Chemo- and atroposelective Boc protection for asymmetric synthesis of NH₂-free axially chiral biaryl amino phenols

Abstract

Enantioselective protection strategies have emerged as powerful tools for creating high-value optically active compounds. Herein, we report an efficient and direct Lewis base-catalyzed enantioselective tert-butoxycarbonyl (Boc) protection strategy for the construction of axial chirality. Employing a chiral isothiourea (ITU) catalyst, a series of amino bisphenols undergo chemo- and atroposelective O-Boc protection with Boc anhydride (Boc₂O), delivering NH₂-free axially chiral biaryl amino phenols with moderate yields and high enantioselectivities. This desymmetrization protocol is scalable to gram level and enables facile downstream transformations of the chiral products. Computational studies reveal that the amino (NH₂) group on the naphthalene ring facilitates intramolecular proton transfer of the hydroxyl (OH) group. Moreover, an unprecedented S⋯C–NH₃⁺ electrostatic interaction, in combination with π–π stacking, is identified as a key stabilizing factor in the transition states.

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Introduction

The strategic use of protecting groups is essential for synthesizing complex molecules containing multiple reactive functionalities.¹ By integrating chiral catalysts with rationally designed substrates, conventional protection methods can be adapted for asymmetric transformations, opening new avenues for the construction of optically active compounds. In 2006, Snapper, Hoveyda, and co-workers pioneered an amino acid-derived imidazole-catalyzed asymmetric silylation of meso diols to afford secondary alcohols with high enantioselectivities (Scheme 1a).² Inspired by this elegant work, the research groups of Snapper and Hoveyda,^3a–c Oestreich,^3d–g Song,^3h He,³ⁱ List,^3j and Franz^3k subsequently developed a range of Lewis base-, transition-metal-, and Brønsted acid-catalyzed enantioselective O-silyl protection protocols to generate carbon-, silicon-, and axial chirality. Despite these advances, there remains an urgent need for the development of versatile enantioselective protection systems. Boc anhydride (Boc₂O), also known as di-tert-butyl dicarbonate, is an easily available and synthetically versatile reagent that has been widely employed for amine protection in peptide, pharmaceutical, and natural product synthesis.^1,4 Occasionally, Boc₂O is also used for the protection of alcohols and thiols (Scheme 1b, left).⁵ We envision that a chiral Lewis base could activate Boc₂O to form a reactive intermediate within a chiral environment, thereby enabling either the kinetic resolution of racemic substrates or the desymmetrization of prochiral substrates to establish new stereogenic centers (Scheme 1b, right). This approach could diversify catalytic asymmetric protection strategies beyond traditional silylation methods.

Scheme 1 Enantioselective protection strategy and our protocol to directly access NH₂-free NOBIN analogs.

Atropisomeric 2-amino-2′-hydroxy-1,1′-binaphthyl (NOBIN) and its analogs are recognized as privileged scaffolds broadly utilized in the development of chiral ligands and catalysts (Scheme 1c).⁶ Consequently, significant efforts have been devoted to the stereoselective preparation of these frameworks.^7–12 To date, there are four primary catalytic asymmetric strategies toward the synthesis of axially chiral NOBIN analogs (Scheme 1d): (i) transition-metal-catalyzed oxidative coupling of 2-naphthols and 2-naphthylamines;⁸ (ii) Brønsted/Lewis acid-catalyzed nucleophilic addition of one aryl fragment to another;⁹ (iii) organocatalytic kinetic resolution of racemic NOBINs;¹⁰ and (iv) N-heterocyclic carbene (NHC)-catalyzed desymmetrization of prochiral amino bisphenols.¹¹ While those protocols are well-established, most of them lead to N-protected axially chiral biaryl amino phenols as final products, necessitating an additional deprotection step that limits their synthetic utility. Our group aims to advance catalytic systems for constructing synthetically valuable molecules.¹³ We envisage that a Lewis base-catalyzed enantioselective O-Boc protection of unprotected biaryl amino bisphenols could offer a highly efficient and direct route to chiral NOBIN analogs. Achieving such a transformation requires addressing two key challenges: chemoselectivity between O-Boc and N-Boc protection, and atroposelectivity between the two identical hydroxy groups. Herein, we disclose an isothiourea (ITU)-promoted chemo- and atroposelective Boc protection of amino bisphenols, allowing for the rapid access to NH₂-free axially chiral amino phenols with excellent enantiocontrol (Scheme 1e).

