COMMUNICATIONS 4a: 1 (473 mg, 1.8 mmol), 3 (430 mg, 1.8 mmol), and bis(triphenylphosphane1- (ethene)platinum (83 mg, 0.12 mmol, 6.2 mol%) in toluene (15 mL) were heated under reflux for 24 h. Afterwards, the precipitate was isolated by filtration, washed several times with THF and CH,Cl,, and dried in vacuum. Yield: 455 mg (50.6%)) m.p.: >310"C, 'HNMR (ZOOMHz, [D,]MeOH): 6 =6.5-6.9 (m); I3C NMR (50 MHz, [DJMeOH): 6 = 146.3,120.9, 116.4(C6H.,), CBnot observed; "BNMR (64 MHz, [DJMeOH): 6 = 34 ( A v , , ~ = 524 Hz), 18 (AvI j , = 58 Hz, C,H,02BOCD,); HR-MS (EI): m / z 500.1209 ( M + , calcd '2C,,1H,,'oB,'60,: 500.1217); correct C, H analysis. 2b, 2c: A suspension of 2a (785 mg, 1 mmol) in toluene (15 mL) and methyllithium (8 mL of a 1.5 M solution in diethyl ether, 12 mmol) was first stirred for 1.5 h at - 5 "C and then for 8 h at 25 "C. After removing the insoluble components, a yellow solution of 2b was obtained which contained minor amounts of boranates. IIB

29 Hz); GC-MS: m / z ("A) 317 (100) [M+], 302 (17.4) [ M + -Me], 41 (92.4) [BMe,+]. An excess of pyridine was added to the yellow solution. Crystalline 2c precipitated from the resulting orange-red solution at room temperature. Yield: 296 mg(33%), m.p.: >250°C (decomp), 'HNMR (200 MHz, CDCI,): 6 = - 0.3 to 0.3 (m, BMe), 5.8 -6.9 (m, C,H,), 7.0 (m, Py), 7.5 (m, Py), 8.2 (m, Py); 13C NMR (50MHz, CDCI,): 6 =13.0 (BMe, br), 114.6, 116.5, 118.2, 120.0, 153.6, 160.0 (C6H,), 123.7, 136.6, 149.6 (Py), 168.0 (CB, br); I l B NMR (64 MHz, CDCI,): 6 = 0.3 ( A V , , ~ = 204 Ht); MS (FD): m / z 982 [ M + - Py - 4Mel.

NMR (64 MHz): 6 = 6 (AVlj2 = 466 Hz), -17.4 (Av,,, = 87 Hz), -20.6 (AVlj2 =

Received: February 12, 1996 [Z8807IE] German version: Angew. Chem. 1996, 108, 1664-1666

Keywords: boron compounds * catalysis * diborylacetylenes

[l] H. Schulz, 0. Gabbert, H. Pritzkow, W Siebert, Chem. Ber. 1993, 126, 1593. [2] K. P. C. Vollhardt, Angew. Chem. 1984, 96, 525; Angew. Chem. Int. Ed. Engl.

[3] D. E. Kaufmann, R. Boese, A. Scheer, Chem. Ber. 1994, 127, 2349. [4] S . Cabiddu, A. Maccioni, M. Secci, Gazz. Chim. Ital. 1972, 102, 555. [5] H. Noth, B. Wrackmeyer, Nuclear Magnelic Resonance Spectroscopy of Boron

Compounds, Springer Berlin, 1978. [6] T. Ishiyama, N. Matsuda, N. Miyaura, A. Suzuki, J. Am. Chem. Soc. 1993, 115,

11018; T. Ishivama, N. Matsuda. M. Murata. F. Ozawa. A. Suzuki. N. Mivau-

1984,23, 539.

creasing number of posttranslational modifications originating from enzymatic, ribosomally synthesized precursor proteins has been detected.[21 Thus, the fascinating 43-peptide antibiotic mi- crocin B17, which inhibits DNA-gyrase, contains four oxazole and four thiazole rings,[3b1 which arise by cyclizations and dehy- drogenations from di- and tripeptide units containing Gly, Ser, and Cys residues of the precursor protein (69 residues) followed by cleavage of a leader peptide (26 residues). A wide variety of oxazole-, thiazole-, and thiazoline-containing natural products has also been isolated from marine organismst4"-'] and mi- croorganisms.~4~-m1 The reported syntheses of o x a ~ o l e [ ~ ~ 6]

and/or thiazole['] derivatives include only few examples of aminoalkyl- or aminoaryloxazolecarboxylic g,

Substituted five-membered heterocycles are pharmacophores of many natural and synthetic, bioactive compounds. In partic- ular, the amino- and carboxy-functionalized heterocycles found in microcin B17 (Scheme 1) are valuable building blocks for

N R-cn,-cb 'c-coon

\ / / s-cn

3

N R-w,-Cd 'c-coon

\ / / 0--cH

I 6

ra, Organometalhcs 1996, i5, 713. N N N [7] C. N. Welch, S G. Shore, Inorg. Chem 1968, 7, 226. [8] C D Cook, G. S . Jauhal, J Am. Chem. Soc 1968,90, 1464. 191 In the EI mass spectrum, only the molecular peak at m / z = 502 was observed

na-cn,-cJ 'c-coon npcn& \c-cd 'c-coon \ / / x-cn

In the higher mass regon A signal for [M' - H,] (4a+) was not observed, compare with ref. [lo].

