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Synthesis of the (E)-dehydrobutyrine–thiazoline–proline–leucine fragment of vioprolides B and D
Tetrahedron Letters 54 (2013) 3150–3153
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Synthesis of the (E)-dehydrobutyrine–thiazoline–proline–leucine fragment of vioprolides B and D Hao Liu, Eric J. Thomas ⇑ School of Chemistry, University of Manchester, Manchester M13 9PL, UK
a r t i c l e
i n f o
Article history: Received 27 February 2013 Revised 4 April 2013 Accepted 5 April 2013 Available online 13 April 2013 Keywords: Peptolides [E]-dehydrobutyrine Thiazoline Vioprolides
a b s t r a c t A procedure is reported for the introduction of both the thiazoline and (E)-dehydrobutyrine components into a tetrapeptide-derived fragment of vioprolides B and D. The (E)-dehydrobutyrine is introduced first but, as the carbon–carbon double-bond of the dehydrobutyrine appeared incompatible with an adjacent thiol, the thiazoline was assembled by dehydration of a serine containing thioamide not by dehydration of a cysteinyl analogue. Ó 2013 Elsevier Ltd. All rights reserved.
The vioprolides A–D 1–4 are a small group of peptolides isolated from Cystobacter violaceus.1 They possess antifungal activity and are highly cytotoxic to mammalian cells with vioprolide D showing the least cytotoxicity and maximum antifungal activity. Of particular interest is the observation that they have a synergistic effect on the murine type 1 inferferon signalling pathway leading to more than a three-fold increase in activity when stimulated by viral infection.2 However, further studies into their biological activity are being delayed because of the extremely limited supply of these natural products.
⇑ Corresponding author. Tel.: +44 (0) 161 275 4613; fax: +44 (0) 161 275 4939. E-mail address: [email protected] (E.J. Thomas). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.04.017
Structural features of interest in the vioprolides are the 4methylazetidinecarboxylic acid (MAZ), the (E)-dehydrobutyrine and the thiazoline. This was shown during the chemical degradation of the natural products to facilitate epimerisation of the neighbouring MAZ or proline-derived fragment. Two conformers were detected for vioprolide A 1 attributed to atropisomers about the N-methylvaline–threonine amide bond.1 To date, the only studies on synthetic approaches to the vioprolides are a non-stereoselective synthesis of MAZ reported during their isolation1 and a synthesis of the azetidinyl-thiazoline component of vioprolides A 1 and C 2.3 We here report studies concerned with the introduction of both the thiazoline and (E)-dehydrobutyrine components into a fragment of the vioprolides. Procedures are available for the dehydration of threonine containing peptides to give either (Z)- or (E)-dehydrobutyrine containing fragments4,5 and cysteine-derived thiazolines can be accessed by dehydration of thioamides adjacent to a serine residue6 or of cysteine containing peptides.7 The latter approach was selected for the initial studies since several, fairly recently developed, procedures are available for this transformation. S-Trityl-N-Fmoc-protected L-cysteine 58 was converted to its allyl ester 6 and Fmoc-removal gave the amine 7. This was coupled with N-nosyl protected L-proline9 to give the protected dipeptide 8, see Scheme 1. Nosyl protection for the proline was used as the 1 H NMR spectra of the corresponding Boc-protected dipeptides were complicated by the presence of rotamers. Triflic anhydride and triphenylphosphine oxide7a effected thiazoline formation from the dipeptide 8 and gave a good yield of the thiazoline 9 as a single diastereoisomer (1H NMR). However, during attempts to continue with the synthesis by conversion of the allyl ester 9 into the corre-
H. Liu, E. J. Thomas / Tetrahedron Letters 54 (2013) 3150–3153
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Scheme 1. Initial studies on thiazoline formation; Reagents and conditions: (i) CH2=CHCH2Br, NaHCO3, DMSO, rt, 16 h (83%); (ii) piperidine, DMF, r.t., 16 h (94%); (iii) Nnosyl-L-proline, HATU, HOBt, DMF, add 7 then EtNiPr2, rt, 16 h (77%); (iv) Tf2O, Ph3PO, DCM, 0 °C, 10 min, add 8, rt, 2 h (68%); (v) Pd(Ph3P)4 (10%), N-methylaniline, THF, rt, 3 h (95%).
