A Stereoselective, Multicomponent Catalytic Carbonylative Approach to a New Class of α , β -Unsaturated γ -Lactam Derivatives

: We report a stereoselective, multicomponent catalytic carbonylative approach to a new class of α , β -unsaturated γ -lactam derivatives with potential biological activity, that are, alkyl ( Z )-2-(2-oxopyrrolidin-3-ylidene)acetates. Our method is based on the catalytic assembly of readily available building blocks, namely, homopropargylic amines, carbon monoxide, an alcohol, and oxy-gen (from air). These simple substrates are efﬁciently activated in ordered sequence under the action of a very simple catalytic system, consisting of PdI 2 in conjunction with KI to give the γ -lactam products in 47–85% yields. Carbonylation reactions are carried out at 100 ◦ C for 2–5 h under 40 atm of a 4:1 mixture of CO-air, with 0.5–5 mol% of PdI 2 and 5–50 mol% of KI.


Introduction
Carbonylation reactions represent an excellent approach for the direct synthesis of carbonyl compounds using the simplest and readily available C-1 source, that is, carbon monoxide [1][2][3][4][5]. CO is produced industrially starting from petroleum hydrocarbons, natural gas, and coal, and, in the future, a growing amount of carbon monoxide is also expected to be available from renewable feedstocks, such as biowastes and CO 2 [6].
In this work, a stereoselective, multicomponent catalytic carbonylative approach to a new class of α,β-unsaturated γ-lactam derivatives, that are, alkyl (Z)-2-(2-oxopyrrolidin-3ylidene)acetates 2, is presented. The synthetic approach is based on the use of a particularly simple catalytic system, consisting of PdI 2 in conjunction with KI [7,8], for the catalytic activation of very simple starting materials (homopropargylic amines 1, carbon monoxide, an alcohol, and oxygen; Scheme 1).
Interestingly, heterocyclic compounds bearing the strictly related 2-(2-oxopyrrolidin-3-yl)acetate moiety (with a saturated exocyclic carbon-carbon bond) have been reported to exhibit different biological activities, including peptidomimetic inhibitory effect [13], and anti-hepatitis B [14], anti-stroke [15], and anti-thrombotic [16] activity. Therefore, the disclosure of an efficient method for the synthesis of 2 can be of interest for the development of novel bioactive small molecules. In particular, the target compounds incorporate into their structure the 2-(2-oxopyrrolidin-3-ylidene)acetate moiety with an exocyclic double bond, which, as a Michael acceptor group, may react with biological thiols (e.g., glutathione, cysteine) [17], thus inducing diverse biological activities. For this purpose, a biochemical and pharmacological investigation on the cytotoxicity and antiproliferative activity of 2 in tumor cell lines is being carried out in our laboratories, and the first noteworthy results will be reported in due course.

Results and Discussion
The synthetic method developed in our work for the preparation of new alkyl (Z)-2-(2-oxopyrrolidin-3-ylidene)acetate derivatives 2 is based on the oxidative carbonylation of readily available 3-yn-1-amines (homopropargylic amines) 1 carried out with the very simple PdI2/KI catalytic system (Scheme 1).
Our initial experiments were based on the use of N-(but-3-yn-1-yl)aniline 1a as model substrate to assess the feasibility of our hypothesis and to optimize the carbonylation conditions. When 1a was allowed to react with CO, MeOH, and O2 (from air), in MeOH as the solvent (0.04 mmol of 1a per mL of MeOH) under 40 atm of a 4:1 mixture of CO-air at 100 °C and in the presence of PdI2 (5 mol%) and KI (0.5 equiv), after 2 h the GLC-MS analysis of the reaction mixture evidenced a complete substrate conversion and the formation of a product, whose MS spectrum was compatible with the desired γ-lactam derivative 2a. This compound was then isolated from the mixture (74% based on starting 1a, Table 1, entry 1) and fully characterized by IR, 1 HNMR, and 13 CNMR spectroscopies and by XRD analysis, which confirmed the structure corresponding to methyl (Z)-2-(2-oxo-1phenylpyrrolidin-3-ylidene)acetate 2a (Figure 1; see the Supplementary Materials for full XRD data). Interestingly, heterocyclic compounds bearing the strictly related 2-(2-oxopyrrolidin-3-yl)acetate moiety (with a saturated exocyclic carbon-carbon bond) have been reported to exhibit different biological activities, including peptidomimetic inhibitory effect [13], and anti-hepatitis B [14], anti-stroke [15], and anti-thrombotic [16] activity. Therefore, the disclosure of an efficient method for the synthesis of 2 can be of interest for the development of novel bioactive small molecules. In particular, the target compounds incorporate into their structure the 2-(2-oxopyrrolidin-3-ylidene)acetate moiety with an exocyclic double bond, which, as a Michael acceptor group, may react with biological thiols (e.g., glutathione, cysteine) [17], thus inducing diverse biological activities. For this purpose, a biochemical and pharmacological investigation on the cytotoxicity and antiproliferative activity of 2 in tumor cell lines is being carried out in our laboratories, and the first noteworthy results will be reported in due course.

