Full Papers doi.org/10.1002/ejoc.202101192 Synthesis of Chiral Scaffolds Based on Polyfunctional Cyclobutane β-Amino Acids Yaroslav Verkh,[a, b] Ona Illa,*[a] and Rosa M. Ortuño*[a] In Memory of Ferenc Fülöp. Chiral scaffolds have been synthesized through selective trans- tional chemical platforms. These can be suitable for the formations from conveniently protected cyclobutane β-amino synthesis of complex molecules, through condensation reac- acids. The obtained derivatives present additional functional tions, such as peptide coupling, dehydration between a groups, such as hydroxyl, carboxyl, or carbon-carbon triple carboxylic acid and an alcohol, or click chemistry. To illustrate bond, linked to the amino acid core through a flexible C1 or C2 some of these possibilities, two hybrid tripeptides have been alkyl fragment. The resultant molecules are highly versatile to prepared as examples of their synthetic potential. be used as chiral precursors for the production of polyfunc- Introduction ing factor concerning the properties of the target compounds.[12] Some selected instances are illustrated as Amino acids, along with carbohydrates and terpenes, are follows. (R,S)- and (S,S)-β-CBAA were combined with l-proline important contributors to the “chiral pool”.[1] Actually, natural affording hybrid α,β-tripeptides, which have been used as and unnatural or modified amino acids can act as building organocatalysts for enantioselective aldol additions.[13] More- blocks in the construction of many attractive molecules that over, they have served as precursors for the synthesis of ligands include bioactive natural products, pharmaceuticals, and chi- for metal catalysis as well.[14] In other cases, β-CBAAs were key rons for complex architectures.[1,2,3] structural units to build peptidic organogelators resulting from Cyclobutane amino acids (CBAAs) are an interesting class of their combination with themselves[15] or with linear molecules that include both naturally occurring[4] and synthetic monomers.[16] They were also used as a self-assembly compo- products and, accordingly, many efforts have been devoted to nent in functional organic fibers.[17] Regarding biomedical the preparation and applications of cyclobutane α-,[5] β-,[6,7] and applications, cell penetrating peptides with ability to translocate γ-amino acids.[8–11] In particular, the derivatives obtained from the HeLa cell membrane have been obtained through the (R,S)- and (S,S)-2-aminocyclobutane-1-carboxylic acids ((R,S)- coupling of (S,S)-β-CBAA with Nα-functionalized 4-amino-L-pro- and (S,S)-β-CBAA, respectively)[6,7] (Figure 1) have been revealed line (Figure 1).[18] Otherwise, the inclusion of β-CBAAs in the as powerful synthetic precursors of manifold products. When neuropeptide Y (NPY) significantly improved its selectivity; NPY incorporated in homo or hybrid peptides, β-CBAAs can be controls central and peripheral processes by activation of the G useful as chiral polyfunctional platforms to produce different protein coupled receptors YXR.[19] Analogues of its C-terminal products as well as soft materials with specific features. In fragment containing β-CBAAs displayed exclusively binding to addition, the rigid cyclobutane moiety promotes the restriction Y4R while the parental peptide shows promiscuous binding of the molecular conformational freedom, which is an interest- among different receptors of this family.