Synthesis of Enantioenriched α-Deuterated α-Amino Acids Enabled by an Organophotocatalytic Radical Approach

Abstract

A mild, versatile organophotoredox protocol has been developed for the preparation of diverse, enantioenriched α-deuterated α-amino acids. Distinct from the well-established two-electron transformations, this radical-based strategy offers the unrivaled capacity of the convergent unification of readily accessible feedstock carboxylic acids and a chiral methyleneoxazolidinone fragment and highly diastereo-, chemo- and regio-selective incorporation of deuterium simultaneously. Furthermore, the approach has addressed the long-standing challenge of the installation of sterically demanding side chains into α-amino acids.

Graphical Abstract

Isotopically labelled amino acids, particularly, the α-deuterated version, are broadly used in almost every sub-discipline in the life sciences for studying biosynthetic pathways,¹ enzymatic mechanisms,² and probing the secondary and tertiary structures of peptides and proteins by NMR and MS techniques.³ Furthermore, the incorporation of deuterium into α-position of amino acids can enhance metabolic stability and reduce the rate of epimerization of peptido and peptidomimetic therapeutics and thus enhance the efficacy and/or decrease the potential toxicity (see representatives in Figure 1).⁴ Therefore, there is a long-standing interest in the synthesis and application of enantioenriched α-deuterated amino acids.^5–7

Figure 1.

Examples of α-deuterated amino acid therapeutics.

In the routes available for the synthesis of chiral α-deuterated amino acids, enzyme-catalyzed approaches including enzyme mediated deuteration of α-amino acids⁸ and enzymatic reductive amination of pyruvates,⁹ are largely limited by narrow substrate scope. The commonly used methods with the capacity of access to unnatural α-amino acids rely on asymmetric alkylation of deuterated glycine derived imines¹⁰ or H/D exchange of amino acids derived imines¹¹ using chiral auxiliary (e.g., Schöllkopf’s bis-lactam ether) or chiral promoter catalyzed enolization (Scheme 1A1). Transitional metal-catalyzed C-H activation followed by H/D exchange¹² or 1,3-deuteride transfer¹³ provides an alternative to incorporate the isotope into α-position of amino acids (Scheme 1A2). Although these techniques represent the state-of-the-art strategies for the synthesis of α-deuterated amino acids, they all rely on a polar bond connection, and therefore carrying inherent limitations such as poor chemo-, regio- and/or enantio-selectivity, and in many cases, moderate level of deuteration. Furthermore, an intrinsic limitation of these ionic strategies is difficult to synthesize highly sterically demanding amino acids, a class of structures widely used in the field of peptides and peptidomimetics to constrain their conformations, and thus improve their potency and/or selectivity, lipophilicity, and metabolic stability.¹⁴ To overcome these issues, it is clear that a new design paradigm is needed.

Scheme 1.

Synthesis of enantioenriched α-deuterated amino acids.

An open shell radical process would offer a distinct and pragmatic approach for introducing the bulky groups into amino acids by virtue of favorable formation of 3° radicals.¹⁵ The radical addition to dehydroalanine (Dha) derivatives has been demonstrated as a viable approach for the synthesis of α-amino acids.¹⁶ In recent efforts, notably, an efficient Giese-type reaction of tertiary amines or halogenated pyridine with Dha derivatives is realized with photoredox catalysis by Jui and coworkers.¹⁷ Molander and colleagues elegantly introduced fluorine at the α-position of amino acids by regioselective carbofluorination of Dha compounds using alkyl trifluoroborate reagents as radical precursors.¹⁸ We envisioned that direct addition of a decarboxylative radical 4 to Dha derivatives such as (S)-methyleneoxazolidinone 2¹⁹ as a chiral inducer could lead to enantioenriched amino acids 3 by the employment of ubiquitous, readily accessible alkyl carboxylic acids 1 as radical progenitors (Scheme 1B).²⁰ The ready accessibility of feedstock alkyl carboxylic acids 1 make possible for the synthesis of more structurally diverse amino acids. Furthermore, drawing from the mechanistic evidence amassed in these and our studies,^17,18,21 we conceived that Re-face selective deuteration of the chiral anion intermediate 6 would potentially provide a novel approach to enantioenriched α-deuterated amino acids 3. It is expected that the power of the strategy is fueled by the chemo-, regio- and diastereo-selective incorporation of bulky side chains and deuterium into α-amino acids simultaneously. To our knowledge, a strategy of this type has not been documented previously.