Results and Discussion
The synthetic method developed in our work for the preparation of new alkyl (Z)-2-(2-oxopyrrolidin-3-ylidene)acetate derivatives 2 is based on the oxidative carbonylation of readily available 3-yn-1-amines (homopropargylic amines) 1 carried out with the very simple PdI 2 /KI catalytic system (Scheme 1).
Our initial experiments were based on the use of N-(but-3-yn-1-yl)aniline 1a as model substrate to assess the feasibility of our hypothesis and to optimize the carbonylation conditions. When 1a was allowed to react with CO, MeOH, and O 2 (from air), in MeOH as the solvent (0.04 mmol of 1a per mL of MeOH) under 40 atm of a 4:1 mixture of CO-air at 100 • C and in the presence of PdI 2 (5 mol%) and KI (0.5 equiv), after 2 h the GLC-MS analysis of the reaction mixture evidenced a complete substrate conversion and the formation of a product, whose MS spectrum was compatible with the desired γ-lactam derivative 2a. This compound was then isolated from the mixture (74% based on starting 1a, Table 1, entry 1) and fully characterized by IR, 1 HNMR, and 13 CNMR spectroscopies and by XRD analysis, which confirmed the structure corresponding to methyl (Z)-2-(2-oxo-1-phenylpyrrolidin-3-ylidene)acetate 2a (Figure 1; see the Supplementary Materials for full XRD data).
With the aim of obtaining a higher yield of 2a, we then changed the reaction parameters, such as the KI amount, substrate concentration, temperature, and pressure. After this brief optimization study (Table 1, Entries 2-10), 2a was obtained in 85% isolated yield working under the same conditions as those of entry 1, but with a substrate concentration of 0.1 mmol/mL of MeOH (Table 1, entry 7, and Table 2, entry 1). Good yields were still obtained when the process was carried out with lower catalyst loadings (72% yield with 1 mol% of PdI 2 , and 64% yield with 0.5 mol% of PdI 2 ; Table 2, entries 2 and 3, respectively). The use of EtOH instead of MeOH did not cause a significant change in product yield, the corresponding ethyl ester 2a being obtained in 82% yield with 5 mol% PdI 2 ( working under the same conditions as those of entry 1, but with a substrate concentration of 0.1 mmol/mL of MeOH (Table 1, entry 7, and Table 2, entry 1). Good yields were still obtained when the process was carried out with lower catalyst loadings (72% yield with 1 mol% of PdI2, and 64% yield with 0.5 mol% of PdI2; Table 2, entries 2 and 3, respectively). The use of EtOH instead of MeOH did not cause a significant change in product yield, the corresponding ethyl ester 2a' being obtained in 82% yield with 5 mol% PdI2 (   Our next step was to verify the generality of the process using other differently substituted homopropargylic amines 1b-n ( Table 2, entries 8-27). Very good results were consistently obtained with all the tested N-aryl-substituted substrates 1b-1k, bearing electron-withdrawing as well as electron-donating substituents (entries [8][9][10][11][12][13][14][15][16][17][18][19][20]. The structure of another representative product, methyl (Z)-2-(1-(4-chlorophenyl)-2-oxopyrrolidin-3ylidene)acetate 2b, was confirmed again by XRD analysis (Figure 2; see the Supplementary Materials for full XRD data). We assessed the possibility to work with a lower catalyst loading (1 mol% of PdI 2 ) also for substrates 1b ( Table 2, entry 9), 1c ( Table 2, entry 11), and 1e (Table 2, entry 14), still with satisfactory results. In fact, the yields of products 2b, 2c, and 2e were only slightly inferior with respect to those obtained with 5 mol% of PdI 2 ( Table 2; compare entries 9, 11, and 14 with entries 8, 10, and 13, respectively).              The carbonylation protocol led to lower γ-lactam yields when starting from N-alkyl substituted substrates, such as N-benzylbut-3-yn-1-amine 1l and N-(1-phenylethyl)but-3-yn-1-amine 1m, as shown in entries 21-23 and 25, respectively. Interestingly, however, the yields of products 2l and 2m could be significantly improved by carrying out the catalytic process under more diluted conditions (entries 24 and 26, respectively). This effect of substrate concentration on the reaction outcome is difficult to rationalize, and we refrain from proposing speculative interpretations. These conditions permitted to achieve an acceptable product yield even when starting from substrate 1n, bearing a highly bulky alkyl group on nitrogen (yield of methyl (Z)-2-(1-(tert-butyl)-2-oxopyrrolidin-3-ylidene)acetate 2n was 47%; Table 2, entry 27).
Based on the existing knowledge on carbonylations [1][2][3][4][5] and on our expertise in PdI 2 /KI-catalyzed carbonylation reactions [7,8], we can propose the mechanistic pathway shown in Scheme 2 for the formation of (Z)-2-(2-oxopyrrolidin-3-ylidene)acetates 2. Palladation of the nitrogen of 1 initially leads to alkynylaminopalladium intermediate I, stabilized by the intramolecular triple bond coordination (anionic iodide ligands are omitted for clarity). Coordination of CO followed by migratory insertion then affords the carbamoylpalladium complex II, which undergoes intramolecular syn triple bond insertion to yield III, followed by a second insertion of carbon monoxide to give IV. Nucleophilic displacement by ROH on IV finally yields 2 and Pd(0), which is readily oxidized back to catalytically active PdI 2 by the action of O 2 .