[20] Furthermore, some β-CBAA-based cationic surfactants have been revealed as DNA- condensing agents with enhanced properties and absence of [a] Dr. Y. Verkh, Prof. Dr. O. Illa, Prof. Dr. R. M. Ortuño cell toxicity, making them promising candidates to be used as Departament de Química Universitat Autònoma de Barcelona vectors in gene therapy.[21] 08193 Cerdanyola del Vallès, Barcelona, Spain Otherwise, hydroxy-modified (S,S)-β-CBAA has been incor- E-mail: ona.illa@uab.cat porated in conformationally constrained pyrrolidinyl peptide rosa.ortuno@uab.cat [b] Dr. Y. Verkh nucleic acids (PNA)[22] that exhibited improved binding with The Boston Consulting Group respect to the parent PNA.[23] Indeed, further functionalization RWTH Aachen University of β-CBAA provide polyfunctional small molecules, which are 52078 Aachen, North Rhine-Westphalia, Germany interesting scaffolds for the construction of complex chiral Supporting information for this article is available on the WWW under https://doi.org/10.1002/ejoc.202101192 structures.[24] Part of the Special Collection in memory of Ferenc Fülöp. Now, in this work, we present the synthesis of β-CBAAs © 2021 The Authors. European Journal of Organic Chemistry published by containing an additional functional group (FG, Figure 1) linked Wiley-VCH GmbH. This is an open access article under the terms of the to the amino acid through a flexible C1 or C2 spacer. As Creative Commons Attribution Non-Commercial NoDerivs License, which representative examples, a primary alcohol, a carboxylic acid, permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adap- and a terminal alkyne have been chosen as targets because of tations are made. their versatility to be transformed into different functions or for Eur. J. Org. Chem. 2021, 6022 – 6027 6022 © 2021 The Authors. European Journal of Organic Chemistry published by Wiley-VCH GmbH Full Papers doi.org/10.1002/ejoc.202101192 Figure 1. (R,S)- and (S,S)-β-CBAA as synthetic precursors of some peptide derivatives with defined properties, and general structure of the target molecules in this work. the coupling with other structural units via condensation pound 3 (83 % yield), which was previously described,[27] and processes or, in the case of alkynes, to be used in click new product 4 (quantitative), respectively. Subsequently, sepa- chemistry.[25] Moreover, the terminal alkyne group has been rate reduction of the methyl ester with lithium borohydride discovered in natural products made by plants, fungi and (LiBH4) in methanol at 0° C gave alcohols 5 and 6 in good microorganisms and, recently, a pathway for terminal-alkyne yields. Unprotected amino alcohol 7 was then obtained in amino acid biosynthesis has been reported.[26] Then, these quantitative manner by hydrogenolysis of the benzyl carbamate amino acids have been utilized to prepare hybrid tripeptides (Cbz) in 5 in the presence of palladium hydroxide on charcoal. providing polyfunctional chemical platforms suitable for build- Alternatively, 7 was prepared by acidolysis of the tert-butyl ing more elaborated compounds. carbamate (Boc) in 6 using trifluoroacetic acid (TFA). Otherwise, mild saponification of the methyl ester in 3 with 0.25 M sodium hydroxide provided quantitatively carboxylic acid 8,[28] which Results and Discussion was later used in peptide coupling reactions (see below). In order to introduce the terminal triple bond, two Scheme 1 shows the synthesis of the target molecules from strategies were assayed. The first one is based on the reaction precursor monoprotected amino acids 1 and 2. They were of an aldehyde with a dimethyl diazophosphonate to create the coupled with glycine (Gly) that afforded the C2 linker. Thus, triple bond affording the terminal carbon atom, also known as reaction of 1[7a] and 2[7b] with MeO-Gly hydrochloride, in the the Ohira-Bestmann modification of the Seyferth-Gilbert presence of (benzotriazol-1-yloxy)tripyrrolidinophosphonium homologation.