Results and discussion

We began our investigation by selecting amino bisphenol 1a as the model substrate to react with Boc₂O 2 in dichloromethane (CH₂Cl₂) at 25 °C for 48 hours, with key results summarized in Table 1. The use of cinchona alkaloid-derived Lewis base catalysts C1 and C2 efficiently generated the desired O-Boc protected amino phenol 3a in moderate yields and enantioselectivities, accompanied by side products including the di-O-Boc and N-Boc protected amino phenols (4a and 5a) (entries 1 and 2). To our delight, when isothiourea (ITU) catalysts C3 ¹⁴ and C4 ¹⁵ were employed, formation of the undesired N-Boc product 5a was completely suppressed. Under these conditions, 3a was obtained in 35% yield with 56% ee and 53% yield with 69% ee, respectively (entries 3 and 4). Utilizing C4 as the optimal catalyst, we next evaluated a variety of solvents (entries 5–9). Both polar and non-polar solvents were compatible, with diethyl ether (Et₂O) providing the best result, affording 3a in 44% yield and 72% ee (entry 6). Remarkably, lowering the reaction temperature to −20 °C significantly enhanced enantioselectivity, furnishing 3a with excellent 98% ee (entries 10–12). Further optimization of stoichiometry and catalyst loading revealed that using 1.1 equiv. of 2 and 5 mol% of C4 was sufficient to deliver the expected axially chiral biaryl amino phenol 3a in 62% yield and 98% ee (entries 13 and 14).

Table 1 Optimization of reaction conditions^a

Entry	Lewis base cat.	Solvent	T (°C)	Yield of 3a^b (%)	Yield of 4a^b (%)	Yield of 5a^b (%)	ee of 3a^c (%)
a Reaction conditions are as follows: 1a (0.2 mmol), 2 (0.3 mmol), Lewis base cat. (0.04 mmol), and solvent (2 mL) at a specific temperature for 48 h. b Isolated yield of product based on 1a. c Enantiomeric excess of 3a, determined via chiral-phase HPLC analysis. d 96 h. e 0.22 mmol of 2 was used. f 0.01 mmol of C4 was used. THF = tetrahydrofuran.
1	C1	CH2Cl2	25	32	55	6	53
2	C2	CH2Cl2	25	39	40	4	66
3	C3	CH2Cl2	25	35	57	Trace	56
4	C4	CH2Cl2	25	53	40	Trace	69
5	C4	THF	25	49	47	Trace	69
6	C4	Et2O	25	44	37	Trace	72
7	C4	EtOAc	25	46	42	Trace	63
8	C4	Toluene	25	35	21	Trace	67
9	C4	CH3CN	25	43	34	Trace	20
10	C4	Et2O	0	49	47	Trace	94
11	C4	Et2O	−20	48	50	Trace	98
12^d	C4	Et2O	−40	48	50	Trace	98
13^e	C4	Et2O	−20	63	18	Trace	98
14^e ^,	C4	Et 2 O	−20	62	16	Trace	98

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Having established the optimized reaction conditions (Table 1, entry 14), we then explored the generality of this enantioselective protection strategy. As shown in Table 2, a series of amino bisphenols 1 bearing diverse substitution patterns were examined. Substrates containing 6-methyl and 6-cyclopropyl groups on the naphthyl ring delivered the corresponding O-Boc protected products 3b and 3c in 57% yield with >99% ee and 53% yield with 97% ee, respectively. Halogen substituent such as 6-bromo was accommodated to afford 3d in 54% yield and 98% ee, indicating the potential for further elaboration of the axially chiral amino phenol scaffold. In contrast, the substrate bearing an electron-withdrawing COOMe group at the 6-position of the naphthyl ring formed the desired product 3e in only 16% yield and 90% ee. When amino bisphenols possessing 6-aryl and 6-(2-naphthyl) groups were subjected to the reaction, the desired products 3f–3i were obtained smoothly in 41–65% yields and 95 to >99% ee. Notably, replacing the aryl group with heteroaryl units had negligible effect on the reaction outcome, both 6-(2-furyl) and 6-(2-thienyl) substituted reactants efficiently furnished products 3j and 3k with moderate yields and outstanding enantioselectivities. Importantly, amino bisphenols comprising functional groups such as phenylethynyl, styryl, and vinyl were all competent substrates, providing the target axially chiral products 3l–3n in 55–65% yields and 98–99% ee. Substitution at the 7-position of the naphthyl ring was equally compatible, giving rise to products 3o–3q with decent yields and high enantioselectivities. Furthermore, when methyl groups were installed at the 5- or 4-position of the naphthyl ring, products 3r and 3s were isolated in approximately 50% yield with 98% ee. However, introducing a methyl group at the 3-position led to a significant drop in yield for product 3t, although high enantioselectivity was retained. Substituents on the bisphenol moiety were also well-tolerated. Reactions of substrates featuring methyl or phenyl groups at the 4′-position offered products 3u and 3v in 55% yield with 98% ee and 58% yield with 98% ee, respectively. However, when a trifluoromethyl group was introduced at the 4′-position, both the yield and enantioselectivity of the resulting product 3w decreased dramatically. Finally, the employment of ent-C4 as the catalyst successfully delivered ent-3a in 60% yield and −99% ee, suggesting that both of the enantiomers were readily accessible in our catalytic system. The absolute configurations of these axially chiral amino phenols were assigned as (S) by analogy to product 3a and 3k, whose structure was unequivocally confirmed through X-ray crystallographic analysis.¹⁶

