Introduction

Enantioselective synthesis using chiral catalysts has undergone tremendous development1,2,3,4,5. However, enantioselective synthesis without additional chiral auxiliaries, which is critical for the understanding of biological homochirality6,7,8, is relatively rare. Several examples have demonstrated the enantioselective fragment couplings for peptides in this context. For instance, the chemical synthesis of dipeptide from N-acyl α-amino acids and α-amino acid amides or esters showed a preference for heterochiral outcomes, which was attributed to the intramolecular steric hindrance between adjacent chiral centers9,10. Blackmond et al.11 demonstrated heterochiral selectivity of thiol-catalyzed peptide ligation between amidonitriles and amino acids, amino amides, or peptides, offering a mechanism for both symmetry breaking and chiral amplification of the reactants and homochiral dipeptide products. Helical oligomers formed from L-Cα-methyl valine preferentially incorporate proteinogenic amino acids with D-configuration12. Lahav et al.13,14,15 showed that antiparallel racemic β-sheets consisting of alternant oligo-L- and oligo-D-amino acid peptides function as intermediate templates to drive homochiral coupling for oligopeptides. Ghadiri et al.16,17,18 reported that an amphiphilic homochiral α-helical oligopeptide induces homochiral peptide coupling via the formation of homochiral supramolecular coiled-coil structure.

In the aforementioned examples, the outcomes have adjacent chiral centers that provide local stereogenic constraints. Achieving enantioselective coupling of remote chiral centers, however, is much more challenging19,20,21,22. Clayden et al.23 demonstrated that helical peptides govern the enantioselective coupling in the absence of local chiral residues. In that example, the N- and C-terminal chiral centers are well separated by 14 atoms, but a consecutive intramolecular hydrogen bonding network establishes the asymmetric communication between the two chiral centers, directing the enantioselective coupling. Such helical peptides with a single remote chiral center can even serve as enantioselective organocatalysts for the conjugate addition reaction24. These structures nicely exemplified remote chiral communication despite being synthetically challenging. The works of Lahav et al.13,14,15 and Ghadiri et al.16,17,18 inspired us that the formation of chiral assembled structures, such as β-sheet and helical structures, could be an alternative means to achieve enantioselective coupling for remote chiral centers.

Here, we intend to create an enantioselective coupling that, without any chiral auxiliaries, involves two remote chiral centers through the formation of chiral assembled structures. We chose N-acylphenylalanine-based bilateral N-amidothioureas, i.e., bilateral azapeptides AcFTU2 (L,L-, L,D-, D,D-), as candidates (Fig. 1a). AcFTU2 has two intramolecular hydrogen-bonded β-turn structures25,26, yet the two chiral centers are separated by 14 atoms (Fig. 1a). A more important feature is that the helical β-turn structures can form chiral assembled structures via intermolecular interactions27,28,29,30,31,32,33, possibly driving enantioselective coupling during the synthesis of AcFTU2 from the enantiomeric or racemic reactants. We did observe a preference for the heterochiral coupling in the one-pot reaction of racemic N-acylphenylalanine-based hydrazides with the achiral 1,4-di(isothiocyanine)benzene, yielding more heterochiral azapeptide L,D-AcFTU2 than the statistical yield, over homochiral counterparts L,L- and D,D-AcFTU2. The heterochiral products exhibit helical-like superstructures. This is more favorable than the β-sheet-like organization from the homochiral counterparts, accounting for the favored heterochiral coupling of the one-pot reaction. As such, chiral amplification of the hydrazide starting material is achieved by treating it with achiral 1,4-di(isothiocyanine)benzene, which selectively takes a pair of the enantiomeric hydrazides.