To investigate the feasibility of this proposal, in the initial attempt, we probed a reaction of deuterated methyl 2,3-O-(1-methylethylidene)-β-D-ribofuranosiduronic acid (1a, 1.5 equiv) as the glycosyl radical precursor and (S)-methyleneoxazolidinone 2 (1.0 equiv) as the amino acid surrogate, and D₂O (80 equiv) as the deuterium source in the presence of a photosensitizer (PS) irradiated by a 40 W Kessil blue LED (Table 1). Commonly used Cs₂CO₃ (1.5 equiv) as base in photoredox decarboxylation was tested in anhydrous dichloroethane (DCE) as solvent for 24 h. It should be noted that the use of deuterated acid and anhydrous solvent (eliminating H₂O) was necessary for achieving higher deuteration level. It was found that the reaction efficiency was PS dependent (entries 1–3). Among the PS probed, mesityl acridinium salt (Mes-Acr-Me⁺•ClO4⁻) delivered the desired product 3a in encouraging 59% yield (entry 1), while PS with low reduction potential Ir[dF(CF₃)ppy]₂(dtbpy)PF₆ (entry 2) and (4CzIPN (entry 3) failed to produce the product. In addition, 85% D-incorporation with excellent diastereometric ratio (dr) >20:1 was achieved. Further optimization reaction conditions including solvent (entry 4), the amount of D₂O (entry 4), 1a and base and amount (entries 5 and 6and 3), and (7revealed the optimized reaction conditions (entry 7): 0.3 equiv of DBU, 80 equiv of D₂O and anhydrous MeCN. The control experiments confirmed that base, light and photocatalyst were prerequisites for this transformation (entries 8–10).

Table 1.

Exploration and optimization^[a]

entry	derivation from standard conditions	yield (%),[b] D-content (%)[c]
1	Cs2CO3 (1.5 equiv) as base, anhydrous DCE as solvent for 24 h	59, 92
2	Ir[dF(CF3)ppy]2(dtbpy)PF6 as PS, Cs2CO3 (1.5 equiv) as base, and D2O (40 equiv) used in anhydrous DCE for 24 h	<5, nd[e]
3	4CzIPN as PS, Cs2CO3 (1.5 equiv) as base, and D2O (40 equiv) used in anhydrous DCE for 24 h	<5, nd[e]
4	Cs2CO3 (1.5 equiv.) as base, D2O (40 equiv) used, anhydrous DCE as solvent for 24 h	52, 85
5	0.6 equiv of DBU used	69, 96
6	1.2 equiv of 1a used	63, 96
7	none	70 (68),[d] 95
8	no base	<5, nd[e]
9	no PS	<5, nd[e]
10	no light	<5, nd[e]

^[a]Reaction conditions: unless specified, a mixture of 1a (0.3 mmol), 2 (0.2 mmol) and catalyst (0.01 mmol) in anhydrous MeCN (2.0 mL) was irradiated with 40W Kessil blue LEDs in N₂ atmosphere at rt for 36 h.

^[b]Yield based on ¹H NMR.

^[c]Determined by ¹H NMR.

^[d]Yield of isolated products.

^[e]not determined.

With optimized reaction conditions in hand, we first evaluated the coupling reactions utilizing glycosyl carboxylic acids 1 with 2 by providing an alternative for the synthesis of β-glycosyl α-deuterated amino acids. The protocol worked well for the tested pentose and hexose to give the desired products 3a-c in moderate yield and with high level of deuterium incorporation at the desired α-position (Scheme 2A). It should be noted that the anomeric effect of the glycosyl radicals delivers highly stereoselective anomeric products, consistent with our previous works.^22,23 Furthermore, the chiral (S)-oxazolidinone controlled the deuteration very well with >20:1 dr by only forming one diastereomer.

Scheme 2.

Scope of the organophotoredox-mediated asymmetric α-deuterated α-amino Acids synthesis with simple carboxylic acids^[a]

[a] Reaction conditions: unless specified, a mixture of 1 (0.3 mmol), 2 (0.2 mmol) and Mes-Acr-Me⁺•ClO₄⁻ (0.01 mmol) in anhydrous MeCN (2.0 mL) was irradiated with 40 W Kessil blue LEDs in N₂ atmosphere at rt for specified time. [b] Yield of isolated products. [c] Deuteration and dr determined by ¹H NMR. [d] No desired product was obtained under the standard reaction conditions. The reaction was carried out, as follows: a mixture of 1 (0.24 mmol), 2 (0.2 mmol) Cs₂CO₃ (0.24 mmol) and 4CzIPN (0.01 mmol) in anhydrous DMF (2.0 mL) was irradiated with 40 W Kessil blue LEDs in N₂ atmosphere at rt for specified time.