sponding acid, epimerisation of the proline occurred and a mixture of the epimeric acids 10 was obtained, see Scheme 1. Although the epimerisation of amino acid residues adjacent to thiazolines in peptide derivatives is well known10 and indeed was observed during attempts to degrade the vioprolides into their constituent amino acids,1 it was not reported during the synthesis of the azetidinyl-thiazoline fragment of the vioprolides.3 In the case of the thiazoline 10 the rapid epimerisation may be due to hydrogen bonding between the nitrogen of the thiazoline and the carboxylic acid, see structure 11. This could facilitate imine–enamine tautomerisation thus leading to epimerisation. Next, procedures for the introduction of the (E)-dehydrobutyrine residue were investigated. Deallylation of the protected dipeptide 8 and coupling the resulting acid 12 with the allyl ester of Otert-butyldimethylsilyl protected L-threonine gave the protected tripeptide 13. This was desilylated to give the alcohol 14 ready for the dehydration to the corresponding alkene. (E)-Dehydrobutyrines are available by dehydration of threonine containing peptides using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and copper(II) chloride5 and indeed this procedure gave a good yield of the required dehydrobutyrine-containing peptide 15, the geometry of the double-bond being assigned at this stage by analogy with the literature.5 However, attempts to effect a subsequent dehydration to form the required thiazoline using the triflic anhydride and triphenylphosphine oxide procedure7 were unsuccessful, complex mixtures of products being obtained, see Scheme 2. The problems encountered during the attempted formation of the thiazoline 16 from the thio-ether 15 were attributed to com-
peting reactions between the thiol formed on deprotection of the trityl thio-ether 15 and the neighbouring double-bond of the dehydrobutyrine. This suggested that, although the thiazoline will have to be introduced as late as possible in the synthesis to minimise epimerisation of the neighbouring amino acid residue, free-thiol containing intermediates would have to be avoided to prevent complications due to the presence of the neighbouring dehydrobutyrine. The alternative procedure for the introduction of cysteinylderived thiazolines into peptides based on the dehydration of serine-derived thioamides was therefore investigated.6 Only low yields of the O-TBS-L-threonine-O-TIPS-L-serine dipeptide 19 were obtained in our hands if O-TIPS-protected serine was coupled with the allyl ester of O-TBS-protected threonine, and so the protected L-threonine was coupled with the Fmoc-protected Lserine 17 with a free hydroxyl group, to give the protected dipeptide 18 that was silylated to give the bis-silylated dipeptide 19, see Scheme 3. Following removal of the Fmoc-group, coupling the resulting amine 20 with the Fmoc-protected thioacylating agent 2111 gave a thioamide that was converted into the corresponding amine 22 by treatment with piperidine under standard conditions. This was now coupled with Boc-protected D-leucine to give the modified tetrapeptide 23. Selective removal of the secondary TBS-group in the presence of the primary TIPS-group proved difficult and so both of the silyl protecting groups were removed using tetrabutylammonium fluoride, and the primary serine-derived alcohol was resilylated to give the monosilyl ether 24. Dehydration of this using EDC and copper(II) chloride under conditions known to be selective for formation of (E)-dehydrobutyrines5 gave a product identified as
Scheme 2. Initial studies on thiazoline formation in the presence of a dehydrobutyrine; Reagents and conditions: (i) Pd(Ph3P)4 (10%), N-methylaniline, THF, rt, 3 h (95%); (ii) 12, HATU, HOBt, DMF, add O-TBS-L-threonine allyl ester, EtNiPr2, rt, 16 h (67%); (iii) Et3N(HF)3, THF, rt, 48 h (62%); (iv) EDC, CuCl2, tol., 80 °C, 30 min (62%); (v) Tf2O, Ph3PO, DCM, 0 °C, 10 min, add 15, rt.
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H. Liu, E. J. Thomas / Tetrahedron Letters 54 (2013) 3150–3153
Scheme 3. Synthesis of the dehydrobutyrine–thiazoline–proline–leucine fragment of vioprolides B and D. Reagents and conditions: (i) HATU, HOBt, DMF, add O-TBSthreonine allyl ester, EtNiPr2, rt, 16 h (97%); (ii) TIPSOTf, 2,6-lutidine, DCM, rt, 16 h (56%); (iii) piperidine, DMF, rt, 16 h (89%); (iv) (a) compound 21, THF, rt, 6 h (b) piperidine, DMF, rt, 16 h (40%); (v) Boc-D-leucine, HATU, HOBt, DMF, add 22, EtNiPr2, rt, 16 h (85%); (vi) (a) TBAF, THF, rt, 16 h (b) TIPSCI, imid., THF (30%); (vii) EDC, CuCL2,tol., 80 °C, 30 min (65%); (viii) TBAF, THF, rt, 16 h (62%); (ix) DAST, DCM, 15 °C, 1 h (75%); (x) DAST, pyridine, DCM, 0 °C, 2 h (31%).