General Experimental Methods
All reactions were analyzed by TLC on silica gel 60 F254 or on neutral alumina and by GLC-MS using a Shimadzu QP-2010 GC-MS apparatus (Shimadzu Italia s.r.l., Milano, Italy) and capillary columns with polymethylsilicone + 5% phenylsilicone as the stationary phase. Column chromatography was performed on silica gel 60 (Merck, 70-230 mesh; Merck Life Science s.r.l., Milano, Italy). Evaporation refers to the removal of solvent under reduced pressure.
Melting points are uncorrected. 1 H NMR and 13 C NMR spectra were recorded at 25 • C in CDCl 3 at 300 MHz and 75 MHz, respectively, with Me 4 Si as internal standard, using a Bruker DPX Avance 300 NMR Spectrometer (Bruker Italia s.r.l., Milano, Italy); chemical shifts (δ) and coupling constants (J) are given in ppm and in Hz, respectively. IR spectra were taken with a JASCO FT-IR 4200 spectrometer (Jasco Europe s.r.l., Cremella, Lecco, Italy). Mass spectra were obtained using a Shimadzu QP-2010 GC-MS apparatus (Shimadzu Italia s.r.l., Milano, Italy) at 70 eV ionization voltage (normal resolution) and by electrospray ionization mass spectrometry (ESI-MS) (high resolution) with an Agilent 1260 Infinity UHD accurate-mass Q-TOF spectrometer (Agilent Technologies Italia s.p.a., Cernusco Sul Naviglio, Milano, Italy), equipped with a Dual AJS ESI source working in positive mode, and were recorded in the 150-1000 m/z range. The LC-MS experimental conditions were as follows: The flow-rate was 0.4 mL/min and the column temperature was set to 30 • C. The eluents were formic acid-water (0.1:99.9, v/v) (phase A) and formic acid-acetonitrile (0.1:99.9, v/v) (phase B). The following gradient was employed: 0-10 min, linear gradient from 5% to 95% B; 10-15 min, washing and reconditioning of the column to 5% B. Injection volume was 10 uL. N 2 was employed as desolvation gas at 300 • C and a flow rate of 8 L/min. The nebulizer was set to 45 psig. The Sheat gas temperature was set at 400 • C and a flow of 12 L/min. A potential of 3.5 kV was used on the capillary for positive ion mode. The fragmentor was set to 175 V.

Preparation of Substrates 1a-g and 1l-n
N-Substituted 3-yn-1-amines 1a-g and 1l-n were prepared by the reaction of 3-yn-1-yl tosylates with a primary amine as described below. All starting materials were commercially available (Merck Life Science s.r.l., Milano, Italy) and were used without further purification.

Preparation of Substrates 1h-k
N-Substituted but-3-yn-1-amines 1h-k were prepared by propargylation of the corresponding imines as described below. All starting materials were commercially available (Merck Life Science s.r.l., Milano, Italy) and were used without further purification.
The aqueous phase was extracted with AcOEt (3 × 100 mL) and the combined organic layers were washed with brine (100 mL), dried over K 2 CO 3 , and concentrated under vacuum. The crude products were purified by column chromatography on silica gel using as eluent hexane−AcOEt from 100:0 to 95:5.

Conclusions
In conclusion, we have shown that our simple PdI 2 -KI catalytic system is able to catalyze the oxidative carbonylation of readily available homopropargylic amines 1 to give (2-(2-oxopyrrolidin-3-ylidene)acetates) 2 under relatively mild conditions (100 • C for 2-5 h under 40 atm of a 4:1 mixture of CO-air). This stereoselective multicomponent catalytic approach occurs through an ordered sequence of steps starting from simple building blocks, involving nitrogen palladation of the homopropargylic amine followed by CO insertion, intramolecular syn triple bond insertion with 5-exo-dig cyclization, further CO insertion and alcoholysis. The method has been successfully applied to a variety of differently substituted alkyne substrates and different alcohols (including MeOH, primary, secondary, and tertiary alcohols) to give the new α,β-unsaturated γ-lactam derivatives in 47-85% yields.
The biological evaluation of some of the novel α,β-unsaturated γ-lactams synthesized in this work, aimed at assessing their cytotoxicity toward tumor cells and their probable biomolecular targets, is currently underway in our laboratories and the results will be reported elsewhere.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.