Fig. 1: Design of the enantioselective bilateral couplings.
figure 1

a Structures of N-acylphenylalanine based bilateral azapeptides L,L-/L,D-/D,D-AcFTU2, containing two hydrogen-bonded β-turns. The asterisks in the structures indicate chiral carbons. b Schematic diagrams of three different fates for the bilateral reaction of L-/D-AcFN2H3 with Ph(NCS)2. c Reactions of L-AcFN2H3 and D-PrFN2H3 with Ph(NCS)2 and HPLC traces of the three products. Experimental conditions: L-AcFN2H3 (0.22 mmol), D-PrFN2H3 (0.22 mmol), and Ph(NCS)2 (0.2 mmol) in 10 mL solvent is stirred at set temperatures for 8 h.

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Results

Heterochiral coupling to bilateral azapeptides

We designed a one-pot reaction of N-acylamino acid-based hydrazide, AcFN2H3, with achiral 1,4-di(isothiocyanine)benzene, Ph(NCS)2, that leads to bis(N-amidothiourea), AcFTU2, in which two chiral residues are far away by 14 atoms or 15 bonds, with three possible diastereomers L,L-, D,D-, and L,D-AcFTU2 whose statistical percentages would be 25%, 25%, and 50%, respectively (Fig. 1b).

Different from the theoretical statistical distributions of the coupling products, we observed a favored heterochirality in the reaction of rac-AcFN2H3 (equal L- and D-AcFN2H3) with Ph(NCS)2, yielding preferentially the L,D-bilateral azapeptide AcFTU2, ca. 74% when reacting in MeCN at 80 °C, a general condition for the synthesis of such bilateral azapeptides27,28, which is evaluated from absorption spectral measurements (Supplementary Fig. 1). In real reaction systems, for a more facile measurement of the distribution of the products, we used D-PrFN2H3, instead of D-AcFN2H3 (Fig. 1b), to form a quasi-racemate13,14,15,34 with the same equivalent of acetylated L-AcFN2H3, enabling the HPLC analysis of the highly polar products (Fig. 1c). The hydrazides were added in 10% excess to ensure complete reactions.

Using MeCN as a solvent, the products precipitate gradually from the solution. At room temperature (25 °C), the heterochiral L,D-product is slightly favored over the homochiral ones (56:44), showing a weak heterochiral selectivity. Significantly, the heterochirality increases dramatically with increasing temperature, up to 84:16 at 100 °C (Table 1 and Supplementary Fig. 2). This means that in the one-pot reaction, when the first step takes place with L-hydrazide, the follow-up step is more likely takes the D-counterpart, despite that the two chiral centers are distant in the product molecule and separated by a central benzene ring.

Table 1 Diastereoselectivity in the reaction of L-AcFN2H3 and D-PrFN2H3 with Ph(NCS)2
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In protic solvent MeOH, similar temperature-dependent diastereoselectivity was found in the distributions of the insoluble products (Table 1). However, no products were detected when the reaction was carried out in a high-pressure reactor at 100 °C. In 1:1 (v/v) MeCN/H2O at 80 °C, the heterochiral selectivity is preserved partly as in pure MeCN, but at a higher temperature of 100 °C the yield drops sharply. Both of those suggest that protic solvents are not suitable for the heterochiral coupling reaction to proceed at high temperatures.

Using highly polar solvents dimethyl formamide (DMF) and DMSO in which no product precipitates, the heterochiral selectivity disappears, indicating that precipitation may play a role in the diastereoselective coupling. In aprotic solvent THF of medium polarity, preferred heterochirality is also found at a temperature at which insoluble products form, but at a higher temperature, such as 100 °C, the reaction is destroyed. In addition, solvents of low polarity are not suitable due to the limited solubility of the hydrazide reactant.

As the propionylated and acetylated reactants may exhibit different reactivity in the one-pot reaction, a control reaction of L-AcFN2H3 and L-PrFN2H3 with Ph(NCS)2 was carried out (Supplementary Fig. 3). No selectivity but a random distribution in the reaction products was found, indicating that the diastereoselectivity shown in Table 1 does not result from the possible difference in the reactivity of those two reactants. This is further supported by the comparable rate constants for the reaction of phenyl isothiocyanate, a mono-NCS analog of Ph(NCS)2, with hydrazide L-AcFN2H3 or D-PrFN2H3 in MeCN (5.34 × 10−3 L mol−1 s−1 versus 5.42 × 10−3 L mol−1 s−1, Supplementary Fig. 4).