1993, 126, 1291. x=s,o [lo] P. Frankhauser, E Kuhlmann, A. Kramer, H. Pritzkow, W Siebert, Chem. Ber.

[Ill C. N. Iverson, M. R. Smith III., J. Am. Chem. Sac. 1995, 117, 4403. [I21 H. E. Katz, J. Am. Chem. SOC. 1985, 107, 1420. [I31 J. E. Dobson, P. M. Tucker, F. G. A. Stone, R. Schaeffer, J. Chem. SOC. A 1969,

12, 1882.

Synthesis of Naturally Occurring, Conforma- tionally Restricted Oxazole- and Thiazole- Containing Di- and Tripeptide Mimetics"" Georgi Videnov, Dietmar Kaiser, Christoph Kempter, and Gunther J u g *

Residues that restrict the peptide backbone are synthesized in Nature inter alia by the following modifications: N- and a-al- kylations,[ll a,b-didehydroamino acid and sulfide ring forma- tions,[21 and synthesis of aromatic five-membered hetero- cycle~.[~] The unusual amino acids of most peptide antibiotics are incorporated by multienzyme complexes.['] However, an in-

[*] Prof. Dr. G. Jung, Dr. G. Videnov, Dip1.-Chem. D. Kaiser, DipLChem. C. Kempter Institut fur Organische Chemie der Universitit Auf der Morgenstelle 18, D-72076 Tiibingen (Germany) Fax: Int. code +(7071)29-6925 e-mail: guenther.jung@uni-tuebingen.de

and 436 EUL-113/68/0). [**I This work was supported by the Deutsche Forschungsgemeinschaft (Ju 103/9-1

X'=S ,O x'=s,o

N N R-cn2-cd 'c-cd 'c-coon

\ / / \ / / 0-cn s-cn 22 I

N N

R-cy-cd \c-cd \c-COO" \ / / \ / / s-cn s-cn

Scheme 1. R = Boc-NH

peptidomimetics, which restrict the w,$-conformational free- dom in peptides. As a part of our efforts towards the total synthesis of microcin B1 7,L31 we developed efficient procedures for the preparation of Boc- and Fmoc-protected amino acids derived from oxazole, thiazofe, bisthiazole, oxazolyl-thiazole, and thiazolyl-oxazole, which can be considered as di- and tripeptide mimetics. We also discovered a novel, useful proce- dure for the oxidative conversion of intermediate oxazoline into corresponding oxazole rings with 1,8-diazabicycl0[5.4.O]undec-

Angew. C h p . Int. Ed. Ehgl. 1996, 35, No. 13/14 0 VCH Verlagsgesellschaji mbH, 0-69451 Weinheim. 1996 0570-083319613513-1503 $ 15.Wf .25/0 1503

COMMUNICATIONS

7-ene (DBU)/CCl,/acetonitrile/pyridine. This reagent proved to be superior to the known reagent CuBr,/DBU/hexa- methylenetetramine (HMTA) .

For the synthesis of 2-aminomethyl thiazole-4-carboxylic acid (Scheme 2), Nx-Boc-glycinamide (1) was converted into thio- amide 2 by Lawesson's reagent.['' Cyclocondensation of 2 with 3-brom-2-0x0-propionic acid['] leads to 3 in 74% yield. After

R-C, OEt 80% R<> COOMe 4NH 2

10

o a b

NH, 87% R - C z - R - C q S NH, --- 7 4 % R < ~ ~ c o o H

3 : R = Boc-NH-CH, h E n t .

1 : R = Boc-NH-CH, 2

7 : R = Fmoc-NH-CH,

12 : R = Boc-NH-CH,

13: R = Fmoc-NH-CH, 10 11 h e

Scheme 2. a) Lawesson's reagent [Sl ; b) 3-brom-2-oxo-prop1onic acid [4d-c,9]; c) iBuOCOCI. MeMorph, NH,OH ; d) Et,OPF, [lo]; e) serine-0Me.HCI [Ilb]; f ) DBU. CCI,. CH,CN. Py; g) NaOH; h) TFA then Fmoc-OSu.

Boc deprotection (50 Yo trifluoroacetic acid (TFA)/CH,CI,) the Fmoc protecting group was introduced by Fmoc-OSu (9-fluo- renylmethoxycarbonyl-0-succinimide) to give 7. The acid 3 was converted via amide 4 into thioamide 5 with isobutyl chlorofor- mate/aqueous ammonia in T H E Reaction of the thioamide 5 with 2-bromopyruvic acid yielded the Boc-bisthiazole 6 (60%), which was converted into the Fmoc compound 8.

The synthesis of the building block 2-aminomethyloxazole-4- carboxylic acid was accomplished by a novel route (Scheme 2). Treatment of the amide 1 with triethyloxonium hexafluoro- phosphate gave the imino ether 9.1lo1 The intermediate oxazo- line 10 was obtained in high yield (80%) by cyclization of the imino ether 9 with serine methyl ester in CH,CI, .[' 'I Oxidation of 10 with the reagent DBU/CCl,/acetonitrile/pyridine gave the oxazole 1 I (43 %), which was saponified to give 12. Removal of the BOG group and reprotection with Fmoc-OSu afforded 13.

