Amino acids with fluorinated olefinic motifs – synthetic approaches

Peptidomimetics are molecules related to natural peptides that have an artificial unit incorporated in their structure. Such structural modifications, mimicking properties of natural amino acids, can be responsible for differently pronounced activity. Among others, amino acid derivatives with fluorinated olefinic motifies can act as building blocks in the synthesis of complex molecules with potential biological activity. Therefore, the synthetic approaches to the fluorinated olefinic moiety amino acids are of interest to organic chemists. There are different synthetic methods yielding fluorinated olefins having significant value in the synthesis of amino acid derivatives. This mini review describes the latest achievements in the synthesis of amino acids bearing mono-, di-or trifluorovinyl moiety


Introduction
The incorporation of fluorine to an organic molecule often induces remarkable changes in their biophysical properties including solubility, lipophilicity, conformational and metabolic stability as well as chemical reactivity. 1,2The changes in reactivity have far-reaching consequences.4][5][6] Therefore, the incorporation of fluorine or fluoroalkyl moieties into molecules is broadly used in the synthesis of drugs and biologically active compounds. 7,8This is one of the reasons why fluorination methods are widely studied in synthetic organic chemistry.
Fluorinated amino acids as well as fluorinated peptides have been widely studied and used in the design and synthesis of novel pharmaceuticals.They are very useful building blocks with regards to the synthesis of bioactive compounds.The big interest in the synthesis of fluorinated amino acids and peptides is also notable in the literature.Recently, in 2019 Professor Koksch published a broad review about the synthetic methods of fluorinated amino acids. 9Additionally, synthetic approaches were also discussed earlier by the Koksch group in 2008. 10Since the synthetic approaches to amino acids having fluorinated olefinic fragments were not well covered, we present herein, a short overview of the synthetic methods of amino acids bearing a fluorinated olefins.There are various successful syntheses that afford fluorinated olefins, and some have significant methodological value to synthetic chemists in the preparation of fluorinated amino acid libraries.

Monofluoroolefinic Amino Acid Derivatives
The monofluorovinyl moiety is often considered isosteric to the peptide bond.As such, due to the electronic similarity, it can replace the amide bond in peptides.Furthermore, the incorporation of fluorovinyl moieties in simple amino acids can strongly affect their bioactive properties and, therefore, this is a common strategy used in the design and synthesis of amino acid derivatives.
The early synthesis of (E)-β-(fluoromethylene)-m-tyrosine was reported by McDonald et al. in 1984. 11 The first step required the bromination followed by further fluorination by KHF2.Compound 2 was obtained in 54% yield and was further treated with triethyl phosphonoacetate in the presence of NaH to yield 3, as a mixture of Z-and E-isomers in 8:1 ratio and 80% yield.After subsequent bromination and dehydrobromination by piperidine, compound 4 was obtained that was further treated with LDA to give the E-bromide 5. Subsequent reaction of bromide 5 with ammonia in DMSO gave amine 6 in 20% yield.After deprotection of 6 the final product 7 bearing monofluorovinyl moiety was synthesized as a colorless powder in good 86% yield (Scheme 1).
McDonald's procedure employed a Wittig-type reaction and led to the E-isomer only.One year later, the same group offered another method called the "isocyanoacetate route" which gave both isomers. 12This procedure involved fluoroketone 8 as a starting material which was first treated with phenyltrimethylammonium tribromide to form product 9 in 87% yield.Subsequent, condensation with ethyl isocyanoacetate in the presence of Cu2O as catalyst gave a mixture of diastereoisomers 10.Next, the formed oxazoline 10 was protonated by trifluoroacetic acid which led to oxazoline ring opening.Furthermore, activated zinc has been used and compounds 11 and 12 were formed.The last step involved the refluxing in 47% aqueous solution of HBr and the treatment of propylene oxide.It finally resulted in the (E)-isomer 13 as a colorless powder and the (Z)-isomer 14 as a yellowish powder in equal 28% yields (Scheme 2).Scheme 2. Synthesis of (E)-and (Z)-fluoromethylene-m-tyrosine via the isocyanoacetate route.
The method established by McDonald was further studied by Lacan et al. 14 The protected derivative of 19 was prepared according to the previously described Wittig-type reaction.Subsequently, treatment of the starting material 19 with trifluoroacetic anhydride yielded (E)-and (Z)-trifluoroacetyl derivatives 20 and 21 in the 85:15 ratio.Next, these isomers were separated by subjecting them to α-Chymotrypsin.This enzyme specifically catalyzed only the hydrolysis of the (E)-isomer 21 while there was no interaction between Chymotrypsin and (Z)-isomer 20.As a result, the compounds 22a and 22b were formed.Deprotection was achieved by HBr-mediated hydrolysis and yielded 23a and 23b (or 23c and 23d) in equal amounts.The product mixture was separated by HPLC (Scheme 4).