The preferential heterochirality was also confirmed by the reaction rate constants of mono-reacted intermediate L-AcFTUNCS (Supplementary Fig. 5), which is fully soluble in the reaction solution, with hydrazides to yield L,L- and L,D-AcFTU2 (kL,L and kL,D), respectively (Fig. 2a). We measured kL,L and kL,D in MeCN over the temperature range of 25–80 °C. Data show that kL,D is higher than kL,L (Fig. 2b and Supplementary Table 1). More importantly, the ratio of reaction rate constants, kL,D/(kL,L + kL,D), increases with temperature, which is consistent with the temperature dependence of heterochiral selectivity in MeCN (Supplementary Fig. 6).

Fig. 2: Reaction rate constants.
figure 2

a The reactions between L-intermediate (L-AcFTUNCS) and L-AcFN2H3 or D-AcFN2H3, yielding homochiral L,L-AcFTU2 and heterochiral L,D-AcFTU2, respectively. b The reaction rate constants kL,L and kL,D in MeCN versus reaction temperature.

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β-Turns and intramolecular chiral transfer

To get insight into the driving forces for the preferred heterochiral coupling, we first investigated the structural characteristics of L,D- and L,L-AcFTU2 by examining their crystal structures. Slow diffusion of diethyl ether vapor into DMF solutions of L,D- and L,L-AcFTU2 in sealed wide-mouthed bottles for 3–4 days leads to crystals suitable for X-ray crystal structure analyses (for crystallographic data, see Supplementary Table 2). Crystal structure of heterochiral L,D-AcFTU2 (Fig. 3a) exhibits a type II (β1, containing L-phenylalanine residue) and a type II’ (β1’, containing D-phenylalanine residue) β-turn structures referring to the torsion angles (Supplementary Table 3)35,36, maintained respectively by two intramolecular ten-membered ring N−Hd∙∙∙eO=C hydrogen bonds. The homochiral counterpart L,L-AcFTU2 (Fig. 3b) contains a type II β-turn structure at each side of the molecule (labeled as β2 and β3), but with different parameters (Supplementary Table 3). Absorption and CD spectra of L,L- and D,D-AcFTU2 in MeCN show exciton-coupled characteristic at 268 and 282 nm, indicating an intramolecular coupling of phenylalanine residue, phenyl thiourea and p-phenylenediamine chromophores37, likely mediated by the β-turn structures in them that afford the intramolecular chirality transfer (Supplementary Figs. 7 and Fig. 3c). The mesomeric L,D-AcFTU2 displays the same absorption spectrum as L,L- and D,D-AcFTU2, but it is CD silent (Fig. 3c). Solvent accessibilities and temperature coefficients of the chemical shifts of the –NH protons in DMSO-d6/CD3CN binary solvents verify the existence of β-turn structures involving –NHd proton in the intramolecular hydrogen bonds in solutions (Supplementary Figs. 8 and 9)38,39. For example, the temperature coefficients of –NHd proton, −3.48 ppb/°C for L,L-AcFTU2, and −3.51 ppb/°C for L,D-AcFTU2 (Fig. 3d), are much lower than those of the rest three -NH protons.

Fig. 3: β-Turn structures.
figure 3

a X-ray crystal structure of L,D-AcFTU2 with atom labeling. b X-ray crystal structure of L,L-AcFTU2. Intramolecular hydrogen bonds are shown by dashed pink lines and β-turn structures are labeled as β1, β1’ for L,D-AcFTU2, and β2 and β3 for L,L-AcFTU2. c Absorption and CD spectra of L,L-AcFTU2 (blue line), D,D-AcFTU2 (red line), and L,D-AcFTU2 (black line) in 99/1 (v/v) MeCN/DMSO. Concentration: 20 μM. d Temperature-dependent chemical shifts and the fitted temperature coefficients (Δδ/ΔT) of –NHd protons of L,D-AcFTU2 and L,L-AcFTU2 in 95:5 (v/v) CD3CN/DMSO-d6 mixtures.