The Boc- and Fmoc-protected 2-(2'-aminomethylthia=ole-4'- ~l)oxazole-4-carboxylic acids were synthesized according to Scheme 3. The amide 4 was converted into the imino ether 14 as described for 9. Formation of the thiazolyloxazoline derivative

11 R =Bw, X = OMe, Y = 0

19 R = B o e , X = N H , , Y = O J d 20 R = B w , X = N H , , Y = S

4 R = B w , X = N H , , Y = O

14 R = B w , X = O E t , Y = N H

ie I b I5

21 R = B o c , X = O E t

11 R = B w , X = O H

23 R = F m o c , X = O H

16 R = B W , X = O M ~

17 R = B o c . X = O H

18 R = F m o c , X = O H

Scheme 3 . For a) , b). and e)-h) see Scheme 2; i) MeOH-NH,OH

15 occurs in high yield (72%). Oxidation of 15 with the novel procedure described for 10 yielded the ester 16, which was sa- ponified without problems to give 17. Deprotection of the aminomethyl group and introduction of the Fmoc group afford- ed 18.

For the synthesis of Boc- and Fmoc-protected 2-(2'-amino- methyloxazole-4'-yl)thiazole-4-carboxylic acid (Scheme 3) I 1 was converted into amide 19 with methanol/ aqueous ammonia and to the thioamide 20. The condensation of thioamide 20 with ethyl bromopyruvate (modified Hantzsch reactionr4a, b,j l ) yield- ed the ethyl ester 21. Basic hydrolysis of 21 gave the acid 22, which was treated successively with 50% TFA/CH,Cl, and Fmoc-OSu to obtain 23.

The preparation of the oxazole- and/or thiazole-derived amino acids described above enabled us to synthesize fully ac- tive microcin B17,[l3I and the use of these and related buildjng blocks for the synthesis of further bioactive products is the object of intense research. During our experiments, we devel- oped a novel method for the oxidation of oxazolines to oxa- zoles, which we hope will prove useful for the synthesis of relat- ed heterocycles. A report on X-ray structures, ab initio calculations, NMR spectroscopy studies and molecular dynam- ics simulation (MDS) studies summarizes the structural proper- ties of the novel amino acids.['41

Experimentui Procedure Thin-layer chromatography: glass sheets coated with silica gel (E. Merck, Darm- stadt); solvent systems: A = CHCI,/MeOH/H20 (80/30/5): B = CHCI,/MeOH/ AcOH (95/5/3): C = CHCIJMeOH (9/1); D = EtOAc/n-hexane ( l / l ) ; E = nBuOH/AcOH/H,O (4/1/1). Silica gel for flash chromatography was from J. T. Baker (Deventer. Holland). Instrumentation and conditions: Mass spectrometry: API 111 trlple quddrupole mass spectrometer with an electrospray ion source at atmospheric pressure (Sciex, Thornhill, Canada); electrospray ionization mass spec- tra IESI-MS) were taken in the positive mode: samples were dissolved in 10% HCOOH/MeOH (1/1). NMR spectroscopy: Bruker AC 250 spectrometer (Bruker Physics. Karlsruhe, Germany); solutions in CDCI, and [D,]DMSO ( c = 40 mgmL-'): chemical shifts referenced to the solvent peaks [6( I3C, CDC13) =77. &"C. [DJDMSO) = 39.5 and 6(1H. [DJDMSO) = 2.491, 1 - GIY-NHZ-HCI (22.0 g, 0.2 mol). triethykamine (28.0 mL, 0.2 mol) and di . fer f . butY' dicarbonate 1121 (48.0 g. 0.22 mol) in THFiwater (4/1) (300 mL) were stirred

1504 Q VCH Verlugsgesellschufr mbH, 0.69451 Wetnliebn, 1996 0570-0833/96/3513-1504 S 15.00f ,2510 Angew. Chem. In!. Ed. Engl 1996, 35, No 13/14

