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Synlett 2013; 24(3): 343-346
DOI: 10.1055/s-0032-1318117
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© Georg Thieme Verlag Stuttgart · New York

Stereocontrolled Synthesis of 5′- and 6′-Epimeric Analogues of Muraymycin Nucleoside Antibiotics

Anatol P. Spork
University of Paderborn, Department of Chemistry, Warburger Str. 100, 33 098 Paderborn, Germany   Fax: +49(5251)603245   Email: christian.ducho@uni-paderborn.de
,
Christian Ducho*
University of Paderborn, Department of Chemistry, Warburger Str. 100, 33 098 Paderborn, Germany   Fax: +49(5251)603245   Email: christian.ducho@uni-paderborn.de
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Publication History

Received: 30 October 2012

Accepted after revision: 07 January 2013

Publication Date:
23 January 2013 (online)

 
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Abstract

Naturally occurring nucleoside antibiotics, for example, Streptomyces-produced muraymycins, represent a promising class of potential lead structures for the development of novel antimicrobial agents. The efficient preparation of muraymycin analogues is an essential prerequisite for detailed structure–activity relationship (SAR) studies, particularly with respect to the variation of the stereochemistry in a controlled manner. In this work, stereoselective syntheses of 5′- as well as 6′-epimers of muraymycins are reported. The obtained target structures also represent useful probes for the elucidation of the biosynthesis of muraymycins and related nucleoside antibiotics.


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Key words

antibiotics - medicinal chemistry - natural products - ­nucleosides - biosynthesis

Bacterial strains displaying resistances towards established antibiotics continue to emerge. They are expected to represent a significant threat to human health within upcoming decades.[ 1 ] Consequently, there is increasing interest in the development of novel antimicrobial agents, which should ideally employ new or yet unexploited modes of action.[ 2 ] The bacterial membrane protein MraY, a key enzyme in the intracellular section of bacterial peptidoglycan biosynthesis, is anticipated to be a valuable candidate target for novel antibacterial drugs.[3] [4]

MraY is inhibited by a class of natural products referred to as ‘nucleoside antibiotics’ with respect to their rather unusual nucleoside-derived core structures.[ 5 ] The muraymycins [e.g., muraymycin A1 (1), Figure [1]] represent a collection of 19 nucleoside antibiotics isolated from a Streptomyces sp.[ 6 ] Their nucleosyl amino acid core appears to be structurally related to other Streptomyces ­produced subclasses of nucleoside antibiotics, among them the caprazamycins[ 7 ] and the A-90289 family of compounds [e.g., A-90289 A (2), Figure [1]].[ 8 ] Some structure–activity relationship (SAR) studies of muraymycin analogues have already been reported.[ 9 ] Remarkably, the protected truncated 5′-epi-muraymycin analogues 3 and 4 (Figure [1]) showed pronounced antibacterial activity, with the epimeric 5′R configuration being essential for their biological potency.[ 9b ]

For the synthesis of the natural-product-like nucleoside core structure with 5′S,6′S configuration, three different strategies have been reported: (i) an aldol-type reaction of a glycine derivative with protected uridine-5′-aldehyde;[ 9b ] (ii) Sharpless aminohydroxylation of a uridine-derived α,β-unsaturated ester;[ 10 ] (iii) reaction of a sulfur ylide with protected uridine-5′-aldehyde furnishing a uridine-­derived trans-epoxide, followed by double inversion at C-6′.[11] [12d] The latter route was developed following some initial confusion regarding the stereochemistry of the ­epoxide key intermediate, which had led to the sulfur ylide approach being incorrectly reported as a method to ­prepare 5′-epi-muraymycins first.[11] [12]

