Synthesis of 1,5-Substituted Pyrrolidin-2-ones from Donor–Acceptor Cyclopropanes and Anilines/Benzylamines

We developed a straightforward synthetic route to pharmacologically important 1,5-substituted pyrrolidin-2-ones from donor–acceptor cyclopropanes bearing an ester group as one of the acceptor substituents. This method includes a Lewis acid-catalyzed opening of the donor–acceptor cyclopropane with primary amines (anilines, benzylamines, etc.) to γ-amino esters, followed by in situ lactamization and dealkoxycarbonylation. The reaction has a broad scope of applicability; a variety of substituted anilines, benzylamines, and other primary amines as well as a wide range of donor–acceptor cyclopropanes bearing (hetero)aromatic or alkenyl donor groups and various acceptor substituents can be involved in this transformation. In this process, donor–acceptor cyclopropanes react as 1,4-C,C-dielectrophiles, and amines react as 1,1-dinucleophiles. The resulting di- and trisubstituted pyrrolidin-2-ones can be also used in subsequent chemistry to obtain various nitrogen-containing polycyclic compounds of interest to medicinal chemistry and pharmacology, such as benz[g]indolizidine derivatives.

In general, two types of transformations of DA cyclopropanes can be used for the synthesis of 1,5-substituted pyrrolidin-2-ones. The first one is the (3 + 2)-cycloaddition of 2-aryl-or 2-alkenylcyclopropane-1,1-diesters with the appropriate isocyanates [40,41]. This process directly afforded the corresponding pyrrolidones (Scheme 1a); however, the Although many methods for the γ-lactam synthesis are known [15][16][17][18], the develo ment of new and simple strategies that also make it possible to introduce desired subs uents into the resulting products remains an urgent task. Our interest in this problem related to the possibility of solving it using the donor-acceptor (DA) cyclopropane [19-reactivity, which has been the subject of our studies in recent years [27,28,[34][35][36][37][38][39].
In general, two types of transformations of DA cyclopropanes can be used for synthesis of 1,5-substituted pyrrolidin-2-ones. The first one is the (3 + 2)-cycloaddition 2-aryl-or 2-alkenylcyclopropane-1,1-diesters with the appropriate isocyanates [40,4 This process directly afforded the corresponding pyrrolidones (Scheme 1a); however, resulting products contain two acceptor substituents at the C(3) atom; these groups m be removed to obtain the aforementioned bioactive compounds. Alternatively, these DA cyclopropanes can undergo a small ring opening with Nnucleophiles followed by cyclization producing the target γ-lactams. For example, we have recently developed a method for the synthesis of 1,5-substituted pyrrolidin-2-ones, the key step of which is the opening of the DA cyclopropane ring with an azide ion (Scheme 1b) [37][38][39]. This method includes isolation and purification of the intermediate azides; a simpler and general approach to the synthesis of 1,5-disubstituted pyrrolidin-2ones 2, 3 can be developed based on the reaction of DA cyclopropanes 1 with the corresponding primary amines, such as anilines, benzylamines, etc.
The reactions of DA cyclopropanes with primary amines affording both acyclic and various cyclic products, depending on the structure of the reagents and reaction conditions, have been well studied [29,. However, only a few examples of the use of this reactivity for the synthesis of 1,5-functionalized pyrrolidin-2-ones have been described [59][60][61][62][63][64][65][66][67][68][69][70]. Usually, these examples were reported as postmodifications of the primary acyclic products [59][60][61][62][63][64] that provides not the principal advantage over other stepwise transformations. The one-step formation of the requisite pyrrolidones was achieved either on specific substrates [65][66][67][68][69][70] (Scheme 1c), i.e., has limited application, or proceeded under harsh conditions, giving pyrrolidones in moderate yields [62,64].  Alternatively, these DA cyclopropanes can undergo a small ring opening with Nnucleophiles followed by cyclization producing the target γ-lactams. For example, we have recently developed a method for the synthesis of 1,5-substituted pyrrolidin-2-ones, the key step of which is the opening of the DA cyclopropane ring with an azide ion (Scheme 1b) [37][38][39]. This method includes isolation and purification of the intermediate azides; a simpler and general approach to the synthesis of 1,5-disubstituted pyrrolidin-2ones 2, 3 can be developed based on the reaction of DA cyclopropanes 1 with the corresponding primary amines, such as anilines, benzylamines, etc. In this paper, we demonstrate that the transformation of DA cyclopropanes to 1,5substituted pyrrolidones can be implemented as a one-pot process via a Lewis acid-initiated, three-membered ring opening with anilines, benzylamines, and other primary amines, followed by lactamization, as well as further modifications of the obtained pyrrolidones to polycyclic molecules such as benz[g]indolizidine derivatives (Scheme 1d).