COMMUNICATIONS at room temperature (RT) until the starting material was consumed (TLC system A). Yield 31.0 g (89%); homogeneous (TLC systems A and C). 2: Lawesson's reagent 181 (30.0 g, 75 mmol) and a solution of 1 (17 4 g. 100 mmol) in dimethoxyethane (400 mL) was stirred at room temperature until 1 was con- sumed (TLC system C). The solvent was removed in vacuo. A solution of the residue in EtOAc was extracted with 10% NaHCO,, and the aqueous phase reextracted with EtOAc (2 x ) The combined organic phases were washed with brine and dried. Evaporation and crystallization from EtOAcjn-hexane afforded a yellow solid. Yield 17.0 g(89O%). homogeneous (TLC systems B andC). ' H NMR ([DJDMSO): 6 = 1 . 3 X (s. 9 H : 3CH,). 3.Yl(d. 'J(H,H)=6,07Hz, 2 H ; CH,). 7.82 (1, 'J = 5.92 HL. I H . N H ) . 9.0, 9.66 (br s. 1 H: CONH,); I3C NMR ([DJDMSO): d = 28 2 (Boc-CH,). 50 5 (CH,). 78.3 (Boc-C,). 155.5 (Boc-CO). 2040 (C=S) . 3' 3-brom-2-0x0-propionic acid (1.93 g. 11.6 mmol), thioamide 2 (1.52 g, 8 mmol). and CaCO, (2.3 p. 23 mmol) were added to dry ethanol (60 mL) and stirred at room temperature undei- Ar for 5 h. The reaction mixture was filtered. and the solid washed with ethanol (2 x 20 mL). The combined filtrate was evaporated, and the residue dissolved in EtOAc (150 mL), the solution washed with 5 % KHSO, and brine, and then dried. After evaporation the solid 3 was crystallized from EtOAc/ n-hexane. Yield 1.48g (74%): homogeneous (TLC system A). 'H NMR ([D,]DMSO):O =1.41 (s,YH;3CH3),4.39(d.'J(H.H) = 5.87Hz,2H;CH2).782 (t. 'J(H.H) = 5.86 Hz. 1 H: NH). 8.34 (s, 1 H; CH,,,), 12.94 (br s, 1 H , COOH); I3C NMR ([D,]DMSO): 6 = 28.6 (Boc-CH,), 43.1 (CH,). 80.9 (Boc-C,). 129.1 (C&7). 148.2 (C:h,). 158.2 (Boc-CO). 164.0 (C&J, 173-7 (COOH) ESI-MS. m / z : 259 [ M + H ] + . 276 [M+NH,]+. 281 [ M + N a ] + . 4: To 3 (5 2 g. 10 mmol) and N-methylmorpholine (2.2 mL, 20 mmol) in T H F (100 mL) was added isobutyl chloroformate (5.2 mL, 20 mmol) at -20 'C. After 4 min aqueous ammonia (6 mL. 0.1 M) was added. After 3 h at room temperature and subsequent evaporation. the residue was precipitated with 10% NaHCO,, filtered. and the solid was washed with water. dried. and crystallized from ethanol/ etherjri-hexane. Yield 4.77 g. (93%), homogeneous (TLC systems A and C); X-ray structure analysis [14]. ' H N M R ([DJDMSO): 6 =1.41 (s, 9 H ; 3CH,), 4.41 id. 'J(H.H) = 6.03. 1H. CH,). 7.53. 7 65 (s. 1 H; CONH,), 7.8 (t. '4H.H) = 5.66. I H: NH). 8.14 (5. 1 H , CH,,,): "C NMR ([DJDMSO). 6 = 28 1 (Boc-CH,), 42.0 (CH,). 7X.7 (Boc-C,). 124.0 (C;,,). 150.0 (C&J. 155.8 (Boc-CO), 162.2 (C&,). 171.5 (CONH2) ESI-MS: mi:: 258 [M+H]'. 275 [M + NH,]+. 280 [M + Na]'.

5 : Prepared from 4 (4.33 g, 16 85 mmol) as described for 2. Crystallization from ethanol/ti-hexane. Yield 4.4g (97%): homogeneous (TLC systems Band C): X-ray structure analysis [14]. ' H N M R ([DJDMSO): 6 =1.41 (s. 9 H ; 3CH,). 4.4 (d, 'J(H.H) = 5 94 HL. ZH: CH,), 7.85 (t. 'J(H.H) = 5.83, 1 H: NH). 8.35 (s. I H; CH,,,). 9.41, 9.94 (br s. I H ; CONH,); ',C NMR ([D,]DMSO): d = 28.2 (Boc- CH,).42 1 (CH,). 7X.8 (Boc-C,). 127.0(C:,,), 153.4(C:,,). 155.8 (Boc-CO), 171.3 (C:,,). 189.2 ( C = S ) . ESI-MS: m / z : 274 [ M + H ] +

6: Prepared from 5 ( 2 0 g. 7.3 mrnol) as described for 3. Crystallization from EtOAc/ ether. Yield 1.5 g (60%); homogeneous (TLC system A); X-ray structure analysis [14]. ' H NMR ([DJDMSO): d =1.43 (s. 9 H . 3CHJ. 4.46 (d. ,J(H.H) = 6.05 Hz.2H. CH,),7 86(t, 'J(H,H) = 5.9.1 H;NH),8.25.8.48(~. IH:C:,,.C$hr). 13.11 (br s. 1 H: COOH), I3C NMR ([DJDMSO): 6 = 28.2 (Boc-CH,), 42.0 (CH,). 78.7 (Boc-C,). 117.9 (C:,,). 128.9 (C:hz). 147.3 (C:;,). 148.2 (C&J, 155.8 (Boc-CO). 162.0(C$,,), 162.2(C:,,). 172.8(COOH). ESI-MS: m / z : 342 [ M + H ] + . 359 [ M + NH,]' . 364 [M + Na]'.

7: 3 (1.3 g, 5 mmol) was deprotected quantitatively in 50% TFA/CH,CI, (20 mL) for 45 min. Fmoc-OSu (2.0 g, 6 mmol) in T H F (30 mL) was added to the TFA salt and Na,CO, (1.0 g. 10 mmol) in water (15 mL). After 12 h at room temperature T H F was removed in vacuo. and the aqueous phase was diluted with water, acidified ( I "i HCI) to pH 3 and filtered. The solid product was recrystallized from ethanoli ether. Yield 1.9g (100%): homogeneous (TLC system A). ESI-MS: m / z : 381 [ M + H ] ' . 403 [ M + Na]'. 425 [ M + ZNa]'.

8 Prepared from 6 (1 .O g. 2.9 mmol) as described for 7. Crystallization from ethanol!ether. Yield 1.3 g (97%). ESI-MS: mi:: 464 [ M f HI', 486 [M + NaI+, 508 [ M + ZNa]'

9: The imino ether 9 was prepared from 1 (3.5 g, 20 mmol) according to ref. [lo], obtained as B oil, and used without purification. Yield 3.4 g ( 8 5 % ) . IR (Nujol): C=l653crn- ' (C=N): ESI-MS: mi:: 203 [M+H]'. 220 [ M + NH4]+. 225 [M + Na]'.