the required (E)-isomer 25 on the basis of precedent. To check that the (E)- and (Z)-isomers could be distinguished, the alcohol 24 was also dehydrated using diethylaminosulfur trifluoride (DAST) under conditions known to be selective for formation of (Z)-dehydrobutyrines.4 Although only a modest yield of alkene was obtained in the latter case, the alkene that was isolated was different by 1H NMR from that obtained using EDC-copper(II) chloride and was identified as the (Z)-isomer 28 [25, =CHCH3, dH (CDCl3) 1.98; 28, =CHCH3, dH (CDCl3) 1.72], see Scheme 3. Indeed following comparison of the 1 H NMR spectra of the two isomers, a small amount of the (Z)-isomer 28 was detected in the (E)-isomer 25; (E)-25: (Z)-28 = 95:5. It now remained to check that the formation of a thiazoline by dehydration of the serine-thioamide was compatible with the (E)dehydrobutyrine component. Deprotection of the tri-isopropylsilyl ether 25 using tetrabutylammonium fluoride gave the corresponding alcohol 26 and this on treatment with DAST6 was dehydrated to give a major product isolated in 75% yield. This was identified as the required thiazoline 27 containing ca. 20% of its epimer due to partial racemisation of the proline-derived residue as indicated in the 1H NMR spectrum of 27 by some doubling of peaks, for example of the doublets attributed to the diastereotopic methyl groups present in the leucine. This work has developed procedures for the assembly of both an (E)-dehydrobutyrine and an adjacent thiazoline into peptides related to fragments of the vioprolides.12 Because of its tendency to promote epimerisation of an adjacent amino acid residue,10 the thiazoline should be introduced as late as possible in any synthetic approach to a vioprolide. However, the compatibility of the introduction of the thiazoline from a thioamide using DAST with the (E)-dehydrobutyrine suggests that this procedure may be appropriate for the synthesis of a natural product. Some optimisation may be necessary to minimise epimerisation at the proline- or MAZ-derived stereogenic centre, although a depsipeptide with a proline derived residue next to the thiazoline has been prepared using this procedure without any racemisation being observed.6
Acknowledgments We thank Novartis for a studentship (to H.L.). References and notes 1. Schummer, D.; Forche, E.; Wray, V.; Domke, T.; Reichenbach, H.; Höfle, G. Leibigs Ann. 1996, 971. 2. Bollati-Fogolín, M.; Oggero, M.; Mirazo, S.; Frank, R.; Kratje, R.; Muller, W. Cells and Culture, ESACT Proceedings 4, 485; Springer, 2010, doi: 10.1007/978-90481-3419-9_86. 3. Chopin, N.; Couty, F.; Evano, G. Lett. Org. Chem. 2010, 7, 353. 4. (a) Ogura, H.; Sato, O.; Takeda, K. Tetrahedron Lett. 1981, 22, 4817; (b) Andruszkiewicz, R.; Czerwínski, A. Synthesis 1982, 968; (c) Berti, F.; Ebert, C.; Gardossi, L. Tetrahedron Lett. 1992, 33, 8145; (d) Goodall, K.; Parsons, A. F. Tetrahedron Lett. 1995, 36, 3259; (e) Yokokawa, T.; Shioiri, T. Tetrahedron Lett. 2002, 43, 8673; (f) Wojciechowska, H.; Pawlowitcz, R.; Andruszkiewicz, R.; Grzybowska, J. Tetrahedron Lett. 1978, 19, 4063; (g) Miller, M. J. J. Org. Chem. 1980, 45, 3131; (h) Srinivasan, A.; Stephenson, R. W.; Olsen, R. K. J. Org. Chem. 1977, 42, 2256; (i) Yokokawa, F.; Shioiri, T. Tetrahedron Lett. 2002, 43, 8679. 5. Sai, H.; Ogiku, T.; Ohmizu, H. Synthesis 2003, 201. 6. McKeever, B.; Pattenden, G. Tetrahedron 2003, 59, 2701. 7. (a) You, S.-L.; Razavi, H.; Kelly, J. W. Angew. Chem., Int. Ed. 2003, 42, 83; (b) Ma, D.; Zou, B.; Cai, G.; Hu, X.; Lin, J. O. Chem. Eur. J. 2006, 12, 7615. 8. Commercially available from Sigma–Alrich. 9. Prepared by reaction of L-proline with 2-nitrobenzenesulfonyl chloride, NaHCO3, H2O, rt, 16 h (85%). 10. (a) North, M.; Pattenden, G. Tetrahedron 1990, 45, 8267; (b) Wipf, P.; Fritch, P. C. Tetrahedron Lett. 1994, 35, 5397. 11. Prepared from Fmoc-protected proline i (see Ref. 10).
Prep. of thioacylating agent 21: (i), 2-amino-4-nitroaniline, iBuOCOCl, THF, 20 °C, add i, rt, 16 h (72%); (ii), P4S10, NaHCO3, THF, 1 h, add ii, 0 °C, rt, 3 h (89%); (iii), NaNO2, AcOH, 5% H2O, 0 °C, 30 min (99%). 12. (E)-Dehydrobutyrine 25: A solution of the alcohol 24 (40 mg, 0.05 mmol), EDC (17 mg, 0.11 mmol) and copper(II) chloride (1 mg, 0.01 mmol) in anhydrous toluene (2 mL) was stirred at 80 °C under an atmosphere of nitrogen for
