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Method Article
Chiral amino alcohols are versatile molecules for use as scaffolds in organic synthesis. Starting from L-lysine, we synthesize amino alcohols by an enzymatic cascade reaction combining diastereoselective C-H oxidation catalyzed by dioxygenase followed by cleavage of the carboxylic acid moiety of the corresponding hydroxyl amino acid by a decarboxylase.
Amino alcohols are versatile compounds with a wide range of applications. For instance, they have been used as chiral scaffolds in organic synthesis. Their synthesis by conventional organic chemistry often requires tedious multi-step synthesis processes, with difficult control of the stereochemical outcome. We present a protocol to enzymatically synthetize amino alcohols starting from the readily available L-lysine in 48 h. This protocol combines two chemical reactions that are very difficult to conduct by conventional organic synthesis. In the first step, the regio- and diastereoselective oxidation of an unactivated C-H bond of the lysine side-chain is catalyzed by a dioxygenase; a second regio- and diastereoselective oxidation catalyzed by a regiodivergent dioxygenase can lead to the formation of the 1,2-diols. In the last step, the carboxylic group of the alpha amino acid is cleaved by a pyridoxal-phosphate (PLP) decarboxylase (DC). This decarboxylative step only affects the alpha carbon of the amino acid, retaining the hydroxy-substituted stereogenic center in a beta/gamma position. The resulting amino alcohols are therefore optically enriched. The protocol was successfully applied to the semipreparative-scale synthesis of four amino alcohols. Monitoring of the reactions was conducted by high performance liquid chromatography (HPLC) after derivatization by 1-fluoro-2,4-dinitrobenzene. Straightforward purification by solid-phase extraction (SPE) afforded the amino alcohols with excellent yields (93% to >95%).
Despite the benefits offered by biocatalysis, the integration of biocatalytic steps in synthetic pathways or total biocatalytic routes remains mostly limited to enzymatic kinetic resolutions. These routes have been widely used as a first step in asymmetric chemo-enzymatic synthesis, but biocatalysis offers many more possibilities in functional group interconversions with high stereoselectivity1,2,3. Moreover, as biocatalytic reactions are conducted in similar conditions, it is therefore feasible to perform cascade reactions in a one-pot fashion4,5.
Chiral amino alcohols are versatile molecules for use as auxiliaries or scaffolds in organic synthesis6. The amino alcohol moiety is frequently found in secondary metabolites and in active pharmaceutical ingredients (API). Primary β-amino alcohols are readily available from the corresponding α-amino acids by conventional chemical synthesis, but access to chiral γ-amino alcohols or secondary amino alcohols often requires tedious synthetic pathways together with sensitive control of the stereochemistry7,8,9,10. Due to its high stereoselectivity, biocatalysis may provide a superior synthetic route to these chiral building blocks11,12,13,14.
We previously reported the synthesis of mono- and di-hydroxy-L-lysines by diastereoselective enzymatic hydroxylation catalyzed by dioxygenases of the iron(II)/α-ketoacid-dependent oxygenase family (αKAO) (Figure 1)15. In particular, starting from L-lysine, the KDO1 dioxygenase catalyzes the formation of the (3S)-hydroxy derivative (1), while the (4R)-derivative (2) is formed by the reaction with KDO2 dioxygenase. Successive regiodivergent hydroxylations by KDO1 and KDO2 lead to the formation of the (3R,4R)-dihydroxy-L-lysine (3) in optically pure form. However, the limited substrate range of these enzymes impedes their large utilization in chemical synthesis, especially in the hydroxylation of simple amines, as a carboxylic acid moiety in the α-position of the amino group is essential for activity16.
Figure 1: Biocatalytic conversions of L-lysine. Conversion into (3S)-hydroxy-L-lysine (1) catalyzed by KDO1 dioxygenase; (4R)-hydroxy-L-lysine (2) catalyzed by KDO2 dioxygenase; and (3R,4R)-dihydroxy-L-lysine (3) by cascade reaction catalyzed successively by KDO1 and KDO2 dioxygenases. Please click here to view a larger version of this figure.