10: Oxazoline 10 was prepared according to ref. [ l lb ] starting from 9 (3.4g, 17 mmol) and H-Ser-OMe-HCI (3.1 g., 20 mmol) in CH,CI, (100 mL) Oxazoline 10 was obtained a s a oil and used without purification. Yield 3.5 g (80%). "C NMR (CDCI,): d = 28.3 (Boc-CH,). 27.9 (CH,), 52 7 (OOCH,), 67.7 (C&,), 70.1 (Csc)xa). 79.9 (Boc-C,). 155 5 (Boc-CO). 167.7 (C&J, 171.2 (COO). ESI-MS: mi:: 259 [ M + H ] ' . 281 [M + Na]'. 297 [ M + K]'. 11: DBU (3 mL. 30 mmol) was added to 10 (2.0 g. 7.75 mmol, about 80% purity) inamixtureofCCI,~10mL),pyridine(15mL),andacetonitrile(15mL) After3 h at room temperature the solvent was removed in vdcuo, the residue dissolved in EtOAc. the solutlon extracted with 0 . 5 ~ HCI and the aqueous phase reextracted with EtOAc (2 x ). The EtOAc phase was washed with brine, dried, and the solvent evaporated. Chromatography on SiO, (EtOAcin-hexane, 1 : 1) afforded oxazole 11. Yield 0.85 g (43%). X-ray structure analysis [14]. 'H NMR ([DJDMSO). 6 = 1.4 (~.9H:3CH,).3.86(s,3H;OCH,).4.45(d.~J(H,H)=5.77,2H;CH~),5.34(br

t, 1 H: N H ) , 8.16 ( s , 1H: CH,,,): I3C NMR ([DJDMSO): d = 28 3 (Boc-CH,) 38.0 (CH,), 52.2 (OOCH,), 80.4 (Boc-C,), 133.4 (C&J. 144.7 (C&). 155.1 (Boc- CO). 161.4 (C&-), 162.3 (COO). ESI-MS: mjz: 257 [M+H]*. 274 [M + NH,]' 279 [M + Na]'. 12. NaOH (0.24 g in 10 inL of water) was added to ester 1 I (0.78 g. 3 mmol) in dioxane (30 mL). After 1 h at room temperature the solution was neutralized with 10% KHSO, to pH 6. Removal of the dioxane in V ~ C U O was followed by acidifica- tion to pH 3 and extraction with EtOAc ( 3 x ) . The extract was dried, and the solvent removed in vacuo Crystallization from EtOAc/n-hexane. Yield 0.66 g (91 YO); homogeneous (TLC systems A and C). ' H NMR ([D,]DMSO)- 6 = 1.37 (s. 9 H ; 3CH,). 4.25 (d, 'J(H.H) = 5.92 Hz. 2 H ; CH,). 7.53 (t. 'J1H.H) = 5.75 Hz. 1 H: NH), 8.66 (s. 1 H; CH,,,), 13.02 (br s, 1 H; COOH): I3C NMR ([DJDMSO). 6 = 28.1 (Boc-CH,), 37.2 (CH,). 78.5 (Boc-C,). 133.2 (C&*). 145.1 (C;,,). 155.6 (Boc-CO), 162.0 (C&J. 162.4 (COOH). ESI-MS: mi:: 143 [ M + H ] + . 260 [M + NH,]', 265 [M + Na]'. 281 [M + K]'. 13. Prepared from 12 (0.27 g. 1 1 mmol) as described for 7. Crystallization from ethanol/ether/n-hexane. Yield 0.34 g (85%); homogeneous (TLC system A); "C NMR ([DJDMSO): 6 = 37 6 (CH,), 46 7 (Fmoc-C9). 65.9 (Fmoc-CH,O). 120.0 (Fmoc-C4.C5), 125.2 (Fmoc-C1, CX). 127.3. 127.6 (Fmoc-C3.C6,C2.C7), 133.7 (C&,), 140.7 (Fmoc-C4a.C4b), 143.8 (Fmoc-CXa.C9a). 145.0 (C&,), 156.3 (Fmoc- CO). 162.0 (C&J. 162.1 (COOH); ESI-MS: m / z : 365 [M+H] ' . 387 [ M + Na]'. 14: The imino ether 14 was prepared from 4 (4.8 g. 18.7 mmol) as described for 9 The product was obtained as a oil and used without purification. Yield 5.1 g (95%). ESI-MS: m!:: 286 [M+H]'. 15: The thiazolyl-oxazoline 15 was obtained from 14 (5.Ig. I8 mmol) and Ser- OMe'HCI (1.55 g. 10 mmol) by heating under reflux for 3 h; after additional 12 h at room temperature, work up as described for 10. Compound 15 was recrystallized from EtOAcln-hexane. Yield 4,4 g (72%). "C NMR ([DJDMSO). 6 = 28.1 (Boc- CH,). 41.9 (CH,). 52.2 (OOCH,), 7.14 (CH<>J. 69.6 (CH&J. 78.7 (Boc-C,). 125.6 (CHThz). 142.2 (N-CThr). 155.8 (Boc-CO). 160.3 (C=N,,,). 171 3 (C=N,,,). 172.2 (COO); ESI-MS. m / z . 342 [M+H]+. 16: The thiarolyl-oxazole 16 was prepared from 15 (2.1 g, 6.1 mmol) as described for 11. After purification on SiO, with EtOAcin-hexane (2.1) as eluent. 16 was obtained as a colorless solid and recrystallized from EtOACiri-hexane. Yield 1.1 g ( 5 5 % ) ; homogeneous (TLC systems Band C). ' H NMR ([DJDMSO): 6 = 1.41 (s. 9 H : 3CH,). 3.83 (s. 3H;CH3).4.44 (brs. 2H;CH,), 7.88 (br t. 1 H ; NH), 8.39(s,

42.0 (CH,). 78.7 (Boc-C,), 122.8 (CHTh7). 133.3 (N-CTbz). 141.3 (N-CclxJ. 145.4 (CH,J. 155.8 (Boc-CO), 157.2 (CHoxd), 161.0 (CHTh,). 173.0 (COO): ESI-MS. m/z: 340 [ . & + H I + , 362 [ M + Na]'.