Results and Discussion
We started our investigation with the study of the reaction of model cyclopropane 1a with aniline, leading to the acyclic product 4a (Table 1). To catalyze the reaction, we tested several available Lewis acids, which are commonly used for initiating reactions of DA cyclopropanes with N-nucleophiles. The reaction was carried out in dichloroethane (DCE) at room temperature for 1 h for all tested initiators. We found that Al(OTf) 3 did not induce the target transformation (Table 1, entry 1). Conversely, in the presence of Fe(OTf) 3 , Sc(OTf) 3 , or Zn(OTf) 2 , cyclopropane 1a reacted with aniline affording acyclic product 4a in reasonable to good yields (Table 1, entries 2 -5). The best results were achieved using 20 mol% Ni(ClO 4 ) 2 ·6H 2 O or Y(OTf) 3 ; with these catalysts, compound 4a was obtained in more than a 90% yield (Table 1, entries 6,9). The decrease in nickel perchlorate loading led to a decrease in the product yield (Table 1, entries 7-9). The yield also decreased with increasing reaction time or when the reaction was carried out with heating; in both cases, the formation of byproducts was detected. When Brønsted acid, TfOH, was used, no reaction occurred at all presumably due to its neutralization with an excess of amine (Table 1, entry 10). With all the studied Lewis acids, only the acyclic product 4a was formed; its cyclization to pyrrolidin-2-one did not occur at room temperature.

Results and Discussion
We started our investigation with the study of the reaction of model cyclopropane 1a with aniline, leading to the acyclic product 4a (Table 1). To catalyze the reaction, we tested several available Lewis acids, which are commonly used for initiating reactions of DA cyclopropanes with N-nucleophiles. The reaction was carried out in dichloroethane (DCE) at room temperature for 1 h for all tested initiators. We found that Al(OTf)3 did not induce the target transformation (Table 1, entry 1). Conversely, in the presence of Fe(OTf)3, Sc(OTf)3, or Zn(OTf)2, cyclopropane 1a reacted with aniline affording acyclic product 4a in reasonable to good yields (Table 1, entries 2 -5). The best results were achieved using 20 mol% Ni(ClO4)2•6H2O or Y(OTf)3; with these catalysts, compound 4a was obtained in more than a 90% yield (Table 1, entries 6,9). The decrease in nickel perchlorate loading led to a decrease in the product yield (Table 1, entries 7-9). The yield also decreased with increasing reaction time or when the reaction was carried out with heating; in both cases, the formation of byproducts was detected. When Brønsted acid, TfOH, was used, no reaction occurred at all presumably due to its neutralization with an excess of amine (Table 1, entry  10). With all the studied Lewis acids, only the acyclic product 4a was formed; its cyclization to pyrrolidin-2-one did not occur at room temperature. Then, the lactamization of γ-aminoester 4a was investigated. We found that the cyclization of compound 4a proceeded under the reflux of its toluene solution with acetic acid. Moreover, we showed that the crude reaction mixture obtained by a nickel perchlorateinduced reaction of cyclopropane 1a with aniline, when refluxing with 2 equiv. acetic acid in toluene efficiently produced the corresponding pyrrolidin-2-one in a one-vessel operation. This compound was obtained as a mixture of two diastereomers due to the presence of an ester group at the C(3) atom of the pyrrolidone ring. To obtain the target bioactive 1,5-diarylpyrrolidin-2-ones, this group must be removed by one of the known dealkoxycarbonylation methods. To further simplify the synthetic sequence and increase the practicality of this strategy, we realized this transformation in one pot using alkaline saponification of the ester group followed by thermolysis (Scheme 2). As a result, pyrrolidone 2a was synthesized by a four-step procedure, requiring chromatographic purification only at the last stage, with an overall yield of 70%.
