{"paper_id":"001b4165-69d7-4594-be6b-3fd097db209a","body_text":"License and Terms: This document is copyright 2020 the Author(s); licensee Beilstein-Institut.\nThis is an open access publication under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0). Please note that the reuse,\nredistribution and reproduction in particular requires that the author(s) and source are credited.\nThe license is subject to the Beilstein Archives terms and conditions: https://www.beilstein-archives.org/xiv/terms.\nThe definitive version of this work can be found at: doi: https://doi.org/10.3762/bxiv.2020.55.v1\nThis open access document is published as a preprint in the Beilstein Archives with doi: 10.3762/bxiv.2020.55.v1 and is\nconsidered to be an early communication for feedback before peer review. Before citing this document, please check if a final,\npeer-reviewed version has been published in the Beilstein Journal of Organic Chemistry.\nThis document is not formatted, has not undergone copyediting or typesetting, and may contain errors, unsubstantiated scientific\nclaims or preliminary data.\nPreprint Title Facile Synthesis of 7-Alkyl-1,2,3,4-tetrahydro-[1,8]-naphthyridines as\nArginine Mimetics Using a Horner–Wadsworth–Emmons Based\nApproach\nAuthors Rhys A. Lippa, John A. Murphy and Tim Barrett\nPublication Date 27 Apr 2020\nArticle Type Full Research Paper\nSupporting Information File 1 THN SI.docx;  2.5 MB\nORCID® iDs Tim Barrett - https://orcid.org/0000-0003-1005-0784\n\n1 \nFacile Synthesis of 7-Alkyl-1,2,3,4-tetrahydro-[1,8]-\nnaphthyridines as Arginine Mimetics Using a Horner–\nWadsworth–Emmons Based Approach \nRhys A. Lippa1, John A. Murphy*1, Tim N. Barrett*2 \n \nAddress: 1Department of Pure &  Applied Chemistry, University of Strathclyde, 295 \nCathedral Street, Glasgow G1 1XL, Scotland, U.K. and 2GlaxoSmithKline Medicines \nResearch Centre, Gunnels Wood Road, Stevenage SG1 2NY, U.K. \n \nEmail: \nJohn A Murphy* john.murphy@strath.ac.uk \nTim N. Barrett* tim.x.barrett@gsk.com \nAbstract \nIntegrin inhibitors based on the tripeptide sequence Arg -Gly-Asp (RGD) are potential \ntherapeutics for the treatment of idiopathic pulmonary fibrosis (IPF). Herein, we \ndescribe a n ex peditious three-step synthetic sequence  of Horner –Wadsworth–\nEmmons olefination, diimide reduction and global deprotection to synthesise cores for \nthese compounds in high yields (63 –83% over 3 steps) with no need for \nchromatography. Key to this transformation is the phosphoramidate protecting group, \nwhich is stable to metalation steps.  \n\n2 \nKeywords \nArginine; Horner–Wadsworth–Emmons; Integrin; Phosphoramidate; \nTetrahydronaphthyridine \nIntroduction \nTetrahydronaphthyridines are prominent in peptidomimet ic pharmaceuticals as  \narginine mimetics; they are widely used in Arg-Gly-Asp (RGD) peptide mimetics such \nas αv integrin inhibitors  [1]. Tetrahydronaphthyridines represent less basic but more \npermeable alternatives to arginine (pKa ≈7 versus 13.8) [1], replicating the side-on salt-\nbridge binding interaction made between the guanidinium functionality of arginine and \nan aspartic acid residue in the protein . Consequently, this moiety has been used in \nvarious integrin inhibitors (Figure 1) [2-7]. \n \nFigure 1. The Arg -Gly-Asp tripeptide sequence and e xamples of tetrahydro-1,8-\nnaphthyridine-containing integrin inhibitors. \n \n\n3 \nCurrent routes to install tetrahydronaphthyridines predominantly revolve around late-\nstage hydrogenation of fully unsaturated 1,8 -naphthyridine derivatives 3, usually \nprepared via an acid  or base-catalysed Friedlander reaction between 2 -\naminonicotinaldehyde 1 and the corresponding ketone  2 (Scheme 1). Both reactions \nemploy harsh conditions with limited functional group tolerance and lack of \nregiochemical control, which present s considerable purification issues during large -\nscale synthesis [8-10].  \n \nScheme 1. Commonly used synthetic routes to tetrahydro-1,8-naphthyridine moieties \nby hydrogenation of saturated naphthyridines 3. \nRecently, GlaxoSmithKline disclosed a route to such a fluoropyrrolidine 6 using a Wittig \nreaction between phosphonium salt 4 and aldehyde 5 [2]. Synthesis of phosphonium \nsalt 4 (itself requiring 6 steps [including partial saturation of a 1,8 -naphthyridine \nmoiety)] and the formation of the triphenylphosphine oxide by-product in the Wittig step \npresented complications on both gram and kilogram scales. Herein, we report a novel \nsynthetic sequence to tetrahydro-1,8-naphthyridines using a Horner -Wadsworth-\nEmmons reaction using diphosphorylated compound 7, proceeding in high yields and \nhigh purities without the need for chromatographic purification (Scheme 2). \n\n4 \n \nScheme 2. Previous synthetic route to fluoropyrrolidine 6 utilising a Wittig reaction and \na novel, higher yielding route using a Horner–Wadsworth–Emmons reaction. \nResults and Discussion \nInvestigation initially began using commercially available N-Boc protected tetrahydro-\n1,8-naphthyridine 8; however, upon deprotonation and quenching with diethyl \nchlorophosphate, migration of the Boc group from the nitrogen atom to the exocyclic \nmethyl group was observed, affording phosphoramidate 9 in low yield with no formation \nof phosphonate 10 seen (Scheme 3). \n \nScheme 3. Synthesis of phosphoramidate 9 from tetrahydro -1,8-naphthyridine 8. \nConditions: sec-BuLi (3 eq.), diethyl chlorophosphate (1.1 eq.), THF, −42 °C, 14% \nyield. \n \n\n5 \nIt was proposed that deprotonation of 7 -methyl-1,2,3,4-tetrahydro-1,8-naphthyridine \n11 with two equivalents of sec-BuLi would afford phosphonate 12 upon quenching with \ndiethyl chlorophosphate via formation of the dianion. This  could then be used in a \nsubsequent Horner–Wadsworth–Emmons reaction to construct the carbon skeleton of \namine 6. Up on addition of a single equivalent of diethyl chlorophosphate, \nphosphoramidate 13 was obtained exclusively at both −42 and −78 °C. Addition of two \nequivalents of the chlorophosphate yielded diphosphorylated compound  7, albeit in \npoor yield (Scheme 4). \n \nScheme 4. Mono- and di -phosphorylation of tetrahydro -1,8-naphthyridine 11. \nConditions: (i) sec-BuLi (2 eq.), diethyl chlorophosphate (1 eq.), THF, −78 °C, 44% \nyield; (ii) sec-BuLi (2 eq.), diethyl chlorophosphate (2 eq.), THF, −78 °C, 27% yield. \nDeprotonation of phosphonate 7 and reaction with aldehyde 5 (formed in situ  by \noxidation of alcohol 14 using T3P®) [11] yielded olefin 15 in 93% yield as a 94:5 mix \nof stereoisomers (presumably E/Z, although this is not conclusive from the 1H NMR \nspectrum) Reduction to compound 16 using diimide, generated in situ, proceeded in \n80% yield and was followed by single-pot carbamate and phosphonate deprotection to \nafford arginine mimetic 6 in 86% yield. This represents a 64% overall yield which was \nincreased to 68% when no column chromatography was undertaken  between \ntransformations, with no loss of purity (Scheme 5).  \n\n6 \n \nScheme 5. Synthesis of amine 6 from phosphonate 7 and aldehyde 5. Conditions: (i) \nT3P® (50% w/w in DCM) (3 eq.), DMSO (3 eq.), DIPEA (2.5 eq.), DCM, 0 °C; (ii) KOtBu \n(6 eq.), THF, 0 °C, 93% yield (relative to phosphonate 7); (iii) PhSO 2NHNH2 (3 eq.), \nK2CO3 (4 eq.), DMF, 100 °C, 80% yield; (iv) 7.4 M HCl, 100 °C, 86% yield. \nOlefin reduction and Cbz deprotection could not be performed simultaneously by \npalladium-catalysed hydrogenation as this results in defluorination , presumably via a \nTsuji–Trost-like elimination of the allylic fluoride  [12-13]. This sequence represents a \nmarked improvement from the Wittig-including route, lowering the number of