molecules Article Enantiomeric Resolution and Absolute Configuration of a Chiral δ-Lactam, Useful Intermediate for the Synthesis of Bioactive Compounds 1, 1, 1 1 Roberta Listro y, Giacomo Rossino y, Serena Della Volpe , Rita Stabile , Massimo Boiocchi 2 , Lorenzo Malavasi 3, Daniela Rossi 1,* and Simona Collina 1 1 Department of Drug Sciences, University of Pavia, via Taramelli 12, 27100 Pavia, Italy; [email protected] (R.L.); [email protected] (G.R.); [email protected] (S.D.V.); [email protected] (R.S.); [email protected] (S.C.) 2 Centro Grandi Strumenti, University of Pavia, via Bassi 21, 27100 Pavia, Italy; [email protected] 3 Department of Chemistry, University of Pavia, via Taramelli 12, 27100 Pavia, Italy; [email protected] * Correspondence: [email protected] These authors contributed equally to this work. y Academic Editor: Józef Drabowicz Received: 2 December 2020; Accepted: 18 December 2020; Published: 19 December 2020 Abstract: During the past several years, the frequency of discovery of new molecular entities based on γ- or δ-lactam scaffolds has increased continuously. Most of them are characterized by the presence of at least one chiral center. Herein, we present the preparation, isolation and the absolute configuration assignment of enantiomeric 2-(4-bromophenyl)-1-isobutyl-6-oxopiperidin-3-carboxylic acid (trans-1). For the preparation of racemic trans-1, the Castagnoli-Cushman reaction was employed. (Semi)-preparative enantioselective HPLC allowed to obtain enantiomerically pure trans-1 whose absolute configuration was assigned by X-ray diffractometry. Compound (+)-(2R,3R)-1 represents a reference compound for the configurational study of structurally related lactams. Keywords: lactam scaffold; enantioselective HPLC; chiral resolution; X-ray diffraction; absolute configuration assignment 1. Introduction Drug discovery is a complex process aimed at identifying new biologically active compounds with a high degree of structural novelty [1]. It can be driven by the exploration of the chemical space around a drug or a selected scaffold, the core structure of the molecular framework. Scaffolds are used as starting points for compound synthesis or diversification and, therefore, their study represents an effective and promising approach for finding new potent drugs [2–5]. In recent years, substituted lactams have emerged as an important pharmacophore for several drug classes, with a large spectrum of biological outcomes and various therapeutic applications [6–8]. From a structural standpoint, they can be four-, five-, six- and seven-membered rings, called β-lactams, γ-lactam, δ-lactam and "-lactam, respectively. Several drugs with a lactam structure have already reached the market, and a few representative examples are reported in Figure1. These include antibiotics with a β-lactam structure (i.e., penicillins, cephalosporins, monobactams, carbapenems) [9], the γ-lactam Lenalidomide (a derivative of Thalidomide endowed with immunomodulatory and antiangiogenic activity) [10,11], the δ-lactam Dolutegravir (a HIV-1 integrase inhibitor) [12] and benzodiazepines based on the "-lactam scaffold (i.e., Prazepam), well known allosteric modulators of the receptor GABA A. [13]. Molecules 2020, 25, 6023; doi:10.3390/molecules25246023 www.mdpi.com/journal/molecules Molecules 2020, 25, x 2 of 16 immunomodulatory and antiangiogenic activity) [10,11], the δ-lactam Dolutegravir (a HIV-1 integrase inhibitor) [12] and benzodiazepines based on the ε-lactam scaffold (i.e., Prazepam), well known allosteric modulators of the receptor GABA A. [13]. Molecules 2020, 25, x 2 of 16 immunomodulatory and antiangiogenic activity) [10,11], the δ-lactam Dolutegravir (a HIV-1 integrase Moleculesinhibitor)2020 [12], 25, 6023 and benzodiazepines based on the ε-lactam scaffold (i.e., Prazepam), well known2 of 15 allosteric modulators of the receptor GABA A. [13]. Figure 1. Examples of marketed drugs based on the lactam scaffold. In the last few years, the interest of MedChem researchers towards lactams as chemical scaffolds has grown, as evidenced by the increasing number of publications (Figure 2), due to their high potential for discovering drug candidates. In fact, lactams possess attractive features such as versatility and easy derivatization, and, additionally, they are conformationally restricted scaffolds with peptidomimetic features. Thus, they can be used to improve the potency, selectivity, and metabolic stability of peptide-based drugs [7]. Several MedChem groups have focused on the synthesis of new molecular entities based on γ- or δ-lactam scaffolds as highly versatile key intermediates, leading to the discovery of new anti-trypanosomal, anti-biofilm and anti-inflammation agents [14–18]. For instance, Delong et al. studied novel α-methylene-FigureFigure 1. 