H. Liu, E. J. Thomas / Tetrahedron Letters 54 (2013) 3150–3153 30 min. Water was added and the mixture was extracted with ethyl acetate (5 mL). The organic extracts were dried (Na2SO4) and concentrated under reduced pressure. Chromatography of the residue eluting with ether gave the title compound 25 (23 mg, 65%) as a colourless gum, Rf 0.6 (light petroleum: ether = 30: 70), ½a20 37.5 (c 2 in CHCl3) (Found: M++H, 711.4189. D C35H63N4O7SiS requires M, 711.4182); mmax/cm1 3294, 1682, 1645, 1507, 1447, 1386, 1366, 1251, 1163, 1106, 1066, 1046, 1014, 994, 920, 881, 851, 778, 735 and 681 cm1; dH (400 MHz, CDCl3) 0.89 and 0.91 (each 3H, m, 400 0 -CH3 and 5000 -H3), 0.99 [21H, m, 3 SiCH(CH3)2], 1.33 [9H, s, C(CH3)3], 1.36 (2H, m, 3000 H2), 1.64 (1H, m, 4000 -H), 1.92 (1H, m, 400 -H), 1.98 (3H, d, J 7.6, 4-H3), 2.04, 2.22 and 2.34 (each 1H, m, 2 300 -H or 400 -H0 ), 3.51 (2H, m, 500 -H2), 3.81 and 3.96 (each 1H, m), 4.21 (1H, dd, J 8.0, 12.0, 30 -H), 4.34 (1H, m, 30 -H0 ), 4.63 (2H, m, OCH2CH=), 4.85 (1H, m), 5.05 (1H, m, NH), 5.18 and 5.28 (each 1H, m, HCH=CH), 5.90 (1H, m, CH2=CH), 6.68 (1H, m, 3-H) and 8.13 and 8.62 (each 1H, br m, NH); dC (125 MHz, CDCl3) 11.79, 14.17, 17.91, 21.93, 23.42, 24.36, 24.64, 28.32, 30.33, 32.37, 41.37, 47.71, 50.85, 62.26, 65.89, 79.99, 82.65, 103.73, 118.55, 126.11, 129.12, 131.96, 148.99, 155.72, 163.59 and 203.33; m/z (ES+) 733 (M++23, 100%). Thiazoline 27: Diethylaminosulfur trifluoride (10 mg, 0.06 mmol) was added dropwise over 1 min to the thioamide 26 (11 mg, 0.02 mmol) in DCM (0.25 mL) at 15 °C and the solution stirred for 1 h. Saturated aqueous sodium hydrogen carbonate was added and the mixture allowed to warm to room temperature. DCM (5 mL) and water (5 mL) were added and the mixture extracted with DCM (10 mL). The organic extracts were dried (Na2SO4) and concentrated under reduced pressure. Chromatography of the residue eluting with ether gave the title compound 27 (8 mg, 75%) as a colourless gum, containing ca. 20% of its epimer at the proline derived centre (1H NMR), Rf 0.5 (ether), ½a20 D 15.6 (c 2 in CHCl3) (Found: M+ H, 535.2596. C26H39N4O6S requires M, 535.2595); mmax/
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cm1 3321, 1702, 1682, 1643, 1510, 1425, 1366, 1343, 1250, 1162, 1015, 988, 932, 854 and 780 cm1; dH (400 MHz, CDCl3) major epimer 0.87 and 0.91 (each 3H, d, J 6.6, 400 0 -CH3 and 500 0 -H3), 1.35 [9H, s, C(CH3)3], 1.41 (2H, m, 300 0 -H2), 1.64 (1H, m, 400 0 -H), 1.85-2.00 (2H, m, 400 -H2), 2.03 (3H, d, J 7.6, 4-H3), 2.05–2.15 (2H, m, 300 -H2), 3.40-3.60 (2H, m, 30 -H2), 3.88 (2H, m, 500 -H2), 4.44 (1H, dt, J 4.8, 9.6, 200 0 -H), 4.67 (2H, m, OCH2CH=), 4.84 (1H, dd, J 4.0, 9.0, 20 -H), 5.03 (2H, m, 200 -H and NH), 5.20 and 5.30 (each 1H, m, HCH=CH), 5.90 (1H, m, CH2=CH), 6.96 (1H, q, J 7.8, 3-H) and 8.62 (1H, br s, NH); minor epimer 0.74 and 0.80 (each 3H, d, J 6.6, 400 0 -CH3 and 500 0 -H3), 4.19 (1H, dt, J 4.8, 9.6, 200 0 -H) and 7.13 (1H, q, J 7.8, 3H); m/z (ES+) 559 (M++23, 100%). (Z)-Dehydrobutyrine 28: Diethylaminosulfur trifuoride (7 mg, 0.04 mmol) was added dropwise over 1 min to the alcohol 24 (20 mg, 0.027 mmol) and pyridine (8 mg, 0.11 mmol) in DCM (0.5 mL) at 0 °C. The reaction was stirred at 0 °C for 2 h then saturated aqueous sodium hydrogen carbonate was added. The organic extracts were washed with brine and dried (Na2SO4) then concentrated under reduced pressure. Column chromatography of the residue eluting with petrol–ether (30:70) gave the title compound 28 (6 mg, 31%) as a colourless gum, Rf 0.5 (30:70 petrol–ether), ½a20 D 11 (c 2.0 in CHCl3) (Found: M++H, 711.4183. C35H63N4O7SiS, requires M, 711.4182); mmax 3323, 1686, 1641, 1509, 1450, 1366, 1250, 1163, 1068, 1046, 1015, 994, 921, 882, 757 and 682 cm1; dH (400 MHz, CDCl3) 0.86 and 0.88 (each 3H, d, J 6.6, 400 0 CH3 and 500 0 -H3), 0.99 [21H, m, 3 SiCH(CH3)2], 1.36 [9H, s, C(CH3)3], 1.42 (3H, m, 300 0 -H2 and 400 0 -H), 1.72 (3H, d, J 7.1, 4-H3), 1.95 (2H, m, 400 -H2), 2.26 (2H, m, 300 -H2), 3.55 (2H, m, 500 -H2), 3.70–3.95 (2H, m), 4.28 and 4.39 (each 1H, m), 4.59 (2H, m, OCH2CH=), 4.85 (1H, dd, J 4.0, 9.0, 200 -H), 5.17 and 5.25 (each 1H, m, HCH=CH), 5.84 (1H, m, CH2=CH), 6.79 (1H, q, J 7.3, 3-H), 7.45 (1H, br d, J 8.6, NH), 7.97 (1H, s, NH) and 8.45 (1H, br d, J 7.3, NH); m/z (ES+) 733 (M++23, 100%).