Scheme 4. Synthesis of (E)-and (Z)-3-fluoromethylene-m-tyrosines.
McDonalds's group continued studies on synthetic methodology, and in 1985 proposed the synthesis of (E)-β-fluoromethyleneglutamic acid according to a similar method. 15The final product was prepared in 11 steps from ethyl 3,3-dimethylacrylate (24) (Scheme 5).First, the substrate was brominated (total 57% yield) and then fluoride exchange took place.The prepared product 25 was obtained as a mixture of isomers in 50% yield.Further the bromination and subsequent dehydrobromination process transformed 25 into 26 in 69% yield.Then, isomerization led to form only the E-isomer 27.The exchange of bromine with phthaloyl dichloride in the presence of dimethyl sulfoxide saturated with ammonia and 4-aminopyridine enabled to obtain Nprotected amino acid derivative 28 in 13% yield.After bromination of methyl group, the compound 29 was synthesized as a mixture of Z-and E-isomers in 2:1 ratio.Finally, the use of sodium cyanide yielded 30, treatment of which with concentrated HCl, then propylene oxide and isopropanol, after several days allowed the formation of the target product 31 in an excellent 95% yield.This multistep synthesis gave access to desired fluoro derivative of glutamic acid.

Scheme 5. Multi-step synthesis of (E)-β-fluoromethyleneglutamic acid.
Similarly, McDonald and Bey presented a general route to fluoroallyl amino acids by modifying the Nprotected substrate bearing fluorovinyl moiety. 16The starting phthalimide 32 (Scheme 6) was treated with NBS affording bromide as a mixture of E/Z-isomers.Next, only the Z-isomer 33 was used in the reaction with sodium cyanide and was transformed to the corresponding nitrile 34.A subsequent hydrolysis with hydrochloric acid yielded amino acid 35 as a final product.Scheme 6. Synthesis of (Z)-fluorovinyl amino acid using phthalimide method.
McCarthy et al. published the synthesis of fluorovinyl amino acids according to HWE (Horner-Wadsworth-Emmons) procedure. 19The starting aldehyde 45 was condensed with McCarthy's reagent to form vinylsulfone, which was subsequently treated with Bu3SnH.The mixture of isomers 47 and 48 was obtained in 60 and 11% yields, respectively (Scheme 9).These isomers were separated and treated with sodium methanolate.The isopropyl group was removed by p-toluenesulfonic acid and 50 and 54 were obtained in 50 and 34% yield, respectively.Finally, the alcohols 50 and 54 were oxidized to the carboxylic acid in the presence of PCC and the Boc protective group was removed.Products 52 and 56 were obtained in 75 and 78% yields, respectively.AUTHOR(S) Scheme 9. Synthesis of amino acids 52 and 56.
Another example of synthetic approach has been presented by Berkowitz et al. 23 They reported the threestep synthesis of six fluorovinyl amino acid derivatives employing Horner-Wadsworth-Emmons reaction protocol.As a starting reactant diethyl α-fluoro-α-(phenylsulfonyl)-methyl phosphonate (67) (McCarthy's reagent) was used with corresponding aldehydes 66a-f in the presence of the LiHMDS as a strong base.The reaction was carried out in -78 °C and led to (E)-α-fluorovinyl sulfones 68a-f in good to excellent yield (52-93%) and unusual diastereoselectivity -each compound was obtained as a single geometric isomer (E).The next step included the stereospecific interchange of sulfone to stannane 69.The last step involved protodestannylation, deprotection of amine and carboxylic group with hydrochloric acid and allowed formation of (Z)-α-(2'fluoro)vinyl amino acids 70a-f in yields ranging from 52% for 70f and 93% for DOPA derivative 70d (Scheme 12).