[29] hexafluorophosphate (PyBOP), afforded fully protected com- Eur. J. Org. Chem. 2021, 6022 – 6027 www.eurjoc.org 6023 © 2021 The Authors. European Journal of Organic Chemistry published by Wiley-VCH GmbH Full Papers doi.org/10.1002/ejoc.202101192 Scheme 1. Synthesis of the polyfunctional cyclobutane monomers. Thus, alcohol 5 was oxidized with oxalyl chloride in the presence of dimethyl sulfoxide (DMSO) to afford aldehyde 9 in 76 % yield, which was used in the next step without further purification. Subsequent reaction with dimethyl (1-diazo-2- oxopropyl)phosphonate (Bestmann-Ohira reagent)[30] provided alkyne 10 in 22 % yield. Finally, quantitative acidolysis of Boc carbamate with TFA gave the ammonium salt 11, in 17 % overall yield from 5, which was identified by its 1H NMR spectroscopic data and used in subsequent reactions without purification. The yield was improved changing the synthetic strategy by coupling Scheme 2. Synthesis of the new peptide derivatives 13 and 14. propargyl amine with carboxylic acids 1 and 2, respectively, in the presence of N,N’-dicyclohexylcarbodiimide (DCC). In this way, compounds 11 and 12 were prepared in ca. 50 % overall yield through a much shorter synthetic sequence. resulting from the possible coupling of the acid with the To show the ability of these modified β-CBAA derivatives to primary alcohol was not detected. On the other hand, coupling produce peptide-based polyfunctional platforms, compounds of acid 8 with the free amine liberated in situ from ammonium 13 and 14 were synthesized (Scheme 2). salt 11, afforded the peptide derivative 14 bearing a terminal Thus, amine 7 was coupled with acid 8 using FDPP in the triple bond, in 46 % yield. Both compounds 13 and 14 have presence of Hünig base (diisopropylethylamine, DIPEA) to terminal functions, protected amine and alcohol or triple bond, provide hydroxy peptide 13 as the only defined product, in allowing their inclusion into complex molecules via coupling 20 % yield after careful purification (see the Experimental reactions or, for instance, click chemistry. Section). The reaction was chemoselective and the compound Eur. J. Org. Chem. 2021, 6022 – 6027 www.eurjoc.org 6024 © 2021 The Authors. European Journal of Organic Chemistry published by Wiley-VCH GmbH Full Papers doi.org/10.1002/ejoc.202101192 Conclusion mixture stirred for 15 min until gas development generally ceased. After, H2O (15 mL) was added and the aqueous layer was extracted By means of selective transformations and starting from with EtOAc (4 × 15 mL). The combined organic layers were dried over MgSO4 and the solvent evaporated. Compound 5 was conveniently protected cyclobutane β-amino acids, the produc- obtained in high purity as a white crystalline solid (85 mg, tion of derivatives bearing a flexible fragment that contains 0.29 mmol, 93 % yield. Rf = 0.33, EtOAc:MeOH (20 : 1)). m.p. 77–78 °C additional functions, such as a primary alcohol, carboxylic acid, (EtOAc). 