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The two chiral centers in AcFTU2 are far separated by 14 atoms or 15 bonds, and the central benzene ring may, to some extent, disturb the intramolecular hydrogen bonding network (Fig. 3a, b), thereby direct intramolecular asymmetric communication between the two chiral centers might be weakened. This is supported by the comparable intramolecular hydrogen bonds that maintain the β-turns in L,D- and L,L-AcFTU2 monomers, deduced from the profiles of the solvent composition and temperature-dependent chemical shifts of –NHd protons involved in the β-turns that are almost overlapped (Supplementary Figs. 10 and Fig. 3d), as well as the close stabilizing energies at optimal geometries (Supplementary Fig. 11).

Absorption and CD spectra of hydrazide substrate L-AcFN2H3 and mono-reacted intermediate L-AcFTUNCS were also recorded (Supplementary Fig. 12). In comparison to the CD spectrum of L-AcFN2H3, CD signals of L-AcFTUNCS at 268 and 300 nm are respectively attributed to phenyl thiourea and phenyl isothiocyanate chromophores, exhibiting characteristics similar to those of L,L-AcFTU2. This indicates an intramolecular chiral transfer in L-AcFTUNCS that is induced by the β-turn structure, agreeing with the solvent accessibilities probed by the solvent-dependent chemical shifts of the –NH protons and with the DFT optimized structure (Supplementary Fig. 13).

Supramolecular architectures

In view of the fact that the preferred heterochirality is accompanied by precipitation, we turn to the intermolecular packing of L,D- and L,L-AcFTU2 in the molecules crystals. Heterochiral L,D-AcFTU2 crystallizes in P21/n space group, indicating a 21 screw axis, whereas homochiral L,L-AcFTU2 crystallizes in P1 space group that indicates a sheet-like organization (Supplementary Table 2).

For clarity of showing the supramolecular network, the L-side with β1-turn in L,D-AcFTU2 is depicted in blue, while D-side with β1’-turn in red, and the central benzene ring in gray (Fig. 4a). Interestingly, the intermolecular hydrogen bonds only occur between the sides of the same configuration of the phenylalanine residues (Fig. 4a), suggesting a high-fidelity of self-sorting dictates as homochiral self-sorting. Viewing from b axis, the L-sides of the L,D-AcFTU2 molecules are bonded by head-to-tail N−Ha∙∙∙gS=C and N−Hc∙∙∙fO=C hydrogen bonds (Fig. 4a, b; for hydrogen bonding parameters, see Supplementary Table 4), leading to a supramolecular M-helix following a 21 screw axis of pitch 8.366 Å. Meanwhile, the D-sides form a P-helix that is a mirror-image of the M-helix formed from the L-sides (Fig. 4a). Therefore, within bc plane, L,D-AcFTU2 molecules form a helix-array superstructure consisting of alternative M- and P-helices that are spaced by the central benzene rings (Supplementary Fig. 14), in which the helicity of β-turn could be well propagated.

Fig. 4: Supramolecular architectures of L,D-AcFTU2 and L,L-AcFTU2.
figure 4

a 2D helix-array superstructure of L,D-AcFTU2 within bc plane, stacking via N−Ha∙∙∙gS=C and N−Hc∙∙∙fO=C hydrogen bonds, to lead in alternative M- and P-helices. The blue ones represent the sides of the molecule containing L-phenylalanine residues while red ones are the D-sides. b Hydrogen-bonded pentamer of L,D-AcFTU2. c 2D β-sheet-like superstructure of L,L-AcFTU2 within ab plane, interacting via double N−Hc∙∙∙gS=C hydrogen bonds along b axis and N−Hb∙∙∙fO=C hydrogen bonds along a axis. Those highlighted in blue represent the sides with β2-turns while those in light blue with β3-turns. d Hydrogen-bonded pentamer of L,L-AcFTU2. Dashed black lines indicate intermolecular hydrogen bonds. For clarity, the hydrogen atoms that are not involved in the intermolecular hydrogen bonding and the benzene rings of phenylalanine residues are omitted.