synthetic \nsteps and increasing overall yield [2]. Furthermore, no problematic by -products are \nformed, and good purity is obtained without the use of any chromatography, which is \nideal for large-scale processes. \nOptimisation of the Synthesis of Phosphoramidate 13 and Phosphonate 7 \nHaving been shown to be a feasible intermediate, attention turned to improving the \nsynthesis of bis-phosphonate 7 via a two-step process, exploiting the base stability of \nthe phosphoramidate protecting group. A variety of bases were trialed at 0 °C for the \ninitial N-phosphorylation, with 10 minutes allowed for complete deprotonation to occur \n(Table 1). \n \n\n7 \nTable 1. Bases surveyed for the formation of phosphoramidate 13a \n \n \nEntry Base \nAmount of  \nphosphoramidate 13 / \nLCMS a/a% \n1 KOtBu (1 M in THF) 4 \n2 LiHMDS (1 M in THF) 58 \n3 LDA (2 M in hexanes/benzene) 58 \n4 sBuLi (1.4 M in cyclohexane) 65 \n5 iPrMgCl (2 M in THF) 88 \naReactions performed on 0.7 mmol scale of compound 11 \nMinimal phosphorylation was observed when using potassium tert-butoxide (Entry 1); \nthis may be  due the disparity in p Ka between the base and the tetrahydro-1,8-\nnaphthyridine (based on 2 -aminopyridine, th e p Ka of the saturated ring nitrogen is \nexpected to be ≈28)  [14]. Similarly, nitrogen -centred bases (Entries 2,3) gave \nmoderate conversions to phosphoramidate 13 due to a close match of the p Ka of \ntetrahydronaphthyridine 11 and the pKaH of the base; sec-BuLi also gave reasonable \nconversion to (Entry 4) albeit with some impurities. This can likely be attributed to the \ntemperature instability of the base  and/or lithiated tetrahydronaphthyridine  in THF at \nthis temperature. Use of iPrMgCl, a strong but room temperature-stable base, gave \nthe greatest conversion to phosphoramidate 13. Further investigation into the use of \niPrMgCl found that quantitative conversion was achieved at ambient temperature with \na metalation time of <1 minute. Pleasingly, premixing of tetrahydronaphthyridine 11 \n\n8 \nwith diethyl chlorophosphate, followed by drop -wise addition of iPrMgCl gave clean \nand total conversion to phosphoramidate 13 as seen by LC-MS. When performed on \na multi -gram scale, a 94% yield of compound 13 was obtained. The re action also \nproceeded well (91% isolated yield) in 2-MeTHF, which offers a preferred alternative if \nperformed on larger scale  due to better partitioning with wa ter, stability and \nsustainability of production [15].  \nOf the bases trialled, only sec-BuLi was efficient in promoting C-phosphorylation. \nOptimisation of the use of this base was then investigated further (Table 2). \n \nTable 2. Optimisation of formation of phosphonate 7 using sec-BuLi as a basea \n \nEntry T / °C Eq. sec-BuLi \nLithiation time / \nmin \nAmount of \nphosphonate 7 / \nLCMS a/a% \n1 −78 1.5 90 53 \n2 −42 2.0 20 62 \n3 −42 2.5 20 81 \n4 −42 3.0 20 86 \n5 −42 3.5 20 67 \n6 −42 4.0 20 48 \n7 −42 3.0 (+ 3 eq. TMEDA) 20 59 \n8 0 2.0 10 0 \naReactions performed on 4.6 mmol scale (Entry 1) , 0.5 mmol scale (Entries 2 –6), 0.3 \nmmol (Entry 7) or 0.4 mmol (Entry 8). \n\n9 \n \nOptimal deprotonation and phosphorylation was found to occur when an excess (3 eq.) \nof base was used at −42 °C (Entry 4), with a lithiation time of 20 min.  