1H-NMR (400 MHz, CDCl3) δ 1.84–2.04 (m, 1H), 2.09–2.20 or terminal alkyne, has been achieved. The introduced func- (m, 1H), 2.21–2.45 (m, 2H), 2.66 (br.s, 1H), 3.15–3.34 (m, 2H), 3.35– tional groups confer these molecules with the possibility to be 3.48 (m, 1H), 3.54–3.68 (m, 2H), 4.39–4.57 (m, 1H), 5.04–5.13 (br. s, 2H), 5.75 (br. s, 1H), 6.11 (br. s, 1H), 7.30–7.46 (m, 5H).13C-NMR used as chiral scaffolds for the production of polyfunctional (100 MHz, CDCl3) δ 18.1, 28.6, 42.1, 46.1, 46.8, 61.9, 66.7 128.0, chemical platforms suitable for the synthesis of complex 128.1, 128.4, 136.3, 156.0, 173.6. HRMS (ESI): C15H20N2NaO4 requires molecules, through condensation reactions or click chemistry. 315.1320, found [M + Na] + 315.1316. IR (ATR) 3307, 3036, 2954, Two hybrid tripeptides have been prepared as some examples 2876, 1689, 1650, 1523 cm 1. to illustrate the synthetic potential of these compounds. tert-Butyl (1S,2R)-2-(2-hydroxyethylcarbamoyl)cyclobutyl- carbamate (6): A flask containing compound 4 (0.60 g, 2.10 mmol, 1.0 eq) was deoxygenated with N2. Et2O (60 mL) was added and the Experimental Section flask cooled down to 0 °C. 2 M LiBH4 in THF (2.5 mL, 5.03 mmol, 2.4 eq) was added slowly to the mixture, causing the appearance of General Information: Starting amino acid 2[7b] and dipeptide 3[27] a white precipitate. The reaction mixture was stirred for 3 h at 0 °C were prepared according to previously reported methods. Conven- until the entire ester reacted (monitored by TLC). Subsequently, sat. tional chemicals were purchased from Sigma-Aldrich. Triethylamine NH4Cl(aq) (40 mL) was slowly added and the mixture stirred for (Et3N) and N,N-Diisopropylethylamine (DIPEA) were distilled from a 15 min. After, H2O (20 mL) was added and the aqueous layer was KOH solution before use. Solvents were used as received from the extracted with EtOAc (4 × 15 mL). The combined organic layers suppliers, unless otherwise stated. DCM and MeOH were dried prior were dried over MgSO4 and the solvent evaporated. Compound 6 to use by distillation under nitrogen from solutions with CaCl2 and was obtained in high purity as a white crystalline solid (0.48 g, CaH2, respectively. Dimethylformamide (DMF) was dried using 4 Å 1.86 mmol, 89 % yield. Rf = 0.16, EtOAc). [α]D 103.6 (c 1, MeOH). molecular sieves overnight and subsequent distillation under m.p. 142 °C (EtOAc). 1H-NMR (360 MHz, CDCl3) δ 1.43 (s, 9H), 1.86– vacuum and low temperature. Melting points were determined 2.00 (m, 1H), 1.85–2.71 (m, 4H), 3.21–3.29 (m, 1H), 3.39–3.48 (m, with a Kofler apparatus. Optical rotations were measured using a 2H),3.70–3.75 (m, 2H), 4.42 (ddd, 3JH H = 8.4 Hz, 1H), 5.31 (br. s, 1H), JASCO-715 spectropolarimeter at 22 � 2 °C. IR spectra of solids were 6.10 (br. s, 1H). 13C-NMR (90 MHz, CDCl3) δ 18.0, 28.3, 28.8, 42.2, recorded using a Sapphire-ATR Sensor 27 Bruker spectrophotom- 46.3, 62.0, 79.6, 155.6, 173.7. HRMS (ESI): C12H22N2NaO4 requires eter. Silica gel 60 for flash chromatography (mean pore diameter: 281.1477, found [M + Na] + 281.1473. IR (ATR) 3363, 2946, 1675, 60 Å; particle size: 0.04–0.06 mm, 230–400 mesh ASTM) was 1630, 1545, 1509 cm 1. purchased from Scharlau. NMR spectra were acquired at 298 K on a AC250 Bruker spectrometer or a AVANCE360 Bruker spectrometer (1R,2S)-2-amino-N-(2-hydroxyethyl)cyclobutanecarboxamide (7): in CDCl3 or CD3OD solution. The 1H and 13C chemical shifts are given Compound 5 (0.12 g, 0.41 mmol) was dissolved in EtOAc (20 mL). relative to TMS in ppm. Pd(OH)2 on activated carbon (20 wt%, 0.02 g) was added and the mixture was stirred under a hydrogen atmosphere for 16 h (p = Methyl 2-((1R,2S)-2-(tert-butoxycarbonylamino)cyclobutane- 6 bar). The reaction mixture was filtered through Celite®, washed carboxamido) acetate (4): A flask containing deprotected amino with EtOAc and MeOH and dried. Compound 7 was obtained in acid 2[7b] (1.11 g, 5.16 mmol, 1.0 eq) and PyBOP (2.98 g, 5.72 mmol, quantitative yield in form of a white wax (0.06 g, 0.41 mmol). 