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Along b axis, parallel L,L-AcFTU2 molecules are stacked by double N−Hc∙∙∙gS=C hydrogen bonds between β2 and β3-turns to form chains, which stack in a parallel manner along a axis via N−Hb∙∙∙fO=C hydrogen bonds, resulting in a 2D superstructure similar to the biological β-sheet structure (Fig. 4c, d).

In the crystals each L,D-AcFTU2 molecule interacts with four neighboring molecules via eight hydrogen bonds, while L,L-AcFTU2 molecule interact with four neighboring ones via six hydrogen bonds, exhibiting significantly different hydrogen bonding networks in the pentamers (Fig. 4b, d). It is also noted that the L,D-AcFTU2 molecule belongs to the Ci point group with zero dipole moment, whereas L,L-AcFTU2 is of C2 symmetry with a dipole moment along C2 rotation axis. Computations show that the L,L-AcFTU2 monomer has a dipole moment of 10.3 Debye (Supplementary Fig. 15). Since L,L-AcFTU2 monomers are packed in the same orientation in the crystal, there would be significant electrostatic repulsion between the dipolar L,L-AcFTU2 molecules that to some extent offsets the local hydrogen bonding. Such long range repulsion does not exist in the packing of L,D-AcFTU2 molecule in the crystal. This dramatic difference in the crystal packings of L,D- and L,L-AcFTU2 can be elucidated with quantum calculations of the binding of a monomer with its four surrounding monomers as described in the scheme of Supplementary Fig. 15, at M062X-D3/6-311 ++ G(d,p) level. Computational results demonstrated that the binding energy in the L,D-AcFTU2 pentamer is 104 kcal/mol, higher than that of the L,L-AcFTU2 pentamer (74.6 kcal/mol).

The difference in the intermolecular interactions in the homo- and heterochiral bilateral azapeptides can be further deduced from their solubilities. L,L-AcFTU2 is much more soluble in MeCN than L,D-AcFTU2 at the same temperature, for example, 60.5 μM for L,L-AcFTU2 whereas 2.43 μM for L,D-AcFTU2 at 25 °C (Supplementary Fig. 16). The increased solubility of L,L-AcFTU2 with increasing temperature is more pronouncing than that of L,D-AcFTU2, leading to a larger difference in their solubilities at higher temperatures. Interestingly, both the heterochiral selectivity and the difference in the solubility of L,L- and L,D-AcFTU2 in MeCN are positively temperature-dependent (Table 1 and Supplementary Fig. 16)40. A positive correlation is observed between the difference in solubilities (sL,L—sL,D) and the heterochiral selectivity in MeCN over the tested temperature range (Supplementary Fig. 17). Aggregation of diastereomers L,L- and L,D-AcFTU2 in 95:5 (v/v) MeCN/DMSO mixtures also reveals distinct intermolecular interactions41,42,43,44,45. Fresh solution of L,D-AcFTU2 at 2 mM, at which the NMR investigation on the intramolecular hydrogen bonding was carried out (Supplementary Fig. 8), exhibits polymeric species of diameter ca. 210 nm, as determined by dynamic light scattering (DLS) measurements (Supplementary Fig. 18). In addition, scanning electron microscopy (SEM) of an air-dried sample on a platinum-coated silicon wafer displays clustered rod-like aggregates (Supplementary Fig. 19). In contrast, DLS and SEM analyses of the L,L-AcFTU2 sample show only monomeric components and amorphous solids. Aggregation of L,D-AcFTU2 at lower concentration was further examined by DLS, which shows that it starts to form aggregates at 0.6 mM, of diameter ca. 60 nm (Supplementary Fig. 18). The fact that the chemical shifts of −NH protons in L,D-AcFTU2 are close to those of L,L-AcFTU2 (Supplementary Fig. 8) in CD3CN/DMSO-d6 mixtures of varying DMSO-d6 composition (5–100% by volume) suggests that the NMR signals observed for L,D-AcFTU2 are those from its monomeric species, since in all those cases L,L-AcFTU2 exists in the monomer form. This also prevents our detailed investigations by NMR of the intermolecular hydrogen bonding interactions between L,D-AcFTU2 molecules in solution.