Scheme 12. Synthesis of amino acids with (2′Z)-fluorovinyl moiety.
In 2004 Qiu et al. described for the first time the dehydrofluorination process of (2S,4S)-tert-butyl-N-tertbutoxycarbonyl-4-difluoromethylpyroglutamate (71). 24The starting material 71 was synthesized from trans-4hydroxy-L-proline. 15,16 The reaction course was tested with the use of different bases in CH2Cl2.Triethylamine led to the formation of the desired product in 90% yield, while the use of pyridine yielded 72 only in less than 3%.The effect caused by the use of different solvent in the reaction was also investigated in presence of NEt3 as a base.The best results (90% yield) were obtained for CH3CN and CH2Cl2.(Scheme 14).The first step included asymmetric alkylation of 74a-b with fluorovinyl tosylate in the presence of LDA as a base and led to the products 75a-b in 89 and 84% yield, respectively.These compounds were then transformed with the use of HCl/MeOH/H2O to N'-methylamides 76a-b in 43 and 63% yields, respectively.The final products 77a-b were obtained after basic hydrolysis and treatment of propylene oxide in ethanol.After basic hydrolysis of the amide bond of 76a, partial racemization was observed in the formed compound 77a.To minimize this effect, 75a was treated with phthalic anhydride and oxalyl chloride which allowed the formation of the amide 78a in 89% yield.The amide bond was further removed by thermal decomposition of the in situ generated N'-nitrosoamide and the desired product methyl ester 79a was obtained in 91% yield (Scheme 14).Scheme 14. Asymmetric synthesis of γ-fluoro-α-amino acids.

Scheme 16. Synthesis of fluoroallyl amino acid by alkylation of imine.
Due to the low yields of the previous synthesis, Haufe prepared methyl N-Boc-(S)-2-amino-5-fluorohex-5enoate (88) according to Jackson's procedure with reagents. 29The alanine iodo-derivative 86 in the presence of activated zinc led to the formation of intermediate 87 and its further treatment with CuBr and alkylation with tosylate yielded product 88 in 61% (Scheme 17).Scheme 17. Zinc-activated synthesis of fluorinated allylalanine derivative 88.
Furthermore, Haufe et al. synthesized amino acid derivatives with monofluorovinylic side chain. 30The first step involved oxidation of the starting material by SeO2 (Scheme 18).Further condensation of the generated fluorinated alcohol 90 with N-Boc-glycine allowed the formation of the amino ester enolate.Subsequently, the Claisen rearrangement occurred affording the desired product 92 in 86% yield.

Scheme 18. Synthesis of amino acid 92.
The synthesis of β,γ-unsaturated amino acids has been reported by Berkowitz. 31In the proposed procedure the carboxyl and amino groups are retained, but also the fluorovinyl side chain bonded to the αcarbon is present.In 2004 Berkowitz et al. described the synthesis of such compounds by the Horner-Wadsworth-Emmons reaction using the McCarthy's reagent (diethyl-2-fluoro-1-(phenylsulfonyl)methyl phosphonate, 67). 23The product contained fluorine atom in the 2' position of vinylic group 96 (Scheme 19).This method was further investigated to obtain fluorine at the 1' position of the vinylic fragment 99.The electrophilic fluorinating agent 100 was used (method B) compared to previous method in which nucleophilic fluorination with 95 took place (method A).