With the optimized conditions in hand, we investigated the reaction scope using diversly substituted DA cyclopropanes and a range of anilines (Scheme 3). We found that this one pot transformation was efficient for a series of DA cyclopropanes where the electron-rich het(aryl) group or styryl group was the donor substituent. A broad variety of substituents on the aromatic moiety of both DA cyclopropanes and anilines, such as halogen, alkyl, and alkoxy, were well-tolerated in these transformations. The yields of the obtained pyrrolidones 2 varied considerably from moderate to good; however, it should be taken into account that these yields were given for four-step procedures. This is also reflected in the complex dependence of the obtained yields on the structure of the starting compounds. For example, DA cyclopropanes bearing electron-abundant aromatic substituents are typically more reactive than DA cyclopropanes with less electron-rich donors; i.e., the conversion time is shorter. However, their side reactions also proceed faster, and that can provide lower yields of the target products. For multistage processes, the overall effect of the substituent on the reaction yields is even more complex and cannot be followed by any simple model.
Then, the lactamization of γ-aminoester 4a was investigated. We found that the cyclization of compound 4a proceeded under the reflux of its toluene solution with acetic acid. Moreover, we showed that the crude reaction mixture obtained by a nickel perchlorate-induced reaction of cyclopropane 1a with aniline, when refluxing with 2 equiv. acetic acid in toluene efficiently produced the corresponding pyrrolidin-2-one in a one-vessel operation. This compound was obtained as a mixture of two diastereomers due to the presence of an ester group at the C(3) atom of the pyrrolidone ring. To obtain the target bioactive 1,5-diarylpyrrolidin-2-ones, this group must be removed by one of the known dealkoxycarbonylation methods. To further simplify the synthetic sequence and increase the practicality of this strategy, we realized this transformation in one pot using alkaline saponification of the ester group followed by thermolysis (Scheme 2). As a result, pyrrolidone 2a was synthesized by a four-step procedure, requiring chromatographic purification only at the last stage, with an overall yield of 70%. With the optimized conditions in hand, we investigated the reaction scope using diversly substituted DA cyclopropanes and a range of anilines (Scheme 3). We found that this one pot transformation was efficient for a series of DA cyclopropanes where the electron-rich het(aryl) group or styryl group was the donor substituent. A broad variety of substituents on the aromatic moiety of both DA cyclopropanes and anilines, such as halogen, alkyl, and alkoxy, were well-tolerated in these transformations. The yields of the obtained pyrrolidones 2 varied considerably from moderate to good; however, it should be taken into account that these yields were given for four-step procedures. This is also reflected in the complex dependence of the obtained yields on the structure of the starting compounds. For example, DA cyclopropanes bearing electron-abundant aromatic substituents are typically more reactive than DA cyclopropanes with less electron-rich donors; i.e., the conversion time is shorter. However, their side reactions also proceed faster, and that can provide lower yields of the target products. For multistage processes, the overall effect of the substituent on the reaction yields is even more complex and cannot be followed by any simple model. For example, the moderate yield of pyrrolidone 2f obtained from highly reactive furyl-substituted DA cyclopropane presumably resulted from the well-known tendency of the furan ring to undergo various acid-induced transformations [71,72]. In contrast, compound 2b was formed in a 79% yield. Other reactive DA cyclopropanes, thienyl-and styryl-derived, produced the corresponding pyrrolidones 2g,h in about a 60% yield. Less reactive 2-phenyl-and 2-(p-tolyl)cyclopropane-1,1-diesters produced the corresponding pyrrolidones 2c,d in 47 and 45% yields. The structure of the compound 2c was unambiguously proven by single-crystal X-ray data [73]. Cyclopropane-1,1-diesters containing the 2-nitrophenyl or 3-pyridyl groups at the C(2) atom of the small ring afforded the expected pyrrolidones 2i,j in low yields. However, the yield of 2i was improved by replacing the nickel perchlorate with 20 mol% Y(OTf) 3 . Anilines containing both electron-withdrawing and electron-donating substituents, including fluorine or bromine in the ortho position, reacted well. The exceptions were 4-nitroaniline, for which the first step occurred only when the reaction mixture was refluxed, and 2-nitroaniline and 1,2-phenylenediamine, which did not afford the desired products 2k,l at all. With these anilines, the process was stopped after the formation of the open-chain products 4b,c (see below); cyclization products were not detected in the reaction mixtures even in trace amounts.