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Synthesis of the (E)-dehydrobutyrine–thiazoline–proline–leucine fragment of vioprolides B and D Hao Liu, Eric J. Thomas ⇑ School of Chemistry, University of Manchester, Manchester M13 9PL, UK
a r t i c l e
i n f o
Article history: Received 27 February 2013 Revised 4 April 2013 Accepted 5 April 2013 Available online 13 April 2013 Keywords: Peptolides [E]-dehydrobutyrine Thiazoline Vioprolides
a b s t r a c t A procedure is reported for the introduction of both the thiazoline and (E)-dehydrobutyrine components into a tetrapeptide-derived fragment of vioprolides B and D. The (E)-dehydrobutyrine is introduced first but, as the carbon–carbon double-bond of the dehydrobutyrine appeared incompatible with an adjacent thiol, the thiazoline was assembled by dehydration of a serine containing thioamide not by dehydration of a cysteinyl analogue. Ó 2013 Elsevier Ltd. All rights reserved.
The vioprolides A–D 1–4 are a small group of peptolides isolated from Cystobacter violaceus.1 They possess antifungal activity and are highly cytotoxic to mammalian cells with vioprolide D showing the least cytotoxicity and maximum antifungal activity. Of particular interest is the observation that they have a synergistic effect on the murine type 1 inferferon signalling pathway leading to more than a three-fold increase in activity when stimulated by viral infection.2 However, further studies into their biological activity are being delayed because of the extremely limited supply of these natural products.
⇑ Corresponding author. Tel.: +44 (0) 161 275 4613; fax: +44 (0) 161 275 4939. E-mail address: [email protected] (E.J. Thomas). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.04.017
Structural features of interest in the vioprolides are the 4methylazetidinecarboxylic acid (MAZ), the (E)-dehydrobutyrine and the thiazoline. This was shown during the chemical degradation of the natural products to facilitate epimerisation of the neighbouring MAZ or proline-derived fragment. Two conformers were detected for vioprolide A 1 attributed to atropisomers about the N-methylvaline–threonine amide bond.1 To date, the only studies on synthetic approaches to the vioprolides are a non-stereoselective synthesis of MAZ reported during their isolation1 and a synthesis of the azetidinyl-thiazoline component of vioprolides A 1 and C 2.3 We here report studies concerned with the introduction of both the thiazoline and (E)-dehydrobutyrine components into a fragment of the vioprolides. Procedures are available for the dehydration of threonine containing peptides to give either (Z)- or (E)-dehydrobutyrine containing fragments4,5 and cysteine-derived thiazolines can be accessed by dehydration of thioamides adjacent to a serine residue6 or of cysteine containing peptides.7 The latter approach was selected for the initial studies since several, fairly recently developed, procedures are available for this transformation. S-Trityl-N-Fmoc-protected L-cysteine 58 was converted to its allyl ester 6 and Fmoc-removal gave the amine 7. This was coupled with N-nosyl protected L-proline9 to give the protected dipeptide 8, see Scheme 1. Nosyl protection for the proline was used as the 1 H NMR spectra of the corresponding Boc-protected dipeptides were complicated by the presence of rotamers. Triflic anhydride and triphenylphosphine oxide7a effected thiazoline formation from the dipeptide 8 and gave a good yield of the thiazoline 9 as a single diastereoisomer (1H NMR). However, during attempts to continue with the synthesis by conversion of the allyl ester 9 into the corre-
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Scheme 1. Initial studies on thiazoline formation; Reagents and conditions: (i) CH2=CHCH2Br, NaHCO3, DMSO, rt, 16 h (83%); (ii) piperidine, DMF, r.t., 16 h (94%); (iii) Nnosyl-L-proline, HATU, HOBt, DMF, add 7 then EtNiPr2, rt, 16 h (77%); (iv) Tf2O, Ph3PO, DCM, 0 °C, 10 min, add 8, rt, 2 h (68%); (v) Pd(Ph3P)4 (10%), N-methylaniline, THF, rt, 3 h (95%).