1H- 1.1 eq) was deoxygenated with N2. 20 mL of DCM and DIPEA NMR (250 MHz, CDCl3) δ 1.81–2.45 (m, 4H), 3.04–3.82 (m, 6H), 6.86 (3.67 mL, 21.08 mmol, 4.1 eq) were added and the mixture was (br. s, 1H). 13C-NMR (62.5 MHz, CDCl3) δ 18.4, 32.0, 42.7, 46.8, 49.4, stirred for 5 min. Glycine methyl ester hydrochloride (0.64 g, 61.5, 174.5. 5.72 mmol, 1.1 eq) was added and the resulting mixture was stirred at room temperature for 18 h. After, the solvent was removed, and tert-Butyl (1S,2R)-2-(2-oxoethylcarbamoyl)cyclobutylcarbamate the residue was purified by column chromatography from EtOAc (9): Oxalyl chloride (0.43 mL, 0.85 mmol, 1.1 eq) was dissolved in (Rf = 0.48). Compound 4 was isolated as a white crystalline solid anhydrous DCM and cooled down to 60 °C. DMSO (0.12 mL, (1.47 g, 5.14 mmol, 99 % yield). [α]D 78.8 (c 1, MeOH). m.p. 103 °C 1.70 mmol, 2.2 eq) was added slowly to the oxalyl chloride mixture. (EtOAc). 1H-NMR (360 MHz, CDCl3) δ 1.40 (s, 9H), 1.84–1.99 (m, 1H), The mixture was stirred for 2 min. Compound 5 (0.20 g, 0.77 mmol, 1.99–2.14 (m, 1H), 2.15–2.38 (m, 2H), 3.29 (m, 1H), 3.75 (s, 3H), 3.96 1.0 eq) was dissolved in a mixture of DMSO (0.2 mL) and DCM (dd, 2JH H = 18.3 Hz, 3JH H = 5.0 Hz, 1H), 4.10 (dd, 2JH H = 18.3 Hz, (1.7 mL) and subsequently added to the “activated” DMSO during 3 JH H = 5.7 Hz, 1H), 4.35–4.48 (m, 1H), 5.47 (br.d, 1H), 6.17 (s, 1H). 13C- 3 min. The white mixture was stirred for 15 min. After, Et3N (0.54 g, NMR (90 MHz, CDCl3) δ 18.1, 28.1, 29.3, 41.0, 45.9, 52.1, 79.1, 155.0, 3.87 mmol, 5.0 eq) was slowly added and the resulting solution was 170.1, 173.2. HRMS (ESI): C13H22N2NaO5 requires 309.1426, found [M stirred for 5 min. Then, the mixture was allowed to warm to room + Na] + 309.1419. IR (ATR) 3336, 2955, 1770, 1683, 1650, 1520, temperature (45 min) and a yellow, opaque liquid was obtained. 1513 cm 1. Water (30 mL) was added and the mixture extracted with DCM (3 × 15 mL). The combined organic layers were washed with brine, dried Benzyl (1S,2R)-2-(2-hydroxyethylcarbamoyl)cyclobutylcarbamate over MgSO4 and filtered. After the evaporation of the solvent, (5): A flask containing compound 3[27] (0.1 g, 0.31 mmol, 1.0 eq) was compound 9 was obtained as a yellowish solid (0.15 g, 0.59 mmol, deoxygenized with N2. Et2O (11 mL) was added, and the resulting 76 % yield), which was used without further purification in the solution was cooled down to 0 °C. 2 M LiBH4 in THF (0.34 mL, following step. 1H-NMR (250 MHz, CDCl3) δ 1.41 (s, 9H), 1.75–2.45 0.68 mmol, 2.2 eq) was added slowly to the mixture, causing the (m, 4H), 3.05–3.54 (m, 1H), 3.99–4.21 (m, 1H), 4.24–4.54 (m, 2H), 5.40 appearance of a white precipitate. The reaction mixture was stirred (br. d, 3JH H = 7.8 Hz, 1H), 6.45 (br. s, 1H), 9.64 (s, 1H). for 3 h at 0 °C until the entire reagent reacted (monitored by TLC). Subsequently, sat. NH4Cl(aq) (15 mL) was slowly added and the Eur. J. Org. Chem. 2021, 6022 – 6027 www.eurjoc.org 6025 © 2021 The Authors. European Journal of Organic Chemistry published by Wiley-VCH GmbH Full Papers doi.org/10.1002/ejoc.202101192 tert-Butyl (1S,2R)-2-(prop-2-ynylcarbamoyl)cyclobutylcarbamate chromatography from EtOAc:MeOH (10 : 1, Rf = 0.19). Additionally, (10): Compound 9 (0.42 g, 1.64 mmol, 1.0 eq) was placed in a dry the product was recrystallized from a 0.02 g/mL DCM solution by flask under a nitrogen atmosphere and dissolved in anhydrous diffusion of Et2O at 4 °C for 48 h and obtained as a white solid MeOH (16 mL). K2CO3 (0.54 g, 3.93 mmol, 2.4 eq) and Bestmann- (0.04 g, 0.09 mmol, 20 % yield). [α]D: 101.1 (c 1, MeOH). m.p. 150– Ohira reagent (10 mol% in acetonitrile, 1.22 mL, 1.97 mmol, 1.2 eq) 153 °C (acetone). 