Addition of \nTMEDA was detrimental to this conversion with a large proportion of starting material \n13 remaining (Entry 7). No product was formed at higher temperatures (Entry 8), likely \ndue to instability of the C-lithiated species at elevated temperatures  as degradation \nwas observed. It is believed that the excess base loading is required to account for \ndeprotonation of the more acidic phosphonate product 7 versus the starting material \n13 and potential lithium sequestration by chelation between an oxygen atom of the \nphosphonate and  the nitrogen atom of the unsaturated ring. Monitoring of \ndeprotonation followed by quenching with CD 3OD by 1H and 13C NMR spectroscopy \nindicated mono -labelled 17 as the major product, demonstrating  that lithiation only \noccurs at a single position of compound 13 (Scheme 6). \n \nScheme 6. Monodeuteration of 13 as observed by 1H and 13C NMR. Conditions: sec-\nBuLi (3 eq.), THF, −42 °C then CD3OD (14 eq.). \nWhen performed on a multi-gram scale, phosphonate 7 was synthesised in 68% yield \nafter purification by chromatography on silica. This, combined with the formation of \nphosphoramidate 13 in 94% yield, represents a marked improvement to the initial \nsimultaneous diphosphorylation (Scheme 7). When performed sequentially in a single-\npot, dip hosphorylated compound 7 was not observed, with phosphoramidate 13 \naccounting for the majority of product formed [16]. \n\n10 \n \nScheme 7. Sequential diphosphorylation of tetrahydronaphthyridine 11. Conditions: (i) \niPrMgCl (1.5 eq.), THF, then diethyl chlorophosphate (1.2 eq.), 94% yield; (ii) sec-BuLi \n(3 eq.), THF, −42 °C then (EtO)2P(O)Cl (1.1 eq.), 68% yield \nReaction Scope \nThe sequence of olefination, reduction and deprotection was tested on othe r arginine \nmimetics of varying amine structure, constituting potential Arg-Gly components of Arg-\nGly-Asp inhibitors (Table 3). Where the aldehyde was not commercially available, N-\nBoc-protected alcohols were oxidised using IBX in refluxing ethyl acetate an d used \ncrude. All amines were formed in high NMR and LC -MS purity without the need for \npurification by column chromatography. \n \nTable 3. Cores synthesised by the sequence of olefination, reduction and deprotection \nand the corresponding starting alcohols and aldehydes.a \n \nEntry Alcohol Aldehyde Product Yield \n1 \n \n  \n  \n82%1 \n\n11 \n2 \n \n  \n  \n83%1 \n3 - \n \n  \n79%2 \n4 - \n \n \n63%3 \n*Conditions: 1: IBX, EtOAc, reflux then KO tBu, THF, 0 °C then K 2CO3, PhSO 2NHNH2, DMF, 100 °C \nthen 7.4 M HCl, 100 °C; 2: KOtBu, THF, 0 °C then K2CO3, PhSO2NHNH2, DMF, 100 °C; 7.4 M HCl, 100 \n°C; 3: NaH, THF, 0 °C; then K2CO3, PhSO2NHNH2, DMF, 100 °C then 7.4 M HCl, 100 °C \n \nPleasingly, the sequence proceeded in high yields (63–83%) for all substrates with no \nchromatography required. Despite relatively high yield s, significant racemisation was \nseen in the synthesis of  piperidine 20 and pyrrolidine cores 23. Pyrrolidine 23 was \nobtained with an e.e. of only 24%, representing a serious loss of enantiopurity. It is \nbelieved that racemisation occurs during the olefination step, caused by base mediated \nketo-enol tautomerisation. As such, this sequence, as currently performed, is suitable \nfor substrates lacking an acidic α-proton (fluoropyrrolidine core 6) or achiral aldehydes \n(azetidine cores 25 and 27). \n \nFurthermore, key to success of the Horner–Wadsworth–Emmons olefination is \npremixing of the aldehyde and phosphonate 7 prior to addition of  KOtBu. Upon \ndeprotonation, phosphonate 7 (in the absence of aldehyde) undergoes dimerisation to \nolefin 28. While the exact mechanism is not known, it is likely to involve a reacti ve \ncarbene intermediate formed by α -elimination of the phosphonate as described in \nprevious reports (Scheme 8) [17]. \n\n12 \n \nScheme 8.  