It is worth noting that, despite the potential ability of anilines to serve as ambident nucleophiles, in the studied reactions, they attacked the three-membered ring exclusively with the nitrogen atom, providing no isomeric products via the Friedel-Crafts alkylation of the electron-rich aromatic ring.
For example, the moderate yield of pyrrolidone 2f obtained from highly reactive furyl-substituted DA cyclopropane presumably resulted from the well-known tendency of the furan ring to undergo various acid-induced transformations [71,72]. In contrast, compound 2b was formed in a 79% yield. Other reactive DA cyclopropanes, thienyl-and styryl-derived, produced the corresponding pyrrolidones 2g,h in about a 60% yield. Less reactive 2-phenyl-and 2-(p-tolyl)cyclopropane-1,1-diesters produced the corresponding pyrrolidones 2c,d in 47 and 45% yields. The structure of the compound 2c was unambiguously proven by single-crystal X-ray data [73]. Cyclopropane-1,1-diesters containing the 2-nitrophenyl or 3-pyridyl groups at the C(2) atom of the small ring afforded the expected pyrrolidones 2i,j in low yields. However, the yield of 2i was improved by replacing the nickel perchlorate with 20 mol% Y(OTf)3. Anilines containing both electron-withdrawing and electron-donating substituents, including fluorine or bromine in the ortho position, reacted well. The exceptions were 4-nitroaniline, for which the first step occurred only when the reaction mixture was refluxed, and 2-nitroaniline and 1,2-phenylenediamine, which did not afford the desired products 2k,l at all. With these anilines, the process was stopped after the formation of the open-chain products 4b,c (see below); cyclization products were not detected in the reaction mixtures even in trace amounts. Benzylamines are known to be more nucleophilic than the corresponding anilines. The increased reactivity of benzylamines allowed us to synthesize pyrrolidones 3 in good yields by their direct reaction with DA cyclopropanes 1 by refluxing a dichloroethane solution in the presence of Ni(ClO 4 ) 2 ·6H 2 O without an additional lactamization step. Next, we used two methods of dealkoxycarbonylation of 3-substituted pyrrolidones. The first one included alkaline hydrolysis followed by decarboxylation according to the method developed for pyrrolidones 2 (method A, Scheme 4). Alternatively, dealkoxycarbonylation using a NaCl-promoted Krapcho reaction in wet DMSO at 160 • C under microwave (MW) irradiation provided pyrrolidones 3 in reasonable yields (method B, Scheme 4). For example, benzylamine and alkoxy-substituted benzylamines produced compounds 3a-d in up to 70% yields. In the reactions of DA cyclopropane with furfurylamine and (1H-indol-3yl)methylamine, the corresponding pyrrolidones 3e and 3f were obtained in 32% and 42% yields. Given that these yields corresponded to a four-step sequence realized as a one pot procedure, these yields can be considered reasonable. tion using a NaCl-promoted Krapcho reaction in wet DMSO at 160 °C under microwave (MW) irradiation provided pyrrolidones 3 in reasonable yields (method B, Scheme 4). For example, benzylamine and alkoxy-substituted benzylamines produced compounds 3a-d in up to 70% yields. In the reactions of DA cyclopropane with furfurylamine and (1Hindol-3-yl)methylamine, the corresponding pyrrolidones 3e and 3f were obtained in 32% and 42% yields. Given that these yields corresponded to a four-step sequence realized as a one pot procedure, these yields can be considered reasonable.  Moreover, we applied the developed approach to the synthesis of 1-alkyl-5arylpyrrolidones from DA cyclopropanes and some aliphatic amines (Scheme 4). Cyclobutylamine and propargylamine were found to participate quite efficiently in this transformation, affording the corresponding pyrrolidones 3h,i in yields close to those of 3c-f, although these substrates required a long reaction time (see Section 3). In contrast, tryptamine gave rise to the corresponding 3g product in only an 11% yield. A significant tarring of the reaction mixture was detected in this reaction. When simple primary aliphatic amines, such as methylamine or ethylamine, were reacted with cyclopropane 1a under the same reaction conditions, only unidentified byproducts were formed.