sponding acid, epimerisation of the proline occurred and a mixture of the epimeric acids 10 was obtained, see Scheme 1. Although the epimerisation of amino acid residues adjacent to thiazolines in peptide derivatives is well known10 and indeed was observed during attempts to degrade the vioprolides into their constituent amino acids,1 it was not reported during the synthesis of the azetidinyl-thiazoline fragment of the vioprolides.3 In the case of the thiazoline 10 the rapid epimerisation may be due to hydrogen bonding between the nitrogen of the thiazoline and the carboxylic acid, see structure 11. This could facilitate imine–enamine tautomerisation thus leading to epimerisation. Next, procedures for the introduction of the (E)-dehydrobutyrine residue were investigated. Deallylation of the protected dipeptide 8 and coupling the resulting acid 12 with the allyl ester of Otert-butyldimethylsilyl protected L-threonine gave the protected tripeptide 13. This was desilylated to give the alcohol 14 ready for the dehydration to the corresponding alkene. (E)-Dehydrobutyrines are available by dehydration of threonine containing peptides using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and copper(II) chloride5 and indeed this procedure gave a good yield of the required dehydrobutyrine-containing peptide 15, the geometry of the double-bond being assigned at this stage by analogy with the literature.5 However, attempts to effect a subsequent dehydration to form the required thiazoline using the triflic anhydride and triphenylphosphine oxide procedure7 were unsuccessful, complex mixtures of products being obtained, see Scheme 2. The problems encountered during the attempted formation of the thiazoline 16 from the thio-ether 15 were attributed to com-
peting reactions between the thiol formed on deprotection of the trityl thio-ether 15 and the neighbouring double-bond of the dehydrobutyrine. This suggested that, although the thiazoline will have to be introduced as late as possible in the synthesis to minimise epimerisation of the neighbouring amino acid residue, free-thiol containing intermediates would have to be avoided to prevent complications due to the presence of the neighbouring dehydrobutyrine. The alternative procedure for the introduction of cysteinylderived thiazolines into peptides based on the dehydration of serine-derived thioamides was therefore investigated.6 Only low yields of the O-TBS-L-threonine-O-TIPS-L-serine dipeptide 19 were obtained in our hands if O-TIPS-protected serine was coupled with the allyl ester of O-TBS-protected threonine, and so the protected L-threonine was coupled with the Fmoc-protected Lserine 17 with a free hydroxyl group, to give the protected dipeptide 18 that was silylated to give the bis-silylated dipeptide 19, see Scheme 3. Following removal of the Fmoc-group, coupling the resulting amine 20 with the Fmoc-protected thioacylating agent 2111 gave a thioamide that was converted into the corresponding amine 22 by treatment with piperidine under standard conditions. This was now coupled with Boc-protected D-leucine to give the modified tetrapeptide 23. Selective removal of the secondary TBS-group in the presence of the primary TIPS-group proved difficult and so both of the silyl protecting groups were removed using tetrabutylammonium fluoride, and the primary serine-derived alcohol was resilylated to give the monosilyl ether 24. Dehydration of this using EDC and copper(II) chloride under conditions known to be selective for formation of (E)-dehydrobutyrines5 gave a product identified as
Scheme 2. Initial studies on thiazoline formation in the presence of a dehydrobutyrine; Reagents and conditions: (i) Pd(Ph3P)4 (10%), N-methylaniline, THF, rt, 3 h (95%); (ii) 12, HATU, HOBt, DMF, add O-TBS-L-threonine allyl ester, EtNiPr2, rt, 16 h (67%); (iii) Et3N(HF)3, THF, rt, 48 h (62%); (iv) EDC, CuCl2, tol., 80 °C, 30 min (62%); (v) Tf2O, Ph3PO, DCM, 0 °C, 10 min, add 15, rt.
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Scheme 3. Synthesis of the dehydrobutyrine–thiazoline–proline–leucine fragment of vioprolides B and D. Reagents and conditions: (i) HATU, HOBt, DMF, add O-TBSthreonine allyl ester, EtNiPr2, rt, 16 h (97%); (ii) TIPSOTf, 2,6-lutidine, DCM, rt, 16 h (56%); (iii) piperidine, DMF, rt, 16 h (89%); (iv) (a) compound 21, THF, rt, 6 h (b) piperidine, DMF, rt, 16 h (40%); (v) Boc-D-leucine, HATU, HOBt, DMF, add 22, EtNiPr2, rt, 16 h (85%); (vi) (a) TBAF, THF, rt, 16 h (b) TIPSCI, imid., THF (30%); (vii) EDC, CuCL2,tol., 80 °C, 30 min (65%); (viii) TBAF, THF, rt, 16 h (62%); (ix) DAST, DCM, 15 °C, 1 h (75%); (x) DAST, pyridine, DCM, 0 °C, 2 h (31%).