1H-NMR (250 MHz, CD3OD) δ 1.87–2.13 (m, 4H), were added. The opaque mixture was stirred for 17.5 h at room 2.13–2.42 (m, 4H), 3.19–3.46 (m, 5H) 3.51–3.69 (m, 2H), 3.69–3.74 temperature. After, the solvent was evaporated, the orange residue (m, 1H), 3.79–3.94 (m, 1H), 4.47 (m, 1H), 4.58 (m, 1H), 4.97–5.24 (m, dissolved in Et2O (12 mL) and washed with H2O (100 mL) and sat. 2H), 7.26–7.42 (m, 5H). 13C-NMR (62.5 MHz, CD3OD) δ 17.2, 17.9, NaHCO3(aq) solution (50 mL). The aqueous layers were additionally 27.3, 27.9, 41.3, 41.7, 45.0, 45.5, 46.3, 46.8, 60.1, 65.8, 127.1, 127.3, extracted with Et2O (2 × 20 mL) and DCM (2 × 20 mL). The organic 127.8, 136.6, 156.2, 169.6, 173.5, 173.6. HRMS (ESI): C22H30N4NaO6 layers were dried over MgSO4 and the solvent was evaporated. The requires 469.2063, found [M + Na] + 469.2069. IR (ATR) 3388, 2949, solid residue was purified by column chromatography from EtOAc: 2413, 1700, 1634, 1453, 1338, 1237 cm 1. Hex (2 : 1) (Rf = 0.46) yielding compound 10 in form of a white solid (0.09 g, 0.36 mmol, 22 % yield). [α]D: 83.8 (c 0.4, DCM). m.p. 150– Functionalized peptide (14): Compound 8 (47 mg, 0.15 mmol, 153 °C (EtOAc). 1H-NMR (250 MHz, CDCl3) δ 1.43 (s, 9H), 1.81–2.42 1.2 eq) and FDPP (56 mg, 0.14 mmol, 1.1 eq) were deoxygenized (m, 5H), 3.16–3.31 (m, 1H), 3.93–4.17 (m, 2H), 4.40 (m, 1H), 5.34 (br. with N2 in a round-bottomed flask. Anhydrous DCM (2 mL) and DIPEA (0.14 mL, 0.79 mmol, 6.0 eq) were added and the mixture d, 3JH H = 8.1 Hz, 1H), 5.99 (br. s, 1H). 13C-NMR (62.5 MHz, CDCl3) δ 17.8, 28.1, 28.9, 29.4, 46.0, 46.1, 71.2, 77.0, 79.2, 155.1, 172.3. HRMS stirred for 5 min. After, compound 11 (20 mg, 0.13 mmol, 1.0 eq) (ESI): C13H20N2NaO3 requires 275.1366, found [M + Na] + 275.1368. IR was dissolved in anhydrous DCM (2 mL) and added to the mixture, (ATR) 3354, 3325, 2994, 2971, 2919, 2852, 2101, 1677, 1642, which was stirred for 16 h at room temperature. Subsequently, the 1513 cm 1. solvent was evaporated, and a yellow solid was obtained. The residue was suspended in water. The aqueous layer was extracted (1S,2R)-2-(prop-2-ynylcarbamoyl)cyclobutanaminium trifluoroace- with DCM (3 × 10 mL) and the combined organic layers dried over tate (11): Compound 10 (0.03 g, 0.12 mmol, 1 eq) was dissolved in MgSO4, filtered and the solvent was evaporated. The yellow solid DCM (2.4 mL). TFA (1.32 mL, 17.25 mmol, 145 eq) was slowly added. was purified by column chromatography from EtOAc: MeOH (10 : 1, The mixture was stirred for 15 min at room temperature until the Rf = 0.53). Compound 14 was obtained in the form of a white solid entire reagent reacted (monitored by TLC). Subsequently, the (32 mg, 0.07 mmol, 46 % yield). [α]D: 148.9 (c 1.2, MeOH). m.p. 1st solvent and TFA were evaporated. Compound 11 was obtained in transition 160–165 °C; 2nd transition 172–174 °C (MeOH). 1H-NMR form of a yellow oil and used in the following reaction without (250 MHz, CD3OD) δ 1.82–2.46 (m, 8H), 2.56–2.62 (m, 1H), 3.18–3.48 further purification (0.018 g, 0.12 mmol, 99 % yield). 1H-NMR (m, 4H), 3.60–4.05 (m, 4H), 4.37–4.68 (m, 2H), 4.97–5.20 (m, 2H), (250 MHz, CDCl3) δ 2.11–2.66 (m, 5H), 3.23–3.48 (m, 1H), 3.92–4.30 7.17–7.44 (m, 5H). 