17- The acid 17 was obtained from 16 (0.47 g, 1.4 mmol) a s described for 12. Crystallization from EtOAciether. Yield 0.42 g (92%); homogeneous (TLC system A). ' H N M R ([DJDMSO). 6 =1.42 (s, 9 H ; 3CH,), 4.41 (d. 'J(H.H) = 5.67 Hz. 2H;CH2).7.86(brt . l H . N H ) . 8 . 3 6 ( ~ . 1H:CH0,,),8.X2(s. 1 H;CH,,,), ESI-MS: mi;: 326 [M+H]+. 348 [ M + Na]+. 18: Fmoc derivative 18 was prepared from 17 (0.55 g, 1.7 mmol) as described for 7. Crystallization from ethanoliether. Yield 0.7 g (92%); homogeneous (TLC system

19: Aqueous ammonia (10 mL) was added in one portion to 1 I (0.51 g. 2 minol) in methanol (30 mL) and stirred at room temperature for 15 h. After evaporation in V ~ C U O 10% NaHCO, (50 mL) was added to the residue, which was then filtered off and washed with water. The solid was recrystallized from EtOAcin-hexane. Yield 0.46g (96%); homogeneous (TLC systems B and C); ' H N M R ([DJDMSO): d=1.38(~.9H;3CH,).4.26(d.~J(H.H)=5.90Hz,2H:CH~),7.46(brs,2H. CONH,). 7.21 (t. ' J (H.H) = 5.58 Hz. 1 H: NH). 8.49 (s. 1 H: CH<l,a). "C NMR ([DJDMSO): 6 = 28.2 (Boc-CH,), 37.3 (CH,), 78.5 (Boc-C,). 136.2 (C&,). 141.9 (C&,). 155.5 (Boc-CO), 161 7 (C&J. 161.8 (CONH,). 20: Prepared from 19 (0.84 g. 3.5 mmol) as described for 2. The thioamide 20 was crystallized from EtOAc/n-hexdne. Yield 0.76 g (84%); homogeneous (TLC sys- tems B and C ) ; ' H N M R ([DJDMSO). ri =1.37 (s, 9 H ; 3CHJ. 4.26 (d, 'J (H.H) = 5.65 Hz, 2 H ; CH,). 7.54 (br t, 1 H; NH). 8.57 (s, 1 H: CH,,,), 9.25. 9.79 (brs, lH;CSNH,);"CNMR([D,]DMSO):6 = 28.1 (Boc-CH,).37.4(CH2),78.5 (Boc-C,). 140.9 (C&,J, 143.7 (C&J. 155.5 (Boc-CO), 161.7 (Ci,,). 187.5 (C=S) 21: Ester 21 was prepared from 20 (0.75 g, 2.9 mmol). KHCO, ( I .5 g, 15 mmol) and ethyl bromopyruvate (1.25 mL. 10 mmol) in DME (50 mL) according to refs. [4a.b.j]. After purification on SiO, with EtOAcin-hexane ( 1 1) aseluent 21 was crystallized from EtOAcjn-hexane. Yield 0.57 g. (57%); homogeneous (TLC sys- tems B and C); 13C NMR ([DJDMSO): 6 =14.2 (OCH,). 28.2 (Boc-CH,), 37.4 (CH,). 60.9 (OCH,). 78.5 (Boc-C,). 129.1 (C:h,). 134.5 (C:,,). 137.2 (C&J. 147.2 (C:,,), 155.7 (Boc-CO), 159 9 (C&J, 160.6 ((&), 162.9(COO). ESI-MS. m:z. 354

22: The Boc compound 22 was prepared from 21 (0.42 g, 1.2 mmol) as described for 12. Crystallization from EtOAcln-hexane. Yield 0.36 g (92%); homogeneous (TLC systems B a n d C ) ; 'HNMR([D,]DMSO):6 = 1 . 4 ( ~ , 9 H : 3 C H , ) . 4 . 3 2 ( b r d . 2 H ; CH,), 7.59 (br t, 1 H; N H ) , 8.49 (s, 1 H ; CH,,,). 8.78 (s, 1 H. CH,,,); "C NMR ([DJDMSO): 6 = 28.2 (Boc-CH,). 37.4 (CH,), 78.6 (Boc-C,). 128 6 (C&-), 134.6 (C&,). 137.0 (C&J, 148.4 (C&), 155.7 (Boc-CO), 159.6 (C&J. 161.9 (C$hF), 162 9 (COOH).

1 H; CH,,,), 8.93 (s, 1 H ; CH,,,); "C NMR ([DJDMSO): d = 28.1 (Boc-CH,),

A): ESI-MS: mi/:: 448 [M'], 465 [M + NH41'. 470 [M + Na] ', 486 [M + K]'

[ M + 1.

