-b] pyrazine-5,7-dione (1e), or 1H-pyrrolo[3,4-c] pyridine- 1,3-dione (1f, 1.0 equiv.) with monohydrate hydrazine (~40 equiv.) in EtOH solution (2.0 mL) at room temperature or in neat at reﬂux for 4 h. When the reaction was completed, the reaction mixture was added water (10 mL) for precipitation. The precipitation was ﬁltered, washed with cold water (10 mL) and n-hexane/EA (1/2, 15 mL) to give the corresponding crude phthalazine 1,4-diones 3a–f. The crude desired products 3a–f were recrystallized in acetone/THF (1/4) solution to obtain the pure phthalazine 1,4-diones 3a–f in 83–91% yields. The low solubility of the compounds 3a–f made the 13C-NMR characterization of quaternary and carbonyl carbons of these substrates unclear [28]. 2,3-Dihydro-phthalazine-1,4-dione (3a)[43]: White solid; 89% yield; mp 227–229 ◦C; 1H NMR (DMSO-d6, 400 MHz) δ 7.82 (dd, J = 5.93, 3.30 Hz, 2H, ArH), 8.06 (dd, J = 5.89, 3.28 Hz, 13 2H, ArH); C NMR (DMSO-d6, 100 MHz) δ 125.78 (2 × C), 128.68 (2 × C), 132.35 (2 × C), 156.41 (2 × C); FT-IR (KBr): 3461, 3207, 2715, 1773, 1749, 1609, 1468, 1386, 1309, 1051, 715 cm−1. 6,7-Fluoroo-2,3-dihydrophthalazine-1,4-dione (3b): White solid; 85% yield; mp 220–222 ◦C; 1H 13 NMR (DMSO-d6, 400 MHz) δ 7.99 (t, J = 9.11 Hz, 2H, ArH); C NMR (DMSO-d6, 100 MHz) δ 114.38 (2 × C), 126.58 (2 × C), 151.48 (d, J = 16.11 Hz), 154.02 (d, J = 16.10 Hz), 154.61 (2 × C); FT-IR (KBr): 3490, 3180, 3070, 2610, 1660, 1590, 1511, 1460, 1354, 1303, 1189, 1067, 899, 804, 565 cm−1; EIMS m/z: 199 (13), 198 (M+, 100), 141 (23), 140 (75), 113 (17), 112 (30), 63 + (13); HRMS m/z: [M] calcd for C8H4F2N2O2: 198.0241; found: 198.0234. 6,7-Dichloro-2,3-dihydrophthalazine-1,4-dione (3c): White solid; 87% yield; mp 237–239 ◦C; 1H 13 NMR (DMSO-d6, 400 MHz) δ 8.15 (s, 2H, ArH); C NMR (DMSO-d6, 100 MHz) δ 128.16 (2 × C), 129.64 (2 × C), 134.98 (2 × C), 156.12 (2 × C); FT-IR (KBr): 3314, 3193, 2984, 2926, 2879, 1666, 1565, 1467, 1450, 1373, 1075, 822 cm−1; EIMS m/z: 232 (M+ + 2, 64), 231 (M+ + 1, 17) 230 (M+, 100), 174 (46), 173 (12), 172 (72), 146 (13), 144 (19), 109 (19), 74 (17); HRMS m/z: + [M] calcd for C8H4Cl2N2O2: 229.9650; found:229.9643. 7-Dihydropyrazino[2,3-d] pyridazine-5,8-dione (3d)[44]: Brown solid; 83% yield; mp 325– ◦ 1 13 327 C; H NMR (D2O, 500 MHz) δ 8.82 (s, 2H, ArH); C NMR (D2O, 125 MHz) δ 145.14 (2 × C), 146.29 (2 × C), 169.64 (2 × C). FT-IR (KBr): 3343, 3204, 1661, 1632, 1543, 1314, 1293, 1100 cm−1; EIMS m/z: 164 (M+, 1), 150 (26) 140 (16), 106 (77), 80 (100), 78 (46), 52 (2). 2,3-Dihydrobenzo[g]phthalazine-1,4-dione (3e): White solid; 91% yield; mp 301–303 ◦C; 1H NMR (DMSO-d6, 400 MHz) δ 7.71(dd, J = 6.29, 3.27 Hz, 2H, ArH), 8.25 (dd, J = 6.28, 3.28 Hz, 2H, ArH), 8.72 (s, 2H, ArH); 13C NMR (DMSO-d6, 100 MHz) δ 125.13 (2 × C), 126.69 (2 × C), 128.84 (2 × C), 129.66 (2 × C), 134.54 (2 × C), 156.22 (2 × C); FT-IR (KBr): 3416, 1663, Molecules 2021, 26, 2907 11 of 15

1626, 1501, 1467, 1440, 1366, 1072, 815 cm−1; EIMS m/z: 213 (14), 212 (M+, 100), 155 (12), 154 + (43), 237 (17), 126 (44); HRMS m/z: [M] calcd for C12H8N2O2: 212.0586; found: 212.0588. 1,3-Diphenyl-7,8-dihydro-3H-pyrazolo [4’,3’:5,6] pyrido[3,4-d] pyridazine-6,9-dione (3f) [28]: ◦ 1 white solid; 84% yield; mp 292–295 C; H NMR (DMSO-d6, 600 MHz) δ 7.43–7.47 (m, 4H, ArH), 7.60–7.64 (m, 4H, ArH), 8.20 (d, J = 7.88 Hz, 2H, ArH), 9.43 (s, 1H, ArH).