13C-NMR (62.5 MHz, CD3OD) δ 17.2, 17.8, 27.3, (m, 3H), 7.99 (br. s, 1H), 9.31 (br. s, 1H). 27.7, 41.8, 44.8, 44.9, 45.4, 46.4, 65.8, 70.4, 79.3, 127.2, 127.4, 127.8, 136.6, 156.2, 169.3, 172.9, 173.6. HRMS (ESI): C23H28N4NaO5 requires Benzyl (1S,2R)-2-(prop-2-ynylcarbamoyl)cyclobutylcarbamate 463.1952, found [M + Na] + 463.1948. IR (ATR) 3296, 2950, 2468, (12): A flask containing acid 1[7a] (0.28 g, 1.12 mmol, 1.0 eq) and 1684, 1633, 1538, 1430 cm 1. N,N’-dicyclohexylcarbodiimide (DCC) (0.25 g, 1.24 mmol, 1.1 eq) was deoxygenized with nitrogen. Anhydrous DCM (12 mL) was added. After the solid was dissolved, propargyl amine (0.08 mL, 1.24 mmol, 1.1 eq) was injected and the reaction mixture stirred for 18 h. Subsequently, the solvent was evaporated and the yellow residue Acknowledgements was purified by column chromatography from hexane:EtOAc (1 : 10, Rf = 0.57). Compound 12 was obtained in form of a white solid Financial support from Spanish Ministry of Economy, Industry and (0.17 g, 0.58 mmol, 52 % yield). [α]D: 76.7 (c 1, DCM). m.p. 148– Competitivity (Grant CTQ2016-77978-R) is gratefully acknowl- 152 °C (EtOAc). 1H-NMR (250 MHz, CDCl3) δ 1.83–2.60 (m, 5H), 3.16– edged. 3.37 (m, 1H), 3.86–4.10 (m, 2H), 4.49 (m, 1H), 5.01–5.27 (m, 2H), 5.73 (br. d, 3JH H = 7.7 Hz, 1H), 6.01 (br. s, 1H), 7.14–7.63 (m, 5H). 13C-NMR (62.5 MHz, CDCl3) δ 17.9, 28.1, 28.8, 45.9, 46.4, 66.5, 71.2, 79.2, 127.8, 127.9, 128.3, 136.1, 155.6, 172.2. HRMS (ESI): C16H18N2NaO3 requires Conflict of Interest 309.1210, found [M + Na] + 309.1211. IR (ATR) 3341, 3319, 3257, 3063, 3040, 3012, 2960, 2945, 2120, 1676, 1643, 1533, 1520 cm 1. The authors declare no conflict of interest. Functionalized peptide (13): Compound 8 (0.14 g, 0.44 mmol, 1.0 eq) and FDPP (0.19 g, 0.50 mmol, 1.1 eq) were deoxygenized Keywords: Amino acids · Chiral scaffolds · Peptidomimetics · with N2 in a round-bottomed flask. 20 mL of anhydrous DCM and Polyfuctional chemical platforms · Unnatural peptides DIPEA (0.32 mL, 1.85 mmol, 4.2 eq) were added and the mixture stirred for 5 min. After, compound 7 (0.07 g, 0.44 mmol, 1.0 eq) dissolved in DMF (1.1 mL) was injected and the mixture was stirred for 20 h at room temperature. Subsequently, the solvent was [1] S. Hanessian. Total Synthesis of Natural Products: The “Chiron” evaporated, and a yellow solid was obtained. The residue was Approach. Pergamon Press Ltd. 1983. lyophilized to remove DMF. EtOAc (20 mL) was added to the solid [2] P. Singh, K. Samanta, S. K. Das, G. Panda, Org. Biomol. Chem. 2014, 12, and the solution was washed with sat. NaHCO3(aq) (20 mL) and water 6297–6339. (20 mL). The aqueous layer was reextracted with EtOAc and the [3] S. M. Paek, M. Jeong, J. Jo, Y. M. Heo, Y. T. Han, Y. Yun, Molecules 2016, combined organic layers dried over MgSO4, filtered and the solvent 21, 951. was evaporated. The residue was lyophilized again to remove traces [4] P. Hughes, J. Clardy, J. Org. Chem. 1988, 53, 4793–4796. [5] A. Avenoza, J. H. Busto, N. Canal, F. Corzana, J. M. Peregrina, M. Pérez- of DMF. The yellowish solid was suspended in a small quantity of Fernández, F. Rodriguez, J. Org. Chem. 2010, 75, 545–552. Et2O at 0 °C and stirred vigorously for 30 min. After the suspension [6] a) C. Fernandes, C. Gauzy, Y. Yang, O. Roy, E. Pereira, S. Phaure, D. J. settled, the solvent was decanted, and the procedure repeated Aitken, Synthesis 2007, 14, 2222–2232 ; b) C. 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