A n p i . C ' h i ~ m . I n / . Ed €ng/. 1996. 35, No. 13i14 \<> VCH Verlu~s~esellsrhafl mhH, 0.69451 Weinheim, 1996 0570-0833/96~3513~1SOS 8 15.00+ .25/0 1505

COMMUNICATIONS

23: The Fmoc compound 23 was prepared from 22 (0.32 g, 1 mmol) as described for 7. Crystallization from ethanol/ether. Yield 0.42 g (93 %); homogeneous (TLC sys- tem A); ESI-MS: m/i 448 [ M f ] , 470 [ M + Na]', 492 [ M + 2Na]+.

Received: January 31. 1996 [Z87741E] German version: Angen. Chem. 1996. 108. 1604-1607

Keywords: amino acids * heterocycles - oxidations - peptide mimetics - synthetic methods

[I] Biochemistry of peptide untibiotics (Eds.: H. Kleinkauf. H. von Dohren). de Gruyter, Berlin. 1990.

[2] a) G. Jung, Angew. Chem. 1991, 103.1067-1084; Angen. Chem. Int. Ed. Engl. 1991, 30, 1051-1068; b) Nisin and novel luntibiorics (Eds.: G. Jung. H.-G. Sahl), Escom, Leiden. 1991; c) R. Jack, F. Gotz. G. Jung in Biotechnology Vol. 7.2nd ed., (Eds.: H.-J. Rehm, G. Reed, H. Kleinkauf, H.von Doeren), VCH. Weinheim, 1996, in press.

[3] a) A. Bayer. S. Stevanovic, S. Freund, J. Metzger, G. Jung in Peprides 1992 (Eds:A. C. H. Schneider,A. N. Eberle), Escom. Leiden. 1993. pp. 117-118; b) A. Bayer. S. Freund. G. N. Nicholson, G. Jung, Angex.. Chem. 1993. 105. 1410--1413: Angew. Chrm. Int. Ed. Engl. 1993.32, 1336-1339; c) A. Bayer, S. Freund, G. Jung, Eur. J Biochem. 1995,234,414-426.

[4] a) N. Fusetani. S. Matsunaga, Chem. Rev. 1993, 93. 1793-1806; h) B. S. Davidson, ihid. 1993.93.1771 -1791; c) J. Kobayashi. M. Ishibashi, ihid 1993. 93,1753-1770; d) M. J. Garson. ibid. 1993,93,1699-1733;e) J. P. Michael, G. Pattenden. Angew. Chem. 1993. 105, 1-24; Angew. Chem. In,. Ed. Engl. 1993, 32, 1-24; f ) G. Pattenden, J. Heterocyclic Chem. 1992. 29. 607-618; g) J.-R. Lewis, Nut. Prod. Rep. 1993. 10. 29-50; h) J. R. Lewis, ihid. 1992, 9, 81-101; i) D. J. Faulkner, ibid. 1992, 9, 323-364; j) H. Drechsel. H. Stephan, R. Lotz, H. Haag. H. ZIhner, K. Hantke, G. Jung. Liehig.7 Ann. Chem. 1995. 1727- 1733; k ) H. G. Floss, J. M. Beale, Angeiv. Chem. 1989, 101. 147-179; Angen. Chem. Int. Ed. Engl. 1989, 28, 146-177; I ) B. T. Breil. P. W. Ludden, E. W. TriDlett. J . Bucteriol. 1993. 175. 3693-3702; m) further citations [3c].

[5] I. J. Turchi in O.xuzoles. (Eds.: A. Weissberger. E. C. Taylor), Wi- ley. New York, 1986, pp. 1-342, and references therein.

[6] For some recent syntheses ofoxa- zoles, see a) E. Aguilar, A. I. Meyers, Tefrahedron Lett. 1994, 35, 2477-2480; b) A. 1. Meyers. F. Tavdres, ibid. 1994, 35, 2481 ~

2484; c) M. Tiecco. L. Testaferri. M. Tingoli, F. Marini, J. Org. Chen?. 1993, 58, 1349; d) M. Yokoyama. Y. Menjo. M. Wata- nabe, H. Togo, Synthesis, 1994. 1467-1470; e) H. Vorhruggen, K. Krolikiewicz, Tetrahedron. 1993, 49, 9353-9372; f) R. D. Connell, M. Tebbe, A. R. Gan- gloff, P. Helquist. ibid. 1993, 49, 5445-5459: g) P. Wipf, C. P. Miller. J Org. Chem. 1993, 58, 3604-3606; h) J .C. Barrish, J.

1 10

Synthesis of the DNA Gyrase Inhibitor Microcin B17, a 43-Peptide Antibiotic with Eight Aromatic Heterocycles in its Backbone** Georgi Videnov, Dietmar Kaiser, Marc Brooks, and Giinther J u g * Dedicated to Professor Helmut Zahn on the occasion of his 80th birthday