The attempted synthesis of 5′- and 6′-epimers of the muraymycin nucleoside moiety is based on two rationales: (i) as exemplified by bioactive 5′-epi-muraymycins,[ 9b ] a variation of the stereochemistry will be essential within thorough SAR investigations; (ii) the obtained 5′- and 6′-epimers represent useful probes for studies on the biosynthesis of muraymycins and related nucleoside antibiotics. The gene clusters for the biosynthetic assembly of muraymycins and caprazamycins have been identified, which has led to initial proposals for biosynthetic pathways.[13] [14] However, the biosynthesis of the identical nucleoside core has already been elucidated in more detail for the related A-90289 antibiotics.[ 8,15 ] In case of the A-90289s, uridine-5′-monophosphate (uridylate, UMP) is 5′-hydroxylated by the non-heme 2-oxoglutarate (2-OG) dependent Fe(II)-oxygenase LipL, furnishing uridine-5′-aldehyde 5 (reaction not displayed).[ 15a ] The subsequent aldol-type reaction with an amino acid derived enolate component is mediated by LipK and provides the 5′S,6′S configured nucleoside unit 6 (Scheme [1]).[ 15b ] Epimeric analogues of 6 can serve as reference compounds to support the stereochemical assignment of the LipK reaction product. In principle, it could not be entirely ruled out that the LipK-mediated reaction would not provide stereoisomer 6, but rather an epimer of it, and that epimerization then would occur at a later stage.[ 16 ]

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Figure 1 Naturally occurring nucleoside antibiotics muraymycin A1 (1) and A-90289 A (2) as well as synthetic truncated 5′-epi-muraymycin analogues 3 and 4 displaying antibacterial activity;[ 9b ] green: shared nucleoside core structure of 1 and 2
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Scheme 1

While different routes for the preparation of the muraymycin nucleoside core are available (vide supra), efficient methods to synthesize either 5′- or 6′-epimeric analogues are limited. Bioactive 5′-epi-muraymycins were initially obtained by the aforementioned aldol-type reaction of a glycine derivative with protected uridine-5′-aldehyde, but a lack of stereocontrol resulted in the tedious separation of diastereomers.[ 9b ] With respect to the stereochemical revision of the sulfur ylide approach (vide supra), this method is not available to synthesize 5′-epi-muraymycins. In contrast, we have already identified the sulfur ylide route to be apparently useful to efficiently obtain 6′-epi-muraymycins,[12d] [17] but a systematic study has not been provided yet. Herein, we report a thorough investigation on the stereocontrolled access to both 5′- and 6′-epimers of muraymycin nucleoside antibiotics.

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Scheme 2

Our synthetic strategy is exemplified by the preparation of the 6′-epimer 7 and the 5′-epimer 8 of nucleosyl amino acid 6, with 7 and 8 representing useful biosynthetic probes (vide supra). It was envisaged to employ suitably configured uridine-derived trans-epoxides as key intermediates for both target structures. In the case of 7, we have used our previously described method to react protected uridine-5′-aldehydes 9 and 10 [12b] [c] [d] , [ 18 ] with a sulfur ylide derived from sulfonium ester 11, diastereoselectively furnishing epoxy esters 12 and 13 in 63% and 79% yield, respectively (dr > 95:5, Scheme [2]).[12c] [d] Epoxide 12 was then treated with tetra(n-butyl)ammonium azide to give azido alcohol 14 [ 19 ] in 94% yield and with excellent regio- and stereocontrol (dr > 98:2). Following acidic cleavage of the tert-butyl ester and the silyl ethers as well as HPLC purification, azide reduction by transfer hydrogenation with 1,4-cyclohexadiene 15 (vide infra) provided target compound 7 in 89% yield over two steps.

For the synthesis of 8, we have been geared to Sarabia’s recently reported stereoselective synthesis of uridine-derived epoxy alcohol 16 from 9, which is based on asymmetric Sharpless epoxidation (dr > 95:5).[ 12e ] After oxidation of 16 using TEMPO and BAIB,[ 12e ] the thus-obtained carboxylic acid was treated with trichloroacetimidate 17, furnishing epoxy tert-butyl ester 18 in 68% yield over two steps (Scheme [2]). PMB protection under rather mild conditions in a biphasic system gave 19 in 81% yield demonstrating the principle feasibility to selectively modify the uracil nucleobase by alkylation at this stage. The similar conversion of 12 into 13 had already been described before.[ 12d ] Along the lines of the synthesis of 14, treatment of 18 with tetra(n-butyl)ammonium azide gave azido alcohol 20 [ 19 ] in 91% yield and with excellent regio- and stereocontrol (dr > 98:2). The aforementioned sequence of acidic deprotection, HPLC purification, and azide reduction then provided target compound 8 in 89% yield over two steps.