It was pointed out that above that, o-nitroaniline and 1,2-phenylenediamine did not produce the target products 2 under standard conditions. We tested their reactivity toward DA cyclopropanes 1 in the presence of the same catalysts in more detail (Scheme 5). We found that the full consumption of 2-phenylcyclopropane-1,1-dicarboxylate 1k in its reaction with o-nitroaniline catalyzed by Ni(ClO 4 ) 2 ·6H 2 O required 2 h of refluxing the solution in dichloroethane. Under these conditions, the expected acyclic product 4b was obtained in a 71% yield. The reaction of 3,4-dimethoxyphenyl-substituted cyclopropane 1j with 1,2-phenylenediamine under the same conditions produced the acyclic product 4c only in a low yield. Heating this reaction mixture at 100 • C in chlorobenzene resulted in a complex mixture of unidentified products. However, the acyclic compound 4c was obtained in a reasonable yield at room temperature using Y(OTf) 3 as an initiator (Scheme 5). This product was unstable, and all attempts to cyclize it were unsuccessful. lution in dichloroethane. Under these conditions, the expected acyclic product 4b was ob-tained in a 71% yield. The reaction of 3,4-dimethoxyphenyl-substituted cyclopropane 1j with 1,2-phenylenediamine under the same conditions produced the acyclic product 4c only in a low yield. Heating this reaction mixture at 100 °C in chlorobenzene resulted in a complex mixture of unidentified products. However, the acyclic compound 4c was obtained in a reasonable yield at room temperature using Y(OTf)3 as an initiator (Scheme 5). This product was unstable, and all attempts to cyclize it were unsuccessful. To test the generality of the DA cyclopropane ring opening with anilines, we tried to involve substrates with other acceptor groups in this reaction (Scheme 5). 2-Phenylcyclopropane-1,1-dicarbonitrile was found to undergo ring opening with aniline or 2-bromo-4-methylaniline under catalysis with 25 mol% Y(OTf) 3 at room temperature for 4 days. The full conversion of these substrates required a significantly longer reaction time compared to the corresponding 1,1-diesters. Despite the mild reaction conditions, 4d,e were isolated only in 41% and 43% yields, presumably due to the competitive realization of the side processes resulting from the coexistence of the amino and cyano groups. 2-Phenyl-1cyanocyclopropanecarboxylate turned out to be a less reactive substrate, which did not undergo conversion to amine 4f even at a high temperature. Other Lewis acids, such as Fe(OTf) 3 , Sc(OTf) 3 , and Ni(ClO 4 ) 2 ·6H 2 O, failed also to induce the reaction of this substrate with anilines.
Most biologically active compounds bearing chiral centers have different activities for different stereoisomers. This means that methods of their preparation in an optically pure form are highly desirable. Obviously, this also applies to bioactive-substituted pyrrolidones of types 2 and 3, which can be considered as cyclic analogs of GABA. We tested the possibility of using the developed procedure for the synthesis of chiral pyrrolidones 2 starting from optically active DA cyclopropane 1 as a substrate. We found that dimethyl (S)-2-(p-tolyl)cyclopropane-1,1-dicarboxylate (S)-1c was converted to the corresponding γ-lactam with a full inversion of the absolute configuration of the chiral center (Scheme 6). This result is consistent with previous investigations demonstrating that the nucleophilic ring opening of DA cyclopropanes under catalysis with moderately activating Lewis acids proceeds by an S N 2-like mechanism, and subsequent stages (cyclization, saponification, and decarboxylation) do not affect the chiral center. starting from optically active DA cyclopropane 1 as a substrate. We found that dimethyl (S)-2-(p-tolyl)cyclopropane-1,1-dicarboxylate (S)-1c was converted to the corresponding γ-lactam with a full inversion of the absolute configuration of the chiral center (Scheme 6). This result is consistent with previous investigations demonstrating that the nucleophilic ring opening of DA cyclopropanes under catalysis with moderately activating Lewis acids proceeds by an SN2-like mechanism, and subsequent stages (cyclization, saponification, and decarboxylation) do not affect the chiral center.