The complete structure elucidation of the most unusual polypeptide microcin B17 ( M C B ~ ~ ) , ~ ' , ~ ~ a DNA gyrase in- hibitor, reveals that it contains four oxazole and four thiazole rings within its backbone of 43 amino acids. McB17 inhibits DNA replication with remarkably high biological activity.[31 The glycine-rich peptide antibiotic is ribosomally synthesized by Escherichia coli strain LP 1 7,L41 and posttranslational enzymatic backbone modifications of glycine, serine, and cysteine residues in the 69 amino acid precursor lead to the formation of 2- aminomethylthiazole-4-carboxylic acid [gly(Thz)] and 2-amino- methyloxazole-4-carboxylic acid [gly(Oxa)], as well as the bicyclic 2-[2'-amino-methyloxazole-4'-yl]thiazole-4-carboxylic acid [gly(OxaThz)] and 2-[2'-amino-methylthiazole-4-yl]- oxazole-4-carboxylic acid [gly(ThzOxa)] (Fig. I) . The synthesis of these novel constraint amino acids and their Boc and Fmoc derivatives is described separately.[6' Here we summarize details of the total synthesis of McB17 that we first reported in 1995.[5]

16 23

VGIGGGGGGGGG-N-~-C+ 'c-c 11 \ 11 A A L C " 11 A A L c n 11 !I A \o-CH S-CH

26 31 31 43

Fig. 1. Formula of the glycine-rich peptide antibiotic microcin B17 containing eight heterocyclic modifications that constrain the backbone [1.2]. Protein amino acids are abbreviated in the one-letter code and numbered according to the unmodified precursor protein.

Songh, S. H. Spergel. W-C Han, T. Kissick, D. R. Kronenthal, R. H. Mueller. ibrd. 1993, S8, 4494-4496; I ) G . McBl7 was built up by 9-fluorenylmethoxycarbonyl (Fmoc) Li, P. M. Warner. D. J. Jeharatnam, ihid. 1996, 61. 778-780. solid-phase synthesis starting from Fmoc-Ile-Wang resin.

Fmoc-protected amino acids were coupled with an equimolar

tetrafluoroborate (TBTU), N-hydroxybenzotriazole (HOBt),

[7] a) U. Schmidt, R. Utz, A. Lieberknecht. H. Griesser. B. Potzolli. J. Bahr, K. Wagner, P. Fischer, Synthesis, 1987, 233-236, 236-241; h) U. Schmidt, P. Gleich, H. Griesser. R. Utz. ibid. 1986, 992-998; c) R. C . Kelly, I. Gebhard. N. Wicnienski, J. Org. Chem. 1986. S t . 4590-4594; d) U. Schmidt, H. Griesser.

ratio of 2-(1H-benzotriazol-l-yl)-l,1,3,3-tetramethyluronium

Tetruhedron Lett. 1986, 27, 163-166; e) R. Houssin. M. Lohez, J.-L. Bernier, and diisopropylethylamine (DIEA), and the reaction was men- itored for completion with ninhydrin. Due to problems in the J.-P. Henichart, J . Org. Chem. 1985. SO, 2787-2788; f) G. R. Pettit, P. S. Nel-

son, G. W. Holzapfe1,ihid. 1985. SO, 2654-2659; g) U. Schmidt, R. Utz, P. Gleich, Tetrahedron Lett. 1985,26,4367-4370; h) C. W Holzapfel. G . R. Pet- stepwise synthesis Of the glycine-rich N-terminal part, we used tit, J Org. Chem. 1985, SO. 2323-2327; i) M. W. Bredenkamp, C. W. Holzapfel. Fmoc-Gly-Gly-OH, Fmoc-Gly-Gly-Gly-OH, Fmoc-Ile3-Gly4- R. M. Snyman. W. J. van Zyl. Synth. Commun. 1992.22. 3029-3039. OH, and B o c - V ~ ~ ' - G ~ ~ ~ - O H in the appropriate positions

(Fig. 1). In particular, Boc-Val'-Gly'-OH, Fmoc-Ile3-Gly4- [8] S. Scheihye. B. S. Petersen, S. 0. Lawesson, Bull. Soc. Chini. Belg. 1978. 87, 229-238.

[9] R. C. Kelly. 1. Gebhard, N. Wicnienski. J . Org. Chern. 1986, S l , 4590-4594. [lo] R. J. Bergeron, J. S. McManis, J. B. Dionis. J. R. Garlich. J Org. Chern. 1985.

[ l l ] a) M. Reuman. A. I. Meyers, Tetrahedron 1985. 41. 837-860; h) K. Yonetani,

[I21 V. Pozdnev. Bioorg. Khini. 1977, 3, 1605-1610. [13] G. Videnov, D. Kaiser. M. Brooks, G. Jung, Angew. Chem. 1996. 108. 1607;

Angew Chern. lnt. Ed. Engl. 1996, 35, 1506. (141 D. Kaiser. G. Videnov, C . Maichle-Mossmer. J. Strlhle, G. Jung. unpub-

lished.

SO. 2780-2782.

Y. Hirotsu, T. Shibd, Bull. Chem. Soc. Jpn 1975, 3302-3305.

[*I Prof. Dr. G. Jung, Dr. G. Videnov, Dipl.-Chem. D. Kaiser, Dipl.-Biol. M. Brooks lnstitut fur Organische Chemie der UniversitPt Auf der Morgenstelle 18. D-72076 Tiihingen (Germany) Fax: Int. code +(7071)29-6925 e-mail: guenther.jung(ii'uni-tuehingen.de

project C2-Jung). [**I This work was supported by the Deutsche Forschungsgemeinschaft (SFB 323,

1506 0 VCH Verlagsgesellschafi mhH, 0.69451 Weinheim, 1996 0570-0833~96/3S13-1SO6 $ lS.OO+ .2S/0 Angex.. Chem. Int. Ed. Engl. 1996, 35, No. 13/14