3.4. Standard Procedure for Preparation of Amidination Products 4 and 6–10 from N-Aminophthalimides 2a–f with Vilsmeier Reagent (POCl3/DMF) The reliable procedure that involved N-aminophthalimides 2a–f (1.0 equiv..) was individually treated with ~3.0 equivalent amount of POCl3 in N, N-dimethylformamide solution (DMF, 2.0 mL) at 50 ◦C or 65 ◦C for 0.5–4 h. When the reaction was completed, the reaction mixture was added to saturate sodium bicarbonate (15 mL) and extracted with dichloromethane (15 mL). The organic extracts were dried over MgSO4, filtered, and con- centrated under reduced pressure. The residues were purified by column chromatography on silica gel to give the corresponding amidination products 4 in 89% yield and 6–10 in 74–88% yields [37–39]. N’-(6,8-Dioxo-1,3-diphenyl-6,8-dihydropyrazolo[3,4-b]pyrrolo[3,4-d]pyridin-7(3H)-yl)-N,N-dime- ◦ 1 thylformimidamide (4): Brown solid; 89% yield; mp 225–227 C; H NMR (CDCl3, 400 MHz) δ 2.97 (s, 3H, NMe), 3.02 (s, 3H, NMe), 7.37 (t, J = 7.36 Hz, 1H, ArH), 7.48–7.50 (m, 3H, ArH), 7.54 (t, J = 7.74 Hz, 2H, ArH), 7.70 (s, 1H, ArH), 7.87 (d, J = 4.40 Hz, 2H, ArH), 8.24 (d, J = 13 8.00 Hz, 2H, ArH), 9.03 (s, 1H, ArH); C NMR (CDCl3, 100 MHz) δ 33.83, 41.06, 108.85, 119.70, 122.21 (2 × C), 127.20, 127.90 (2 × C), 129.14 (2 × C), 129.37, 129.82 (2 × C), 131.52, 133.65, 138.40, 143.04, 146.08, 153.77, 161.77, 163.79, 165.45; FT-IR (KBr): 3064, 2923, 2852, 1714, 1626, 1500, 1413, 846 cm−1; EIMS m/z: 411 (25), 410 (M+, 100), 341 (29), 340 (87), 339 (52), 268 (14), 77 (31); HRMS calcd. For C23H18N6O2: 410.1491; found: 410.1482. N’-(1,3-Dioxo-1,3-dihydro-2H-isoindol-2-yl)-N,N-dimethyliminoformamide hydrochloride (6): Yel- ◦ 1 low solid; 88% yield; mp 177–179 C; H NMR (CDCl3, 400 MHz) δ 3.02 (s, 6H, N(CH3)2), 7.66 (dd, J = 5.37, 3.03, 2H, ArH), 7.75 (s, 1H), 7.79 (dd, J = 5.42, 3.09, 2H, ArH); 13C NMR (CDCl3, 100 MHz) δ 34.84, 41.10, 123.01 (2 × C), 123.58 (2 × C), 130.77 (2 × C), 133.77 (2 × C), 161.52, 166.38 (2 × C); FT-IR (KBr): 3446, 2933, 1699, 1620, 1321, 1138, 706 cm−1; EIMS m/z: 218 (12), 217 (M+, 100), 148 (19), 130 (27), 105 (17), 104 (22), 90 (11), 76 (29), 71 (41), 70 + (21); HRMS m/z: [M] calcd for C11H11N3O2: 217.0851; found: 217.0842. N’-(1,3-Dioxo-5,6-difluoro-2H-isoindolin-2-yl)-N,N-dimethylformimidamide (7): Light yellow ◦ 1 solid; 74% yield; mp 183–185 C; H NMR (CDCl3, 400 MHz) δ 2.88 (s, 3H, NMe), 2.96 (s, 13 3H, NMe), 7.69 (t, J = 8.96 Hz, 2H, ArH), 8.04 (s, 1H); C NMR (CDCl3, 100 MHz) δ 31.83, 36.98, 119.75 (dd, J = 13.4, 7.63 Hz, 2 × C), 129.46 (t, J = 4.77 Hz, 2 × C), 150.11 (d, J = 14.45 Hz), 152.68 (d, J =14.31 Hz), 