Thus, both computational binding energies, experimental solubilities, and aggregation behaviors in solutions reveal that the helical-like superstructure of heterochiral L,D-AcFTU2 is more favorable than the β-sheet-like organization of homochiral L,L-AcFTU2 molecule, accounting for the heterochiral coupling of bilateral azapeptides. A solid support for this conclusion is the disappearance of the diastereoselectivity in DMSO and DMF in which no aggregates exist (Table 1).

Bilateral acylhydrazones

The role of intramolecular helical β-turns for heterochiral coupling has not yet been demonstrated. We designed bilateral acylhydrazones that do not bear the β-turn structures. Replacing Ph(NCS)2 by Ph(CHO)2 (Fig. 5a), the reaction that yields bilateral acylhydrazones shows no heterochiral selectivity but random distribution, despite the formation of precipitation from the solutions (Fig. 5b and Supplementary Table 5). We, therefore, conclude that the β-turn structures contribute to the heterochirality during the formation of AcFTU2. It is the helical fragment that builds supramolecular helices from the heterochiral products, affording chiral architectures to induce diastereoselective couplings. Hence the observed heterochirality is the cooperative result of intramolecular hydrogen-bonded β-turns and intermolecular interactions, i.e., hydrogen bonding and dipole-dipole interactions.

Fig. 5: Reactions to bilateral acylhydrazones.
figure 5

a Reactions of L-AcFN2H3 and D-PrFN2H3 with Ph(CHO)2 to yield bilateral acylhydrazones. b HPLC traces of the products from the reaction of L-AcFN2H3 and D-PrFN2H3 with Ph(CHO)2. Experimental conditions: a solution of L-AcFN2H3 (0.22 mmol), D-PrFN2H3 (0.22 mmol), and Ph(CHO)2 (0.2 mmol) in a MeCN (10 mL) reacted in a high-pressure reactor at 100 °C for 24 h.

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Chiral amplification led by heterochirality

Amplification of a low ee (enantiomeric excess) has been considered a requisite for the origin of homochirality. It has been realized in a variety of experiments, such as crystallization46, asymmetric autocatalysis47, polymerization48, sublimation49, and adsorption50. Consider a sample of low ee, if its racemate portion is removed in the way of precipitation by reaction with an achiral reactant that prefers to forming a heterochiral product, the remaining starting material will be of higher ee and eventually homochiral (Fig. 6a)11,51,52. Therefore, our finding of the heterochiral preference may allow chiral amplification of the amino acid-based hydrazides (Fig. 6b, c).

Fig. 6: Chiral amplification of N-acylamino acid-based hydrazides via favored heterochirality.
figure 6

a Schematic diagram for chiral amplification led by heterochiral selectivity of bilateral reactions. b Structures of N-acylamino acid based hydrazides AcXN2H3 (X = F, A, V, P). The asterisks in the structures indicate chiral carbons. c Reactions of excess AcXN2H3 with Ph(NCS)2. Experimental conditions: a solution of AcXN2H3 (0.5 mmol) of different ee and Ph(NCS)2 (0.2 mmol) in a high-pressure reactor in MeCN (10 mL) at 100 °C for 5 h. d ees of AcXN2H3 before and after its reaction with Ph(NCS)2.

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Amplified ee of the AcFN2H3 is indeed observed after treated with achiral Ph(NCS)2 and removal of the precipitates (Supplementary Table 6). Allow the reaction to take place in MeCN in a high-pressure reactor at 100 °C, ee of AcFN2H3 increases only from 40% to 46% if 0.1 eq Ph(NCS)2 is added, with a further dramatic improvement to 82% or from −40% to −84% when 0.4 eq Ph(NCS)2 is loaded. However, at the lower temperature of 80 °C the ee increases only from 40% to 55% using 0.4 eq Ph(NCS)2, which is far less than that observed at 100 °C, likely due to the temperature-dependent strength of heterochirality (Table 1).