Table 2 Scope of reactions^a

a Reaction conditions are as follows: 1 (0.2 mmol), 2 (0.22 mmol), C4 (0.01 mmol), and Et₂O (2 mL) at −20 °C for 48 h. Isolated yield of product 3 based on 1. The enantiomeric excess of 3 were determined via chiral-phase HPLC analysis. b ent-C4 was used as the catalyst. c CH₂Cl₂ was used as the solvent. d 72 h.

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Since for all the substrates examined in Table 2, the formation of di-O-Boc protected amino phenol 4 was consistently observed as a side product, which largely limited the yield of the desired product 3, we sought to further investigate the role of the second O-Boc protection step. To this end, we conducted an additional experiment to determine whether this second protection event functions as a kinetic resolution (Scheme 2). Under the optimized reaction conditions, treatment of racemic 3a with 2 (0.5 equiv.) led to the recovery of chiral 3a in 51% yield (based on racemic 3a) and 55% ee, along with the generation of di-O-Boc product 4a in 48% yield. These results indicate that the high enantioselectivity observed in our catalytic system primarily originates from the initial desymmetrizing O-Boc protection step. Additionally, the second O-Boc protection may contribute positively to the final enantioselectivity outcome, albeit at the expense of product yield.

Scheme 2 Kinetic resolution of racemic 3a.

Experimentally, the asymmetric Boc protection of aminophenol 1a using isothiourea catalyst C4 predominantly yielded the S-configured product 3a, without requiring additional bases. Notably, the phenol group was selectively acylated over the amino group. To gain further mechanistic insight into the origin of chemo- and atroposelectivity, density functional theory (DFT) calculations were performed, focusing on the key transition state. A simplified model was employed to probe the reaction pathway, highlighting the role of the amino group as an intramolecular base.

As depicted in Fig. 1, the most favorable transition state, TS_(S), exhibited the lowest energy barrier (27.4 kcal mol⁻¹), which was 1.3 kcal mol⁻¹ lower than TS_N (28.7 kcal mol⁻¹) and 2.2 kcal mol⁻¹ lower than TS_(R) (29.6 kcal mol⁻¹). This energy difference is consistent with the experimentally observed high enantioselectivity (98% ee), although trace amounts of the N-acylated byproduct 5a was detected. Interestingly, despite TS_(R) having a slightly lower relative Gibbs free energy—0.3 kcal mol⁻¹ below TS_(S)—its corresponding starting substrate complex (R-complex) was 2.5 kcal mol⁻¹ more stable than the S-complex. This energy landscape may help explain the minor formation of the di-O-Boc protected byproduct 4a, even though the S-configured product 3a remains predominant.

Fig. 1 Computational studies of chemo- and atroposelective Boc protection. (a) Reaction profile to form (S)-3a, (R)-3a and 5a. ΔG at the level of SMD(Et₂O)-wB97M-V/def2-TZVP//IEFPCM (Et₂O)-B3LYP-D3(BJ)/def2-SVP in kcal mol⁻¹. (b) Non-covalent interaction analysis via IGMH with an isosurface of 0.01.

Non-covalent interactions were analyzed using the independent gradient model based on Hirshfeld partition (IGMH).¹⁷ Contrary to previous reports,¹⁸ no O⋯S chalcogen bonding (nO – σ*S–C) was identified in either TS_(S) or TS_(R), both of which led to the O-acylated product (S)-3a and (R)-3a. Instead, the S-atom of the Boc-C4 intermediate engaged in electrostatic interaction with the amino carbon of substrate 1a (S⋯C–NH₃⁺). This interaction is likely driven by intramolecular proton transfer from the OH group to the NH₂ group. In contrast, TS_N, which leads to N-acylation, exhibited O⋯S chalcogen bonding, contributing to its conformational stability. Additionally, π–π stacking interactions played a significant role in stabilizing all three key transition states.