163.68, 168.39 (2 × C); FT-IR (KBr): 3048, 2919, 1695, 1620, 1450, 1355, 1148, 818 cm−1; EIMS m/z: 254 (13), 253 (M+, 100), 166 (20), 141 (25), 140 (40), 139 (16), 126 (22), 125 (12), 113 (13), 112 (52), 111 (20), 109 (16), 97 (23), 95 (17), 85 (16), 83 (21), 81 (17); + HRMS m/z: [M] calcd for C11H9F2N3O2: 253.0663; found: 253.0657. N’-(1,3-Dioxo-5,6-dichloro-2H-isoindolin-2-yl)-N,N-dimethylformimidamide (8): Light orange ◦ 1 solid; 85% yield; mp 192–194 C; H NMR (CDCl3, 400 MHz) δ 3.04 (s, 6H, N(CH3)2), 7.74 13 (s, 1H), 7.87 (s, 2H, ArH); C NMR (CDCl3, 100 MHz) δ 34.80, 41.07, 125.09 (2 × C), 129.90 (2 × C), 138.54 (2 × C), 161.35, 164.42 (2 × C). FT-IR (KBr): 3092, 3024, 2926. 1773, 1705, 1624, 1345, 1145, 774 cm−1; EIMS m/z: 287 (M+ + 2, 61), 286 (M+ +1, 12), 285 (M+, 100), 198 (13), 175 (12), 174 (14), 173 (22), 172 (20), 146 (16), 144 (24), 109 (11), 71 (98), 70 (36), 69 (10); + HRMS m/z: [M] calcd for C11H9Cl2N3O2: 285.0072; found: 285.0064. N’-(1,3-Dioxo-5,7-dihydro-6H-pyrrolo[3,4-b]pyrazin-6-yl)-N,N-dimethylformimidamide (9): Light ◦ 1 yellow solid; 88% yield, mp 219–221 C; H NMR (CDCl3, 400 MHz) δ 3.03 (s, 3H, NMe), 13 3.05 (s, 3H, NMe), 7.81 (s, 1H), 8.84 (s, 2H, ArH); C NMR (CDCl3, 100 MHz) δ 34.83, 41.18, 146.17 (2 × C), 148.52 (2 × C), 161.28, 162.31 (2 × C); FT-IR (KBr): 1654, 1289, 1545, 1293, 1104, 825 cm−1; EIMS m/z: 220 (12), 219 (M+, 56), 193 (10), 179 (13), 178 (13), 169 (12), 168 Molecules 2021, 26, 2907 12 of 15

(12), 167 (14), 165 (11), 155 (11), 152 (11), 151 (26), 150 (14), 149 (23), 147 (11), 141 (12), 139 (15), 137 (13), 135 (11), 127 (12), 125 (21), 123 (19), 121 (11), 119 (11), 115 (14), 113 (14), 112 (14), 111 (36), 110 (11), 109 (27), 107 (15), 106 (16), 105 (16), 99 (17), 98 (13), 97 (52), 96 (16), + 95 (36), 93 (12), 91 (27); HRMS m/z: [M] calcd for C9H9N5O2: 219.0756; found: 219.0748. N’-(1,3-Dioxo-5,6-dihydro-2H-benzo[f]isoindol-2-yl)-N,N-dimethylformimidamide (10): Light ◦ 1 yellow solid; 77% yield; mp 199–201 C; H NMR (CDCl3, 400 MHz) δ 3.05 (s, 3H, NMe), 3.07 (s, 3H, NMe), 7.64–7.69 (m, 2H, ArH), 7.84 (s, 1H), 7.99–8.02 (m, 2H, ArH), 8.28 (s, 2H, 13 ArH); C NMR (CDCl3, 100 MHz) δ 34.89, 41.10, 124.33 (2 × C), 126.68 (2 × C), 128.99 (2 × C), 130.20 (2 × C), 135.45 (2 × C), 161.23, 166.00 (2 × C) cm−1; FT-IR (KBr): 2918, 2807, 1757, 1693, 1621, 1521, 1425, 1411, 1321, 1154, 1118, 1004, 900, 754, 479 cm−1; EIMS m/z: 268 (19), 267 (M+, 100), 225 (13), 210 (10), 198 (26), 197 (49), 180 (28), 155 (64), 154 (24), 153 (20), + 152 (13), 140 (21), 127 (26), 126 (74), 71 (17), 57 (11); HRMS m/z: [M] calcd for C15H13N3O2: 267.1008; found: 267.1015.