Encouraged by the above studies, we extended the strategy for the synthesis of highly valued, structurally diverse and unique unnatural α-deuterated amino acids, which are difficult to be accessed by the established polar bond connection methods (Scheme 2B–D). The results from the studies show that the protocol serves as a general approach to various unnatural α-deuterated amino acids. In the view of biological importance of the bulky side chains of amino acids in peptido- and peptidomimetic relevant drug discovery and biological studies, the difficulty in accessing them using prior methods made them an ideal starting point. We first paied our attention on the sterically demanding tertiary alkyl carboxylic acids (Scheme 2B). To our delight, despite their high steric hindrance, the tested tertiary carboxylic acids including adamantyl group and analogue (3d-f), cyclohexyl derivatives (3g-l), tert-butyl group bearing various functional groups (3k-o) gave good to excellent yield with uniformly high diastereoselectivity (dr >20:1) and high deuteration level (91–99%). Moreover, the bridged structures (3p- q) could also be incorporated with high efficiency. Next, cyclic secondary alkyl radicals (3r-v) bearing five-, six-, and seven-membered rings were probed (Scheme 2C). The less hindered structures gave rise to higher yield (80–91%) without sacrificing deuteration level (93–98%) and diastereoselectivity (>20:1 dr). The same trend was observed for acyclic secondary carboxylic acid (3w-y), including the natural amino acid d-leucine and aldehyde precursor-acetal. This study was further expanded to primary carboxylic acids (3z-ae, Scheme 2D) as alkyl radical precursors, which are generally difficult to generate. As shown, the protocol worked smoothly for the cases of 3z-3ae in terms of reaction yield, dr and deuteration. It should also be aware that under the mild reaction conditions, this radical-based method exhibits broad functional group tolerance, as demonstrated for protected amines (3m, 3u and 3ae), free hydroxyl (3n), alkene (3l), ester (3q), ether (3a-c, 3o and 3aa), acetal (3y), carbonyl (3ac, ad), and heteroaromatic (3ab). No desired products were obtained under the standard reaction conditions for 3f, 3u, 3v, 3y, and 3ae. However, the reaction could proceed smoothly with a mixture of 1 (0.24 mmol), 2 (0.2 mmol) and 4CzIPN (0.01 mmol) in anhydrous DMF (2.0 mL) irradiated with 40 W Kessil blue LEDs in N₂ atmosphere at rt.

To further demonstrate the utility of this mild decarboxylative deuteration methodology, we performed a series of late-stage modifications on medicinal agents and natural products. As shown in Scheme 3A, the standard protocol (for detailed experiments, see Scheme 2, footnote [a] and SI) was successfully applied to natively and selectively modify bezafibrate and drug gemfibrozil, clinically used lipid lowering agents, to give amino acid derivatives 7 and 8 in 79 and 68% yield, and 96 and 97% D-incorporation, respectively and with >20:1 dr. Moreover, an anti-inflammatory agent 3-indolacetic acid, indomethacin was efficiently transformed into corresponding isotopically labelled amino acid (9) in good yield (85%), high deuteration (97%) and excellent dr (>20:1). Finally, enoxolone (10) containing a secondary alcohol and an α, β-unsaturated ketone, was tolerated. Of note, a modified protocol using 0.6 equiv. of DBU with a 0.05M concentration was used to improve the reaction efficiency.

Scheme 3.

Late-stage functionalization of pharmaceutics and natural products, conversion to amino acids and radical clock reaction.

[a] Reaction conditions: unless specified, see footnote [a] in Scheme 2 and see SI. [b] Yield of isolated products. [c] Deuteration and dr determined by ¹H NMR.

The synthesized products 3 could be conveniently transformed into α-deuterated α-amino acids, as showcased in the synthesis of α-deuterated Leu (11) by reacting with con. HCl for 30 min without the erosion of deuteration level (Scheme 3B). Intrigued by the apparent breadth of scope, a preliminary mechanistic inquiry was conducted (Scheme 3C). Radical clock experiments–cyclopropyl ring-opening (12) by forming alkenyl derived amino acid 13 suggest the presence of alkyl radicals. This observation is consistent with previously reported decarboxylative coupling studies.^20,22,23

In summary, a mild, versatile organophotoredox protocol has been developed for the preparation of diverse, enantioenriched, α-deuterated α-amino acids. The distinct radical approach represents a significant departure from the two-electron transformations so often prescribed in the literature. This radical-based strategy offers the unrivaled capacity of the convergent unification of readily accessible feedstock carboxylic acids and a chiral methyleneoxazolidinone fragment and highly diastereo-,chemo- and regio-selective incorporation of deuterium simultaneously, which could vastly expand the domain of highly biologically and medicinally valued α-deuterated amino acids. Furthermore, the approach has addressed the long-standing challenge of the installation of sterically bulky side chains into α-amino acids. Customizable by design, the simplicity and efficiency of this procedure should resonate with medicinal chemists requiring rapid access to these highly sought building blocks.