Following the preparation of 7 and 8, it was envisaged to employ the described route for the synthesis of further 5′- and 6′-epi-analogues of truncated muraymycin derivatives (Scheme [3]). The oxirane ring-opening reaction of uridine-derived trans-epoxides is also feasible with primary amines showing equally excellent regio- and stereoselectivities. This had been demonstrated before by the conversion of 5′R,6′S epoxide 12 with amine 21 [ 12c ] furnishing 6′-epimer 22 [ 19 ] in 52% yield.[ 12d ] Exemplified by the synthesis of the fully deprotected 6′-epi-analogue 23, the orthogonality of the protecting-group scheme was surveyed in general, being of particular importance with respect to further selective modifications of derivatives for future SAR studies. Consequently, both silyl ethers were cleaved first using TBAF. After the removal of the Cbz group under transfer hydrogenation conditions with 15 (vide infra), acidic cleavage of the tert-butyl ester and HPLC purification furnished the target compound 23 as the bis-TFA salt in 57% yield over three steps and 30% overall yield from trans-epoxide 12 over four steps.

The stereochemical configuration of the 6′-epimer 22 had been proven before by X-ray crystal structure analysis. This led to an unambiguous assignment of the 5′R,6′S configuration for both sulfur ylide derived trans-epoxides 12 and 13 (12 can be readily converted into 13, vide supra).[12d] [19] Thus, the diastereomeric trans-epoxide 18 as well as its PMB-protected congener 19 (vide supra) could be identified as 5′S,6′R isomers.[ 17 ] Consequently, SN2-type epoxide opening of 18 and 19 at the 6′-position with N-nucleophiles furnished the 5′-epimers as proposed.

The transformations of epoxides 12 and 13 as well as 18 and 19 with amine 21 [ 12c ] proceeded with excellent regio- and stereocontrol (dr > 98:2, Scheme [3]). However, in the case of 18, an unwanted partial conversion of the tert-butyl ester into the methyl ester was observed. This limitation could be overcome by replacing MeOH with i-PrOH as solvent. Hydrogenolytic Cbz deprotection furnished 6′- as well as 5′-epimeric target compounds 24, 25 and 3, 4 in 39–61% yield over two steps. Transfer hydrogenation conditions using 15 as hydrogen source proved to be advantageous over conventional hydrogenation to prevent the undesired C5–C6 reduction of the uracil nucleobase.[18b] [20] Finally, the fully deprotected 5′-epi-congener 26 was obtained by acidic cleavage of the tert-butyl ester and the silyl ethers of 3. Thus, target compound 26 was isolated in 38% yield from trans-epoxide 18 over three steps after final HPLC purification.

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Scheme 3

In conclusion, we report the first systematic and thorough study on stereocontrolled and concise syntheses of 5′- as well as 6′-epimeric analogues of the core structure of several nucleoside antibiotics including muraymycins. Our strategy was based on oxirane ring opening of trans-epoxide key intermediates, proceeding with excellent regio- and stereocontrol. The practical capability of this approach was demonstrated by the first stereocontrolled synthesis of biologically active[ 9b ] truncated muraymycin 5′-epi-analogues 3, 4 and their 6′-epimeric congeners 24, 25 as well as unprecedented fully deprotected derivatives 23 and 26. The prepared target structures as well as potential further analogues will be investigated for their inhibitory properties towards the target protein MraY in the course of detailed SAR studies. Furthermore, the prepared epimers 7 and 8 have recently been employed as useful reference compounds in a study on the LipK-mediated assembly of biosynthetic intermediate 6.[ 21 ]


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Acknowledgment

The authors thank Professor S. Van Lanen, University of Kentucky, for helpful discussions. Financial support by the Deutsche Forschungsgemeinschaft (DFG, SFB 803 ‘Functionality controlled by organization in and between membranes’) and the Fonds der Chemischen Industrie (FCI, Sachkostenzuschuss) is gratefully acknowledged.

Supporting Information

    for this article is available online at http://www.thieme-connect.com.accesdistant.sorbonne-universite.fr/ejournals/toc/synlett.
  • Supporting Information


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Zoom Image
Figure 1 Naturally occurring nucleoside antibiotics muraymycin A1 (1) and A-90289 A (2) as well as synthetic truncated 5′-epi-muraymycin analogues 3 and 4 displaying antibacterial activity;[ 9b ] green: shared nucleoside core structure of 1 and 2
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Scheme 1
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Scheme 2
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Scheme 3