By adding 0.4 eq Ph(NCS)2, the amplification at 100 °C is not complete in one step, but with several cycles, the ee can be improved gradually from 20% to 93% or from −20% to −95% (Supplementary Table 7). Relating the ee before and after the reaction, an S-shaped curve is obtained (Fig. 6c, d), which looks similar to that observed in the classic chiral amplification termed the “majority rules effect”53,54. Compared with that of the reaction at 100 °C, chiral amplification at 80 °C decreases substantially (Supplementary Fig. 20). The observed chiral amplification again supports the heterochirality found in the reaction of racemate AcFN2H3 with Ph(NCS)2.

Similar chiral amplification is also observed for the non-aromatic amino acid (alanine, valine) based hydrazides, AcAN2H3 and AcVN2H3, after treated with Ph(NCS)2 in MeCN (Fig. 6b–d), indicating that the preferred heterochirality could occur in the random reactions of these N-acylamino acid based hydrazides with Ph(NCS)2. According to this chiral amplification mechanism, the overall yield of the enantiomerically enriched substrates, i.e., AcFN2H3, AcAN2H3, and AcVN2H3, is low because of the loss of most of the substrates in the formation of precipitate products (Fig. 6a, c). This loss is not advantageous for the preparation of enantiomerically pure substrates. Yet the chiral amplification mechanism remains significant in terms of understanding the origin of prebiotic homochirality, a compelling but challenging question.

Proline, a natural amino acid known to facilitate the formation of folded β-turn structures35,55,56,57, is also explored in its hydrazide form, AcPN2H3 (Fig. 6b). However, an almost linear relationship is observed for the ee before and after the reaction (Fig. 6d), indicating a substantially decreased or a lack of the heterochirality in the reaction of AcPN2H3 with Ph(NCS)2. This absence can be attributed to the lack of the acetamido –NHa proton in the proline-based azapeptides (Fig. 6b), which would make it impossible to form favorable supramolecular architectures from the heterochiral products, for example the helix-array superstructures. Furthermore, when Ph(CHO)2 is used instead of Ph(NCS)2, the chiral amplification effect of the hydrazide substrates is not observed (Supplementary Table 8), due also to the observed absence of the heterochiral selectivity in the reactions of the bilateral acylhydrazones (Fig. 5).

Discussion

In summary, we report a heterochiral coupling to bilateral β-turn structured azapeptides, in which the two chiral centers are separated by 14 atoms or 15 bonds. Notably, the central benzene ring linker breaks the intramolecular hydrogen bonding network and destroys the direct intramolecular asymmetric communication between the two remote chiral centers. However, a preference of heterochiral coupling was observed in the one-pot reaction of racemic N-acylphenylalanine based hydrazides with the achiral 1,4-di(isothiocyanine)benzene in MeCN that precipitates formed, yielding more heterochiral azapeptides than the homochiral counterparts. But in DMSO or DMF solutions no precipitates formed, and the couplings to bilateral azapeptides were essentially random. The solid-state aggregates of the heterochiral azapeptides feature a helical-like superstructure through intermolecular hydrogen bonding between homochiral β-turns, consisting of alternative M- and P-helices that allow the propagation of β-turn helicity. This helical-like superstructure is more favorable than the β-sheet-like superstructure of homochiral azapeptides, providing a mechanism for the favored heterochiral coupling of the one-pot reaction. Moreover, the heterochiral coupling leads to chiral amplification of hydrazides after being treated with a certain amount of the achiral reactant Ph(NCS)2, resulting in hydrazides of higher ee or enantiopurity. The present findings confirm the enantioselective synthesis induced by biomimetic supramolecular structures and provide a new avenue for the amplification of chirality.