Decarboxylation is a common reaction in metabolism17. In particular, amino acid DCs (EC 4.1.1) are cofactor-free (pyruvoyl-dependent) or PLP-dependent enzymes, and catalyze the decarboxylation of amino acids into the corresponding polyamines in bacteria and higher organisms18,19,20,21,22. The mono- and dihydroxy compounds (Figure 3) 4-7, 10-11 correspond to hydroxylated cadaverine, the diamine obtained by decarboxylation of L-lysine. Cadaverine is a key building block for the chemical industry, specifically it is a component of polyamide and polyurethane polymers. Therefore, bio-based production of this diamine from renewable resources has attracted attention as an alternative to the petroleum-based route, and various microorganisms have been engineered for this purpose. In these metabolic pathways, lysine DC (LDC) is the key enzyme. LDC is a PLP-dependent enzyme belonging to the alanine racemase (AR) structural family23. The PLP-dependent DCs (PLP-DCs) are known to be highly substrate-specific. However, a few enzymes own the capability of slight promiscuity, being active towards both L-ornithine and L-lysine amino acids, as for example the LDC from Selenomonas rumirantium (LDCSrum), which has similar kinetic constants for lysine and ornithine decarboxylation24,25. This extended substrate specificity makes this enzyme a good candidate for the decarboxylation of mono- and di-hydroxy-L-lysine. In addition, to find DCs active towards the hydroxyl derivatives of lysine, we examined the genomic context of the genes encoding the αKAO enzymes. Indeed, in prokaryotic genomes the genes encoding enzymes involved in the same biosynthetic pathway are generally co-localized in gene clusters. The KDO2 (from Chitinophaga pinensis) gene was found co-localized with a gene encoding putative PLP-DC (Figure 2). In contrast, no gene encoding for DC has been found when analyzing the genomic context of the KDO1 dioxygenase. The PLP-DC protein from C. pinensis (DCCpin) was therefore selected as a promising candidate to catalyze the decarboxylation step of the cascade reaction.
Figure 2: Genomic context of KDO2 gene in C. pinensis. Please click here to view a larger version of this figure.
Consequently, we designed enzymatic cascade reactions involving dioxygenases and DCs to achieve the synthesis of aliphatic chiral β- and γ-amino alcohols from amino acids (Figure 3). As previously reported, the C-H oxidation catalyzed by the αKAO introduces the hydroxy-substituted stereogenic center with total diastereoselectivity; the Cβ/γ chirality will be preserved in the decarboxylative step, which only affects the Cα carbon of the amino acid moiety16.
Figure 3: Retrosynthetic analysis. (A) Retrosynthesis of β- and γ-amino alcohols (R)-1,5- diaminopentan-2-ol (4) from (5R)-hydroxy-L-Lysine, and (S)-1,5-diaminopentan-2-ol (5) and 1,5-diaminopentan-3-ol (6) from L-lysine. (B) Retrosynthesis of β,γ- and β,δ-amino diols (2S,3S)-1,5-diaminopentane-2,3-diol (10) and (2R,4S)-1,5-diaminopentane-2,4-diol (11) starting from (5R)-hydroxy-L-lysine, and (2R,3R)-1,5-diaminopentane-2,3-diol (7) starting from L-lysine. Please click here to view a larger version of this figure.
Starting from L-lysine and its (5R)-hydroxy derivative, we herein report a two/three step, one pot, enzymatic procedure combining dioxygenases and PLP-DCs to obtain the target amino alcohols. Prior the synthesis at the laboratory scale of the target molecules, the method was developed at the analytical scale to adjust the reaction conditions, e.g., the enzyme concentrations, required to allow full conversion of the starting materials; we present this procedure as well.
1. Enzyme Preparation
2. Preparation of Solutions
NOTE: All the solutions below are prepared in deionized water.
3. Analytical-scale Reactions
4. Semi-preparative One-pot Biocatalytic Reaction
NOTE: Enzymatic reactions are carried out on 0.1 mmol of L-lysine in an open-air 250 mL glass Erlenmeyer flask for a total volume of 10 mL.