3.5. Standard Procedure for Preparation of Chlorination Products 5, and 11–15 from Phthalazine 1,4-Diones 3a–f with Vilsmeier Reagent (POCl3/DMF) The reliable procedure that involved phthalazine 1,4-diones 3a–f (1.0 equiv.) was individually treated with ~3.0 equivalent amount of POCl3 in N, N-dimethylformamide solution (DMF, 2.0 mL) at 65 ◦C or 80 ◦C for 2–4 h. When the reaction was completed, the reaction mixture was added to saturate sodium bicarbonate (15 mL) and extracted with dichloromethane (15 mL). The organic extracts were dried over MgSO4, filtered, and con- centrated under reduced pressure. The residues were purified by column chromatography on silica gel to give the corresponding chlorinated products 5 in 80% yield and 11–15 in 31–90% yields [29]. 6,9-Dichloro-1,3-diphenyl-3H-pyrazolo[4’,3’:5,6]pyrido[3,4-d]pyridazine (5): Light yellow solid; ◦ 1 80% yield; mp 196–197 C; H NMR (CDCl3, 500 MHz) δ 7.46 (t, J = 7.43 Hz, 1H, ArH), 7.49–7.51 (m, 3H, ArH), 7.57–7.60 (m, 4H, ArH), 8.17 (dd, J = 8.60, 1.09 Hz, 2H, ArH), 9.68 13 (s, 1H, ArH); C NMR (CDCl3, 125 MHz) δ 103.92, 118.46, 123.39 (2 × C), 127.91, 127.95 (2 × C), 128.19, 129.13, 129.33 (2 × C), 130.01 (2 × C), 135.22, 137.87, 147.79, 149.88, 151.00, 152.05, 153.49; FT-IR (KBr): 3064, 2923, 2846, 1571, 1501, 1413, 1243, 1119, 863 cm−1; EIMS m/z: 395 (12), 394 (17), 393 (M+ + 2, 65), 392 (36), 391 (M+, 99), 390 (20), 356 (20), 321 (35), 320 (60), 288 (14), 263 (12), 244 (33), 218 (17), 91 (19), 77 (100); HRMS calcd. For C20H11Cl2N5: 391.0392; found: 391.0397. ◦ 1 1,4-Dichlorophthalazine (11): Yellow solid; 90% yield; mp 162–164 C; H NMR (CDCl3, 400 13 MHz) δ 7.74–7.76 (m, 2H, ArH), 7.85–7.7.87 (m, 2H, ArH); C NMR (CDCl3, 100 MHz) δ 125.86 (2 × C), 127.21 (2 × C), 134.49 (2 × C), 155.03 (2 × C); IR (KBr): 3157, 3000, 2896, 2875, 1671, 1346, 1289, 1157, 993, 775, 664 cm−1; EIMS m/z: 202 (M+ + 4, 10), 200 (M+ + 2, 63), 198 (M+, 100), 182 (25), 180 (77), 172 (17), 170 (26), 151 (17), 135 (20), 128 (14), 125 (11), 123 (29), 102 (17), 101 (11), 99 (20), 90 (11). HRMS calcd. For C8H4Cl2N2: 197.9752; found: 197.9746. 1,4-Dichloro-2,3-difluorophthalazine (12): White solid; 82% yield; mp 75–76 ◦C; 1H NMR 13 (CDCl3, 400 MHz) δ 8.09 (t, J = 8.34 Hz, 2H, ArH); C NMR (CDCl3, 100 MHz) δ 113.97 (dd, J = 14.25, 7.66 Hz, 2 × C), 125.19 (t, J = 5.23 Hz, 2 × C), 153.50 (d, J = 16.46 Hz), 153.68 153.50 (2 × C), 156.15 (d, J = 16.39 Hz); IR (KBr): 3143, 3068, 2836, 2782, 2718, 2611, 1625, 1571, 1539, 1511, 1389, 1218, 1161, 1104, 893, 814 cm−1; EIMS m/z: 238 (M+ + 4, 11), 236 (M+ + 2, 78), 234 (M+, 100), 234 (30), 218 (12), 216 (36), 208 (24), 206 (38), 171 (27), 164 (30), 159 (18), 138 (11), 136 (16), 124 (10), 88 (15), 75 (11); HRMS calcd. For C8H2Cl2F2N2: 233.9563; found: 233.9568. 1,4,6,7-Tetrachlorophthalazine (13): Light orange solid; 84% yield; mp 135–136 ◦C; 1H NMR 13 (CDCl3, 400 MHz) δ 8.40 (s, 2H, ArH); C NMR (CDCl3, 100 MHz) δ 126.07 (2 × C), 127.39 (2 × C), 140.2 (2 × C), 153.45 (2 × C); IR (KBr): 3094, 1596, 1535, 1453, 1501, 1380, 1268, 1242, 1130, 1035, 702, 663 cm−1; EIMS m/z: 270 (M+ + 2, 26), 270 (M+ + 2, 16), 268 (M+, 100), Molecules 2021, 26, 2907 13 of 15