Methods

General methods

1H NMR and 13C NMR spectra were obtained on a Bruker AV500 or AV600 spectrometer. Absorption spectra were recorded on a Thermo Scientific Evolution 300 UV/Vis spectrophotometer. CD spectra were recorded with a JASCO J-1500 spectropolarimeter. High-resolution mass spectra were obtained on a Bruker En Apex ultra 7.0 FT-MS. Infrared spectra were carried out using a Nicolet AVATAR FT-IR330 spectrometer. DLS was collected with a Malvern Zetasizer Nano-ZS90. SEM images were obtained on a ThermoFisher Helios 5 UC microscope. X-ray crystallography data of compounds L,D-AcFTU2 and L,L-AcFTU2 were collected on an Oxford Gemini S Ultra system. Absorption corrections were applied by using the program CrysAlis (multi-scan). The structure was solved by direct methods and non-hydrogen atoms except solvent molecules were refined anisotropically by least-squares on F2 using the SHELXTL program. HPLC traces were recorded on Shimadzu Corporation LC-20AD chromatographic instrument with a UV detector and C18 or Chiralpak ID column.

Synthesis of the compounds

Detailed procedures for the synthesis of all compounds and their characterization data are given in the Supplementary Information.

Determination of diastereoselectivity by HPLC analysis

To determine the diastereoselectivity of the pseudo-racemic reaction system, the solvent was removed through vacuum distillation. 10 mg of the reaction product was dissolved in 0.5 mL DMSO, and then 50 μL of the resultant solution was transferred into 2 mL methanol to obtain a diluted sample. The sample was then analyzed using the following conditions: HPLC column C18 (250 × 4.6 mm), injection volume 10 μL, eluent methanol/water (50:50, v/v), flow 1 mL/min, room temperature, λ = 310 nm (azapeptides) or 330 nm (acylhydrazones). Retention times: L,L-AcFTU2 9.7 min, L,D-AcFTUFPr 13.2 min, D,D-PrFTU2 16.3 min; L,L-AcFAZ2 20.9 min, L,D-AcFAZFPr 22.4 min, D,D-PrFAZ2 23.6 min.

Determination of reaction rate constants

Compound L-intermediate (16.5 mg, 0.04 mmol) and L-AcFN2H3 or D-AcFN2H3 (8.84 mg, 0.04 mmol) were mixed in 20 mL MeCN and then stirred at different temperatures. Small portions of the reaction mixture were taken out at different reaction durations with quick filtrating, and 40 μL of the filtrate was added 1960 μL MeCN, and the resultant solution was subject to UV-Vis absorption measurements. The concentration of product (x) at reaction time (t) can be calculated based on absorption spectral measurements. The reaction rate constant k was obtained by fitting the integral form of the second-order reaction rate equation kt = x/[c(c–x)].

Determination of solubility

Supersaturated solutions of L,L-AcFTU2 or L,D-AcFTU2 in MeCN were stirred at a certain temperature for several hours. The turbid solution was filtered to obtain saturated solutions of L,L-AcFTU2 or L,D-AcFTU2. Then the saturated solutions were subjected to concentration determination by UV-Vis absorption spectra.

Determination of ee by chiral HPLC analysis

To determine the ee of AcFN2H3, AcAN2H3, and AcVN2H3 after reactions, the precipitation was removed by filtration. The filtrate was then analyzed using the following conditions: HPLC column Chiralpak ID (250 × 4.6 mm), injection volume 10 μL, eluent n-hexane/2-propanol (75:25 for AcFN2H3 and AcAN2H3, 80:20 for AcVN2H3, 65:35 for AcPN2H3, v/v), flow 1 mL/min, room temperature, λ = 220 nm. Retention times: L-AcFN2H3 33.1 min, D-AcFN2H3 17.0 min; L-AcAN2H3 20.5 min, D-AcAN2H3 13.1 min; L-AcVN2H3 21.7 min, D-AcVN2H3 13.1 min; L-AcPN2H3 36.5 min, D-AcPN2H3 23.1 min.