5. Purification of Amino Alcohols from Crude Enzymatic Reaction Mixture
6. Reaction Monitoring and Product Analysis
7. NMR Analysis of Purified Amino Alcohols
We have previously reported the synthesis of mono- and di-hydroxy-L-lysines by diastereoselective enzymatic hydroxylation catalyzed by dioxygenases of the iron(II)/αKAO family (Figure 1)16. To optimize the protocol of the entire cascades presented here, which combine one or two hydroxylation steps catalyzed by an αKAO followed by a decarboxylation step catalyzed by a PLP-DC, the reaction conditions were adjusted to satisfy th...
Chiral amino alcohols and derivatives have a wide range of applications, from chiral auxiliaries for organic synthesis to pharmaceutical therapy. Multistep synthesis for producing amino alcohols by conventional organic synthesis are numerous, but may not always be efficient because of tedious protection/deprotection steps together with a sensitive control of the stereochemistry16. A biocatalytic approach that dispenses with the protection/deprotection steps and is usually highly stereoselective re...
The authors have nothing to disclose.
The authors thank Véronique de Berardinis for fruitful discussion and Alain Perret, Christine Pellé, and Peggy Sirvain for technical support.
Name | Company | Catalog Number | Comments |
HEPES | Sigma Aldrich | H3375 | |
L-lysine hydrochloride | Sigma Aldrich | L5626 | |
(5S)-hydroxy-L-lysine | Sigma Aldrich | GPS NONH | Out sourcing |
α-ketoglutaric acid | Sigma Aldrich | 75892 | |
Sodium ascorbate | Sigma Aldrich | A7631 | |
Ammonium Iron(II) sulfate hexahydrate | Acros | 201370250 | |
Pyridoxal phosphate (PLP) | Sigma Aldrich | 82870 | |
3,4-dimercaptobutane-1,2-diol (DTT) | Sigma Aldrich | D0632 | |
1-fluoro-2,4-dinitrobenzene (DNFB) | Sigma Aldrich | D1529 | |
Ethanol | VWR | 20825.290 | |
Sodium hydrogen carbonate | Sigma Aldrich | 71631 | |
HCl 37% | Sigma Aldrich | 435570 | |
HCl 0.1M | Fluka | 35335 | |
Acetonitrile HiPerSolv CHROMANORM for LC-MS | VWR | 83640.320 | |
2,2,2-trifluoroacetic acid | VWR | 153112E | |
Ammonia 28% | VWR | 21182.294 | |
Methanol HiPerSolv CHROMANORM for LC-MS | VWR | 83638.32 | |
Formic acid | Acros | 270480010 | |
Phosphoric acid 85% | Acros | 201145000 | |
Deuterium oxide | Acros | 320,710,075 | |
NaOH | Sigma Aldrich | S5881 | |
C18 HPLC column | Phenomenex | 00F-4601-Y0 | |
Accela UHPLC System | ThermoFisher Scientific | ||
Accela PDA detector | ThermoFisher Scientific | ||
4mm syringe filters - 0,22µm - PVDF | Merck | SLGVR04NL | |
Single-use tuberculin syringe with ml graduation, Luer tip | VWR | HSWA5010.200V0 | |
Cation exchange resin 100-200 mesh | Sigma Aldrich | 217506 | |
Mixed mode cation-exchange solid-phase extraction cartridge 6 mL | Waters | 186000776 | |
Extraction manifold | Waters | WAT200609 | |
Rotary evaporator | Büchi | 531-0103 | |
Lyophilizer alpha 1-2 LDplus | Christ | L083302 | |
Micropipette 20 µL | Eppendorf | 3121000031 | |
Micropipette 100 µL | Eppendorf | 3121000074 | |
Micropipette 500 µL | Eppendorf | 3121000112 | |
Micropipette 1000 µL | Eppendorf | 3121000120 | |
300 MHz spectrometer | Bruker | ||
2 mL microtube | CLEARLine | CL20.002.0500 | |
50 mL conical-bottom centrifuge tube | Fischer Scientific | 05-539-8 | |
25 mL round-bottom flask 14/23 | Fischer Scientific | 10353331 | |
100 mL round-bottom flask 29/32 | Fischer Scientific | 11786183 | |
250 mL round-bottom flask 29/32 | Fischer Scientific | 11786183 | |
250 mL erlenmeyer flask | Fischerbrand | 15496143 |
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