266 (77), 240 (19), 238 (14), 205 (14), 203 (14), 196 (13), 84 (10); HRMS calcd. For C8H2Cl4N2: 265.8972; found: 265.8979. 6,7-Dichloropyrazino[2,3-d] pyridazine (14)[28]: Black solid; 31% yield; mp 204–206 ◦C; 1H NMR (CDCl3, 400 MHz) δ 9.14 (s, 1H, ArH), 9.17 (s, 1H, ArH)[28]. ◦ 1 1,4-Dichlorobenzo[g]phthalazine (15): Gray solid; 85% yield; mp 212–214 C; H NMR (CDCl3, 400 MHz) δ 7.78–7.81 (m, 2H, ArH), 8.18–8.21 (m, 2H, ArH), 8.82 (s, 2H, ArH); 13C NMR (CDCl3, 100 MHz) δ 123.23 (2 × C), 126.83 (2 × C), 129.22 (2 × C), 129.79 (2 × C), 135.46 (2 × C), 155.39 (2 × C); IR (KBr): 2961, 2932, 2857, 1739,1725, 1461, 1282, 1264, 1121, 1075, 739 cm−1; EIMS m/z: 250 (M+ + 2, 59), 249 (M+ + 1, 11), 248 (M+, 100), 178 (31), 152 (19), 151 (14), 150 (18); HRMS calcd. For C23H18N6O2: 247.9908; found: 247.9906.

4. Conclusions N-aminophthalimides 2 and phthalazine 1,4-diones 3 were successfully and selectively synthesized from isobenzo-furan-1,3-diones (1a–c), isoindoline-1,3-dione (1d), furo[3,4-b] pyrazine-5,7-dione (1e), and 1H-pyrrolo[3,4-c] pyridine-1,3-dione (1f) with monohydrate hydrazine under the different reaction condition. The structural divergence between N-aminophthalimides 2f and phthalazine 1,4-diones 3f was effectively identiﬁed via Vilsmeier reaction methodology. Furthermore, the thermodynamically transformation from N-aminophthalimides 2a–f to phthalazine 1,4-diones 3a–f was successfully found, which provides the good conversion ratio from 6/94 to 1/99 of 2a–f /3a–f under acetic acid mediated solution.

Supplementary Materials: The following are available online, copies of 1H and 13C-NMR spectra of compounds 2a–2e, 3a–3e, and 4–15, Table S1: Crystal data and structure refinement for N’-(6,8- dioxo6,8-dihydropyrazolopyrrolo-pyridine-yl)-N,N-dimethylformimidamide 4 (CCDC No. 1954819). Author Contributions: F.F.W. conceived and designed the experiments; C.-Y.C., C.-C.T., S.-M.L. and S.-E.T. performed the experiments; C.-Y.C., C.-C.T. and S.-M.L. analyzed the data; F.F.W., H.-Y.L., contributed reagents/materials/analysis tools; F.F.W., C.-Y.C., C.-C.T. and S.-E.T. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the China Medical University (CMU109-ASIA-08 and CMU109-MF-86) and the Ministry of Science and Technology of Taiwan (MOST 109-2113-M-039-003). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in the article and the Supplementary Materials. Acknowledgments: We are grateful to the China Medical University (CMU109-ASIA-08 and CMU109- MF-86) and the Ministry of Science and Technology of Taiwan (MOST 109-2113-M-039-003) for financial support. Conflicts of Interest: The authors declare no conflict of interest. Sample Availability: Samples of the compounds are available from the authors.

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