Possible mechanism for the formation of dimer 28 via a reactive carbene \nintermediate. \nIn order to circumvent racemisation of aldehyde 22 during the Horner–Wadsworth–\nEmmons olefination, alkylation of phosphoramidate 13 was explored using \ncommercially available iodide 29. Formation of compound 30 proceeded in 21% yield, \nwith alcohol 31 and dimer 32 also formed in 20% and 5% yield respectively ( Scheme \n9). Indeed, when iodide 29 was replaced with bromide 33 and tosylate 34 no formation \nof compound 30 was observed, with alcohol 31 and dimer 32 accounting for the major \nproducts. Acidic deprotection of phosphoramidate 30 afforded amine (R)-23 in 92% \nyield in 99% e.e., offering an alternative route to the Horner –Wadsworth–Emmons-\nbased approach. \n \nScheme 9. Alkylation of phosphoramidate 13 by iodide 29 to afford compound 30 and \nby-products alcohol 31 and dimer 32. Use of bromide 33 or tosylate 34 afforded only \ncompounds 31 and 32. Conditions: (i) sec-BuLi (1.3 eq.), iodide 29, THF, −78 °C; (ii) \n7.4 M HCl, 100 °C, 92% yield. \n \n\n13 \n \nThe mechanism of the formation of alcohol 31 and dimer 32 was not fully explored; \nhowever, when iodide 29 was replaced by  a superior oxidant in  1,2-dibromoethane, \nformation of dimer 32 increased (17% isolated yield). This supports previously reported \nproposals that oxidative coupling of the anion can take place, involving a radical \npathway [18-20]. \nConclusion  \nIn conclusion, a novel method for the assembly of 7-alkyl-1,2,3,4-tetrahydro-[1,8]-\nnaphthyridine-based arginine mimetics has been developed. Synthesis of \nphosphonate 7 has been optimised, with a sequential diphosphorylation process using \ncommercially available starting materials affording the desired comp ound in 64% \noverall yield. A Horner –Wadsworth–Emmons/reduction/deprotection procedure has \nbeen used to synthesise amines in good yield requiring no chromatography. This \nmethodology utilised the underused base -stable phosphoramidate protecting group , \nwhich was superior to the more commonly applied Boc protecting group which was \nunstable to the lithiation. This synthetic route replaces traditional Wittig and tandem \nalkylation/reduction methodologies , which suffer from complications arising from \ntroublesome by-products and reaction selectivity; the new procedure  proceeds in a \nhigher yield than previously obtained, providing benefits in large-scale manufacture of \nintegrin inhibitors and other arginine peptidomimetics. \nSupporting Information  \nSupporting Information File 1 \n\n14 \nDetailed experimental procedures, and product characterisation data, along with 1H \nand 13C NMR spectra.  \nAcknowledgements \nThanks to Sean M. Lynn for assistance with NMR spectroscopy analysis and the GSK \nUK Discovery Analytical team for acquiring HRMS data \nFunding \nFinancial support for this work was provided by GSK via the GSK/University of \nStrathclyde Centre for Doctoral Training in Synthetic and Medicinal Chemistry. We \nthank EPSRC for further funding via Prosperity Partnership EP/S035990/1. \nReferences \n1. Hatley, R., J. D.; Macdonald, S. J. F.; Slack, R. J.; Le, J.; Ludbrook, S. B.; Lukey, P. \nT.; Angew. Chem. Int. Ed., 2018, 57, 3298–3321. \n2. Anderson, N. A.; Campbell -Crawford, M. H. J.; Hancock, A. P.; Pritchard, J. M.; \nRedmond, J. M.; Patent WO2016046226 (31 Mar 2016). \n3. Procopiou, P. A.; Barrett, T. N.; Copley, R. C. B.; Tame, C.  \nJ.; Tetrahedron Asymmetry, 2017, 28, 1384–1393. \n4. Procopiou, P. A.; Anderson, N. A.; Barrett, J.; Barrett, T. N.; Crawford, M. H. J.; \nFallon, B. J.; Hancock, A. P.; Le, J.; Lemma, S.; Marshall R. P.; Morrell, J.; Pritchard, \nJ. M.; Rowedder, J. 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LC-MS shows peaks consistent with phosphoramidate 13 (m/z = 285 [M+H]+) and \na triphosphorylated species (m/z = 557 [M+H]+). \n\n16 \n17. Subramanyam, V.; Silver, E. H.; Soloway, A. H.; J. Org. Chem., 1976, 41, 1272–\n1273. \n18. Lehn, J.-M.; Ziessel, R.; Helv. Chim. Acta, 1988, 71, 1511–1516. \n19. Zorin, A. V.; Zaynashev, A. T.; Chanysheva, A. R.; Zorin, V. V.; Russ. J. Gen. \nChem., 2015, 85, 1382–1385. \n20. Barham, J. P.; Coulthard, G.; Kane, R. G.; Delgado, N.; John, M. P.; Murphy, J. A. \nAngew. Chem. Int. Ed., 2016, 55, 4492-4496.","source_license":"CC-BY-4.0","license_restricted":false}