[a]D = +100 (c 0.024, CH 3 OH ) The synthetic utility of the developed transformations can be significantly extended by diverse postmodifications of their multiple functionalities of the synthesized pyrrolidones 2,3, that allows for preparing complex azaheterocycles. In particular, the treatment of pyrrolidone 2h with PPA at 100 °C produced benz[g]indolizidine 5 in an acceptable yield (Scheme 7). Based on NOESY spectroscopy [73] and a comparison of its spectra with spectral data for the related compounds described earlier [74], the hydrogen atoms at the stereogenic centers in compound 5 have a cis arrangement. The synthetic utility of the developed transformations can be significantly extended by diverse postmodifications of their multiple functionalities of the synthesized pyrrolidones 2,3, that allows for preparing complex azaheterocycles. In particular, the treatment of pyrrolidone 2h with PPA at 100 • C produced benz[g]indolizidine 5 in an acceptable yield (Scheme 7). Based on NOESY spectroscopy [73] and a comparison of its spectra with spectral data for the related compounds described earlier [74], the hydrogen atoms at the stereogenic centers in compound 5 have a cis arrangement.
(S)-2-(p-tolyl)cyclopropane-1,1-dicarboxylate (S)-1c was converted to the corresponding γ-lactam with a full inversion of the absolute configuration of the chiral center (Scheme 6). This result is consistent with previous investigations demonstrating that the nucleophilic ring opening of DA cyclopropanes under catalysis with moderately activating Lewis acids proceeds by an SN2-like mechanism, and subsequent stages (cyclization, saponification, and decarboxylation) do not affect the chiral center.
The synthetic utility of the developed transformations can be significantly extended by diverse postmodifications of their multiple functionalities of the synthesized pyrrolidones 2,3, that allows for preparing complex azaheterocycles. In particular, the treatment of pyrrolidone 2h with PPA at 100 °C produced benz[g]indolizidine 5 in an acceptable yield (Scheme 7). Based on NOESY spectroscopy [73] and a comparison of its spectra with spectral data for the related compounds described earlier [74], the hydrogen atoms at the stereogenic centers in compound 5 have a cis arrangement.

General Information
The structures of synthesized compounds were elucidated with the aid of 1D NMR ( 1 H, 13 C) and 2D NMR (NOESY, HSQC and HMBC 1 H-13 C) spectroscopy. NMR spectra were acquired on Avance 600 and Avance 500 (Bruker, Billerica, MA, USA) and 400-MR (Agilent, Santa Clara, CA, USA) spectrometers at room temperature; the chemical shifts δ were measured in ppm with respect to solvent (1H: CDCl 3 , δ = 7.27 ppm; CD 3 OD, δ = 3.35 ppm; 13 C: CDCl 3 , δ = 77.0 ppm; CD 3 OD: 13 C: δ = 49.9 ppm). Splitting patterns were designated as s, singlet; d, doublet; m, multiplet; dd, double doublet; and br, broad. Coupling constants (J) were in Hertz. Infrared spectra were recorded on an FTIR spectrometer ALPHA II (Bruker, Billerica, MA, USA) in KBr for solid substances and as thin film for oils. High resolution and accurate mass measurements were carried out using a micrOTOF-Q TM ESI-TOF (Electrospray Ionization/Time of Flight, Bruker, Billerica, MA, USA). Elemental analyses were performed with an EA-1108 CHNS elemental analyzer instrument (Fisons, Ipswich, UK). Melting points (mp) were measured using a Stuart ® SMP3 melting point apparatus (Cole-Parmer, Stone, Staffordshire, UK). Microwave reactions were performed in a Monowave 200-Anton Paar microwave reactor in sealed reaction vessels. The temperature