the required (E)-isomer 25 on the basis of precedent. To check that the (E)- and (Z)-isomers could be distinguished, the alcohol 24 was also dehydrated using diethylaminosulfur trifluoride (DAST) under conditions known to be selective for formation of (Z)-dehydrobutyrines.4 Although only a modest yield of alkene was obtained in the latter case, the alkene that was isolated was different by 1H NMR from that obtained using EDC-copper(II) chloride and was identified as the (Z)-isomer 28 [25, =CHCH3, dH (CDCl3) 1.98; 28, =CHCH3, dH (CDCl3) 1.72], see Scheme 3. Indeed following comparison of the 1 H NMR spectra of the two isomers, a small amount of the (Z)-isomer 28 was detected in the (E)-isomer 25; (E)-25: (Z)-28 = 95:5. It now remained to check that the formation of a thiazoline by dehydration of the serine-thioamide was compatible with the (E)dehydrobutyrine component. Deprotection of the tri-isopropylsilyl ether 25 using tetrabutylammonium fluoride gave the corresponding alcohol 26 and this on treatment with DAST6 was dehydrated to give a major product isolated in 75% yield. This was identified as the required thiazoline 27 containing ca. 20% of its epimer due to partial racemisation of the proline-derived residue as indicated in the 1H NMR spectrum of 27 by some doubling of peaks, for example of the doublets attributed to the diastereotopic methyl groups present in the leucine. This work has developed procedures for the assembly of both an (E)-dehydrobutyrine and an adjacent thiazoline into peptides related to fragments of the vioprolides.12 Because of its tendency to promote epimerisation of an adjacent amino acid residue,10 the thiazoline should be introduced as late as possible in any synthetic approach to a vioprolide. However, the compatibility of the introduction of the thiazoline from a thioamide using DAST with the (E)-dehydrobutyrine suggests that this procedure may be appropriate for the synthesis of a natural product. Some optimisation may be necessary to minimise epimerisation at the proline- or MAZ-derived stereogenic centre, although a depsipeptide with a proline derived residue next to the thiazoline has been prepared using this procedure without any racemisation being observed.6
Acknowledgments We thank Novartis for a studentship (to H.L.). References and notes 1. Schummer, D.; Forche, E.; Wray, V.; Domke, T.; Reichenbach, H.; Höfle, G. Leibigs Ann. 1996, 971. 2. Bollati-Fogolín, M.; Oggero, M.; Mirazo, S.; Frank, R.; Kratje, R.; Muller, W. Cells and Culture, ESACT Proceedings 4, 485; Springer, 2010, doi: 10.1007/978-90481-3419-9_86. 3. Chopin, N.; Couty, F.; Evano, G. Lett. Org. Chem. 2010, 7, 353. 4. (a) Ogura, H.; Sato, O.; Takeda, K. Tetrahedron Lett. 1981, 22, 4817; (b) Andruszkiewicz, R.; Czerwínski, A. Synthesis 1982, 968; (c) Berti, F.; Ebert, C.; Gardossi, L. Tetrahedron Lett. 1992, 33, 8145; (d) Goodall, K.; Parsons, A. F. Tetrahedron Lett. 1995, 36, 3259; (e) Yokokawa, T.; Shioiri, T. Tetrahedron Lett. 2002, 43, 8673; (f) Wojciechowska, H.; Pawlowitcz, R.; Andruszkiewicz, R.; Grzybowska, J. Tetrahedron Lett. 1978, 19, 4063; (g) Miller, M. J. J. Org. Chem. 1980, 45, 3131; (h) Srinivasan, A.; Stephenson, R. W.; Olsen, R. K. J. Org. Chem. 1977, 42, 2256; (i) Yokokawa, F.; Shioiri, T. Tetrahedron Lett. 2002, 43, 8679. 5. Sai, H.; Ogiku, T.; Ohmizu, H. Synthesis 2003, 201. 6. McKeever, B.; Pattenden, G. Tetrahedron 2003, 59, 2701. 7. (a) You, S.-L.; Razavi, H.; Kelly, J. W. Angew. Chem., Int. Ed. 2003, 42, 83; (b) Ma, D.; Zou, B.; Cai, G.; Hu, X.; Lin, J. O. Chem. Eur. J. 2006, 12, 7615. 8. Commercially available from Sigma–Alrich. 9. Prepared by reaction of L-proline with 2-nitrobenzenesulfonyl chloride, NaHCO3, H2O, rt, 16 h (85%). 10. (a) North, M.; Pattenden, G. Tetrahedron 1990, 45, 8267; (b) Wipf, P.; Fritch, P. C. Tetrahedron Lett. 1994, 35, 5397. 11. Prepared from Fmoc-protected proline i (see Ref. 10).
Prep. of thioacylating agent 21: (i), 2-amino-4-nitroaniline, iBuOCOCl, THF, 20 °C, add i, rt, 16 h (72%); (ii), P4S10, NaHCO3, THF, 1 h, add ii, 0 °C, rt, 3 h (89%); (iii), NaNO2, AcOH, 5% H2O, 0 °C, 30 min (99%). 12. (E)-Dehydrobutyrine 25: A solution of the alcohol 24 (40 mg, 0.05 mmol), EDC (17 mg, 0.11 mmol) and copper(II) chloride (1 mg, 0.01 mmol) in anhydrous toluene (2 mL) was stirred at 80 °C under an atmosphere of nitrogen for