1.γExamples Examples-lactams, of ofα marketed-arylidene-marketed drugsdrugsγ and based ba δsed-lactams, on on the the lactam lactamwith novel sca scaffold.ffold. antifungal properties against Colletotrichum orbiculare [19], whereas Davoren et al. prepared a series of molecules with lactamInIn thescaffolds the last last few few and years, years, identified thethe interestinterest a number ofof MedChemMedChem of γ- and researchersresearchers δ-lactams towards abletowards to function lactams lactams as as chemicalpositivechemical allosteric sca scaffoldsffolds hasmodulatorshas grown, grown, as (PAMs)as evidenced evidenced of muscarinic by theby increasingthe increasingreceptors number (M1)number of [20]. publications of Lastly, publications by (Figure applying 2(Figure), due a to combined2), their due high to moleculartheir potential high formodellingpotential discovering -forSTD discovering drug NMR candidates. approach, drug In ourcandidates. fact, group lactams recently In possess fact identified, lactams attractive apossess featurespromising attractive such ligand as versatility featureswith a δ and -lactamsuch easy as derivatization,scaffoldversatility able and to and, easybind additionally, derivatization,the RNA-binding they and, are protein additionally, conformationally HuR [21–23], they restrictedare which conformationally scainteractsffolds withwith restricted peptidomimetictarget mRNAs,scaffolds features.leadingwith peptidomimetic to Thus, the formation they canfeatures. beof usedHuR–mRNA Thus, to improvethey cancomple the be potency,usedxes involvedto selectivity,improve in theseveral and potency, metabolic physio-pathological selectivity, stability and of peptide-basedconditionsmetabolic [24–26].stability drugs of [ 7peptide-based]. drugs [7]. Several MedChem groups have focused on the synthesis of new molecular entities based on γ- or δ-lactam scaffolds as highly versatile key intermediates, leading to the discovery of new 25 anti-trypanosomal, anti-biofilm and anti-inflammation agents [14–18]. For instance, Delong et al. studied novel α-methylene-20 γ-lactams, α-arylidene-γ and δ-lactams, with novel antifungal properties against Colletotrichum orbiculare [19], whereas Davoren et al. prepared a series of molecules with lactam scaffolds and15 identified a number of γ- and δ-lactams able to function as positive allosteric modulators (PAMs)10 of muscarinic receptors (M1) [20]. Lastly, by applying a combined molecular modelling-STD NMR approach, our group recently identified a promising ligand with a δ-lactam 5 scaffold able to bind Number publications of the RNA-binding protein HuR [21–23], which interacts with target mRNAs, leading to the formation0 of HuR–mRNA complexes involved in several physio-pathological conditions [24–26]. 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 Years 25 Figure 2. Papers published20 per year. Source Scopus (31 October 2020), key words “lactams” and “medicinal chemistry”. 15 Several MedChem groups have focused on the synthesis of new molecular entities based on γ- or δ-lactam scaffolds10 as highly versatile key intermediates, leading to the discovery of new anti-trypanosomal, anti-biofilm and anti-inflammation agents [14–18]. For instance, Delong et al. 5 studied novel α-methylene- Number publications of γ-lactams, α-arylidene-γ and δ-lactams, with novel antifungal properties against Colletotrichum orbiculare0 [19], whereas Davoren et al. prepared a series of molecules with lactam scaffolds and identified a2000 number 2002 of 2004γ- and 2006δ-lactams 2008 2010 able 2012to function 2014 2016 as positive 2018 2020 allosteric modulators (PAMs) of muscarinic receptors (M1) [20]. Lastly, byYears applying a combined molecular modelling-STD NMR approach, our group recently identified a promising ligand with a δ-lactam scaffold able to bind the RNA-binding protein HuR [21–23], which interacts with target mRNAs, leading to the formation of HuR–mRNA complexes involved in several physio-pathological conditions [24–26]. Molecules 2020, 25, x 3 of 16 MoleculesFigure2020 ,2.25 Papers, 6023 published per year. Source Scopus (31 October 2020), key words “lactams” and3 of 15 “medicinal chemistry”. Given the high interest in la lactams,ctams, extensive eeffortsfforts have been directed towards the identification identification of versatile syntheticsynthetic strategies strategies for for obtaining obtaining lactams. lactams. Among Among the the di differentfferent methodologies methodologies exploited exploited so far,so far, it is it worth is worth mentioning mentioning the Beckmann the Beckmann rearrangement, rearrangement, the Staudinger the Staudinger procedure procedure