was monitored with installed IR detector. X-Ray analysis was performed on STOE STADI VARI PILATUS-100K diffractometer (Stoe & Sie, Darmstadt, Germany). Analytical thin-layer chromatography (TLC) was carried out with silica gel plates (silica gel 60, F 254 , supported on aluminum); visualization was performed using a UV lamp (365 nm). Column chromatography was performed on silica gel 60 (230-400 mesh, Merck, Darmstadt, Germany). Enantiomeric purity of the optically active compounds was determined by chiral HPLC with a Hitachi LaChrome Elite-2000 chromatograph (Hitachi Hugh-Tech Corp, Toranomonon Minato-Ku, Japan) using a Daicel (Daicel Corp, Osaka, Japan) Chiralcel OD-H column (0.46 × 25 cm) at room temperature. The column was eluted with heptane/i-PrOH = 70:30 at a flow rate of 1 mL/min, and peak detection was accomplished using a UV detector at 219 nm. Optical rotation was measured on a Krüss P8000 polarimeter (A. Krüss Optronic GmbH, Hamburg, Germany). All reactions were carried out using freshly distilled and dry solvents. Cyclopropanes 1 were prepared by Knoevenagel/Corey-Chaykovsky reaction sequences from the corresponding aldehydes [75,76]. Compounds 2c,j, 3b,c, and (2S)-1d were described previously [15][16][17]39,77,78]. Commercial reagents employed in the synthesis were analytical-grade, obtained from Sigma-Aldrich (St. Louis, MO, USA) or Alfa Aesar (Ward Hill, MA, USA). The 1 H NMR and 13 C NMR for synthesized compounds as well as 2D (HSQC and HMBC) NMR spectra for selected compounds are available in the Supplementary Materials.

General Procedure 1
To a 0.2 M solution of aniline or benzylamine (1.0-1.2 equiv.) in DCE in the presence of molecular sieves, 4 Å Ni(ClO 4 ) 2 ·6H 2 O or Y(OTf) 3 (0.2 equiv.) was added under Ar atmosphere; then, cyclopropane 1 (1-4 mmol, 1.0 equiv.) was added. The resulting mixture was stirred at room temperature for 1-3 h, diluted with dichloromethane (DCM), and filtered through a short pad of silica gel using EtOAc as the eluent. The filtrate was concentrated under vacuum; the residue was dissolved in toluene (0.13 M). Next, acetic acid (2.0 equiv.) was added, and the reaction mixture was stirred under reflux for 7 h. Then, solvent was removed under vacuum, and residue was dissolved in ethanol (0.17 M); 1M aq. solution of sodium hydroxide (2.0 equiv.) was added in one portion. The reaction mixture was stirred at room temperature for 2 h, and after that, ethanol was removed under vacuum. The residue was diluted with water, and 1M HCl was added until pH 1. The resulting mixture was extracted with ethyl acetate (3 × 10 mL). Combined organic layers were dried with Na 2 SO 4 and concentrated in vacuo. The residue was dissolved in toluene (0.07 M) and was refluxed for 7 h. The solvent was removed under vacuum; the pure product was isolated by silica gel column chromatography.

Conclusions
In summary, we developed a convenient general method for the synthesis of substituted γ-lactams based on Lewis acid-catalyzed DA cyclopropane ring opening with primary amines as the key step. Various 1,5-disubstituted γ-lactams were synthesized in moderate to good yields in three or four steps, requiring only a single purification procedure. We also demonstrated the potential of our method in the synthesis of an optically pure γ-lactam derivative from optically active DA cyclopropane. Additionally, the presence of reactive functionalities at the C(1) and C(5) atoms of γ-lactams ensured the possibility for postmodifications of the obtained products to convert them into more complex azaheterocycles, such as benz[g]indolizidine derivatives.