H. Liu, E. J. Thomas / Tetrahedron Letters 54 (2013) 3150–3153 30 min. Water was added and the mixture was extracted with ethyl acetate (5 mL). The organic extracts were dried (Na2SO4) and concentrated under reduced pressure. Chromatography of the residue eluting with ether gave the title compound 25 (23 mg, 65%) as a colourless gum, Rf 0.6 (light petroleum: ether = 30: 70), ½a20 37.5 (c 2 in CHCl3) (Found: M++H, 711.4189. D C35H63N4O7SiS requires M, 711.4182); mmax/cm1 3294, 1682, 1645, 1507, 1447, 1386, 1366, 1251, 1163, 1106, 1066, 1046, 1014, 994, 920, 881, 851, 778, 735 and 681 cm1; dH (400 MHz, CDCl3) 0.89 and 0.91 (each 3H, m, 400 0 -CH3 and 5000 -H3), 0.99 [21H, m, 3 SiCH(CH3)2], 1.33 [9H, s, C(CH3)3], 1.36 (2H, m, 3000 H2), 1.64 (1H, m, 4000 -H), 1.92 (1H, m, 400 -H), 1.98 (3H, d, J 7.6, 4-H3), 2.04, 2.22 and 2.34 (each 1H, m, 2 300 -H or 400 -H0 ), 3.51 (2H, m, 500 -H2), 3.81 and 3.96 (each 1H, m), 4.21 (1H, dd, J 8.0, 12.0, 30 -H), 4.34 (1H, m, 30 -H0 ), 4.63 (2H, m, OCH2CH=), 4.85 (1H, m), 5.05 (1H, m, NH), 5.18 and 5.28 (each 1H, m, HCH=CH), 5.90 (1H, m, CH2=CH), 6.68 (1H, m, 3-H) and 8.13 and 8.62 (each 1H, br m, NH); dC (125 MHz, CDCl3) 11.79, 14.17, 17.91, 21.93, 23.42, 24.36, 24.64, 28.32, 30.33, 32.37, 41.37, 47.71, 50.85, 62.26, 65.89, 79.99, 82.65, 103.73, 118.55, 126.11, 129.12, 131.96, 148.99, 155.72, 163.59 and 203.33; m/z (ES+) 733 (M++23, 100%). Thiazoline 27: Diethylaminosulfur trifluoride (10 mg, 0.06 mmol) was added dropwise over 1 min to the thioamide 26 (11 mg, 0.02 mmol) in DCM (0.25 mL) at 15 °C and the solution stirred for 1 h. Saturated aqueous sodium hydrogen carbonate was added and the mixture allowed to warm to room temperature. DCM (5 mL) and water (5 mL) were added and the mixture extracted with DCM (10 mL). The organic extracts were dried (Na2SO4) and concentrated under reduced pressure. Chromatography of the residue eluting with ether gave the title compound 27 (8 mg, 75%) as a colourless gum, containing ca. 20% of its epimer at the proline derived centre (1H NMR), Rf 0.5 (ether), ½a20 D 15.6 (c 2 in CHCl3) (Found: M+ H, 535.2596. C26H39N4O6S requires M, 535.2595); mmax/
3153
cm1 3321, 1702, 1682, 1643, 1510, 1425, 1366, 1343, 1250, 1162, 1015, 988, 932, 854 and 780 cm1; dH (400 MHz, CDCl3) major epimer 0.87 and 0.91 (each 3H, d, J 6.6, 400 0 -CH3 and 500 0 -H3), 1.35 [9H, s, C(CH3)3], 1.41 (2H, m, 300 0 -H2), 1.64 (1H, m, 400 0 -H), 1.85-2.00 (2H, m, 400 -H2), 2.03 (3H, d, J 7.6, 4-H3), 2.05–2.15 (2H, m, 300 -H2), 3.40-3.60 (2H, m, 30 -H2), 3.88 (2H, m, 500 -H2), 4.44 (1H, dt, J 4.8, 9.6, 200 0 -H), 4.67 (2H, m, OCH2CH=), 4.84 (1H, dd, J 4.0, 9.0, 20 -H), 5.03 (2H, m, 200 -H and NH), 5.20 and 5.30 (each 1H, m, HCH=CH), 5.90 (1H, m, CH2=CH), 6.96 (1H, q, J 7.8, 3-H) and 8.62 (1H, br s, NH); minor epimer 0.74 and 0.80 (each 3H, d, J 6.6, 400 0 -CH3 and 500 0 -H3), 4.19 (1H, dt, J 4.8, 9.6, 200 0 -H) and 7.13 (1H, q, J 7.8, 3H); m/z (ES+) 559 (M++23, 100%). (Z)-Dehydrobutyrine 28: Diethylaminosulfur trifuoride (7 mg, 0.04 mmol) was added dropwise over 1 min to the alcohol 24 (20 mg, 0.027 mmol) and pyridine (8 mg, 0.11 mmol) in DCM (0.5 mL) at 0 °C. The reaction was stirred at 0 °C for 2 h then saturated aqueous sodium hydrogen carbonate was added. The organic extracts were washed with brine and dried (Na2SO4) then concentrated under reduced pressure. Column chromatography of the residue eluting with petrol–ether (30:70) gave the title compound 28 (6 mg, 31%) as a colourless gum, Rf 0.5 (30:70 petrol–ether), ½a20 D 11 (c 2.0 in CHCl3) (Found: M++H, 711.4183. C35H63N4O7SiS, requires M, 711.4182); mmax 3323, 1686, 1641, 1509, 1450, 1366, 1250, 1163, 1068, 1046, 1015, 994, 921, 882, 757 and 682 cm1; dH (400 MHz, CDCl3) 0.86 and 0.88 (each 3H, d, J 6.6, 400 0 CH3 and 500 0 -H3), 0.99 [21H, m, 3 SiCH(CH3)2], 1.36 [9H, s, C(CH3)3], 1.42 (3H, m, 300 0 -H2 and 400 0 -H), 1.72 (3H, d, J 7.1, 4-H3), 1.95 (2H, m, 400 -H2), 2.26 (2H, m, 300 -H2), 3.55 (2H, m, 500 -H2), 3.70–3.95 (2H, m), 4.28 and 4.39 (each 1H, m), 4.59 (2H, m, OCH2CH=), 4.85 (1H, dd, J 4.0, 9.0, 200 -H), 5.17 and 5.25 (each 1H, m, HCH=CH), 5.84 (1H, m, CH2=CH), 6.79 (1H, q, J 7.3, 3-H), 7.45 (1H, br d, J 8.6, NH), 7.97 (1H, s, NH) and 8.45 (1H, br d, J 7.3, NH); m/z (ES+) 733 (M++23, 100%).