To determine whether the NH₂ group is essential for maintaining a stable conformation and achieving high enantioselectivity, we conducted three control experiments. As illustrated in Scheme 3, the secondary (NHMe) and tertiary amine (NMe₂) substituted substrates afforded the corresponding products 3x and 3y in moderate yields with excellent enantioselectivities. These results indicate that protonation of the nitrogen atom facilitates electrostatic interactions between the sulfur and phenyl carbon, which are crucial regardless of whether the nitrogen substituent is NH₂, NHMe, or NMe₂. Further transition-state calculations for 3x and 3y also reveal the presence of such noncovalent interactions (please see SI for details). It is also noteworthy that, in the case of 3y, the lower reaction energy barrier (21.0 kcal mol⁻¹) compared to TS_(S) may be attributed to its higher enantioselectivity (ee > 99%).

Scheme 3 Control experiments.

Finally, the practicality and applicability of the developed protocol were demonstrated by gram-scale preparation and downstream transformations of 3a. Under the optimized catalytic conditions, the reaction was efficiently scaled up to an 8 mmol scale using only 2 mol% of C4, affording 1.71 g of 3a without any loss in enantiopurity (Scheme 4a). The versatility of 3a was showcased through various derivatizations. Treatment of enantioenriched 3a with isothiocyanate and isocyanate smoothly delivered the corresponding thiourea 6 and urea 7 in 86% yield with 98% ee and 81% yield with 98% ee, respectively. Reactions of 3a with pyridine derivatives provided the chiral amino phenol analogs 8 and 9 in acceptable yields with retention of the enantioselectivities. Moreover, condensation of 3a with Boc₂O 2 in the absence of a catalyst offered N-Boc protected amino phenol 10. Subsequent O-functionalization of 10, including O-methylation, O-phenylation, O-benzoylation, and O-silylation, followed by Boc deprotection, furnished the anticipated derivatives 11–14 in moderate to good yields and excellent enantioselectivities, thereby significantly enriching the library of axially chiral amino phenols (Scheme 4b). To further highlight the synthetic potential of this methodology, a Cu-catalyzed enantioselective coupling reaction of α-substituted cyanoacetate 15 with aryl iodide 16 was carried out utilizing the previously synthesized amino phenol derivative 9 as a chiral ligand (Scheme 4c).^6d Gratifyingly, preliminary result showed that the corresponding product 17 bearing an all carbon quaternary stereocenter could be obtained in 73% yield with a promising 80% ee, revealing the utility of these axially chiral amino phenols in asymmetric catalysis.

Scheme 4 Synthetic utility of the current protocol.

Conclusions

In summary, we have developed the first enantioselective Boc protection strategy for the construction of optically pure compounds. In the presence of a chiral isothiourea catalyst, a broad range of bisphenols featuring diverse substitution patterns undergo reaction with Boc anhydride to afford NH₂-free axially chiral biaryl amino phenols with excellent enantiocontrol. This operationally simple and efficient method shows strong potential for application in preparative synthesis, and the resulting chiral products can be readily derivatized into valuable functional molecules. Complementary DFT studies provide mechanistic insight into the origins of the observed chemo- and atroposelectivities. Further investigations aimed at extending this enantioselective Boc protection strategy to the synthesis of additional types of chiral elements are currently underway in our laboratory.

Author contributions

Y. W. performed and analysed the experiments. Y. W., Z. X. and K. C. contributed to the data analysis. Y.-G. L. conducted the DFT calculations. X. C. and J. X. conceptualized and directed the project. H. Z. and J. X. drafted the manuscript. All authors contributed to the discussions.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental procedures and characterization data. See DOI: https://doi.org/10.1039/d5sc06233k.

CCDC 2433022 and 2434593 contain the supplementary crystallographic data for this paper.^16a,b

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (Grant No. 21602203, 22071082, 22101266), the Natural Science Foundation of Guangdong Province (No. 2024A1515011116), and the Pearl River talent program of Guangdong Province (Youth Top-Notch Talent, 2021QN020472), the Excellent Youth Program of Hennan Province (242300421119), China Scholarship Council and Zhengzhou University for financial support.

Notes and references

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