Scheme 19. Synthesis of fluorovinyl amino acids.
The amino acids derivatives (Scheme 20, 101a-j) reacted with LiTMP and sulfone 100 to form compounds 102a-j in 60 to 91% yields.Subsequently they were treated with Bu3SnH in the presence of AIBN and the products 103a-i were obtained in good yields (70-91%).Finally, the -SnBu3 group was removed by acid hydrolysis and the carboxylic moiety was deprotected.The desired α-(1'-fluoro)vinyl amino acids hydrochlorides 99a-f and 99k-l were obtained in 52 to 99% yields.

Difluoro-and Trifluoro Amino Acid Derivatives
Access to the difluoro-and trifluorofinyl derivatives is discussed below.The di-and trifluorovinyl groups are very special components in medicinal chemistry and can be widely used in the design of drugs, owing to their metabolic stability.
Qui et al. proposed the synthesis of benzyl (2S)-N-Boc-4-difluoromethyleneprolinate (111) (Scheme 22). 25 The reaction proceeded by addition of CF2Br2 to the carbonyl moiety of the substrate 110 and subsequent treatment with zinc and HMPT which yielded the target derivative of proline 111 in 48% yield.This compound has been obtained as an intermediate for further synthesis of its hydrogenated analogue N-Boc-cis-4-difluoromethyl-L-proline.The synthesis of 4-difluoromethylene proline 119 was described by Ichikawa et al. 33 The three-step procedure involved treating the starting material 116 with NaH, removal of PMB group with DDQ and oxidation of the generated alcohol 118 with NaClO2 in the presence of TEMPO.This route led to the desired product 119 in 93% yield and 99% enantioselectivity (Scheme 24).Scheme 24.Synthesis of 4-difluoromethylene proline 119.
Recently, Juncosa et al. reported the synthesis of (S)-3-amino-4-(difluoromethylenyl)cyclopent-1-ene-1carboxylic acid from its saturated analogue CPP-115 (Scheme 26, cf.Scheme 23). 35The first step included the protection of both amino and carboxylic group.Then, after using potassium hexamethyldisilazane and PhSeCl, the phenylselenyl group was introduced to the α-carbon.Next, deprotection of amino and carboxylic groups and elimination of PhSe-group furnished a mixture of the hydrochlorides of isomers 127 and 128 in a 5:3 ratio, respectively.One of the isomers, 128, turned out to be more reactive and was removed by a selective modification using a soft thiol nucleophile.Finally, the isomer 127 was isolated as a pure product.Scheme 26.Synthesis of (S)-3-Amino-4-(difluoromethylenyl)cyclopent-1-ene-1-carboxylic acid (127).
As shown before, in 1987 Kolb et al. 17  The preparation of trifluorovinyl amino acid 136 (Scheme 28) was presented by Castelhano et al. 36 This route involved condensation of the Grignard reagent 133 with N-protected 2-chloroglycine methyl ester 134 and further acidic hydrolysis.

Conclusions
The idea, that peptidomimetics, as analogues of natural peptides, can be useful alternative in therapy is well known.The expected differences in their biological activity are attributed to the "artificial" unit incorporated to the original peptide chain.Also, the fact, that fluorine introduced to organic molecules, can dramatically change their properties is generally known.Besides changes of physical properties such as hydrophilicity, lipophilicity, the reactivity and stability can also be influenced by the incorporation of fluorine(s) in a molecule.It affects bond energy, acidity and basicity, hydrogen bond formation and, what is very important -the geometry of the molecule.Among others, introducing fluorine substituted vinyl units into poly-amino acid chain are of special interest.The presence of fluorine(s) at vinylic moieties and their precise location can significantly impact their properties.This general idea, however, requires quite challenging synthetic methods to prepare such "imaginary beautiful" molecules.This short review recent synthetic approaches and achievements on synthesis of amino acid bearing fluorine-substituted olefinic motifs.