lilt. .I Peptide Protein Res. 39, 1992, 273-377

“Carba” peptide bond surrogates Different approaches to Gly- $(CHZ-CHz)-D,L-Xaa pseudo-dipeptide units

MARC RODRIGUEZ. ANNIE HEITZ and JEAN MARTINEZ

CCIPE, Faculty of Pharmacy, Moiitpellier, France

Received 29 August, accepted for publication 10 November 1991

Raceniic “carba” pseudo-dipeptide units such as Gly-$(CH2-CH2)-D,L-Xaa were obtained either through the Horner-Emmons condensation of N-tevt.-butyloxycarbonyl-/3alaninal with the appropriate substituted triethyl phosphonoacetate, or from commercially available 3-carbethoxy-2-piperidone.

Ke!, words.: N-rerr.-but~loxycarbonyl-palaninal; “carba” peptide bond surrogates; 3-carbethoxy-2-piperidone; triethyl phosphonoacetate

During the last decade, isosteric peptide bond replace- ments have attracted considerable interest. Their intro- duction in a peptide sequence usually affords hormone analogues capable of exhibiting increased stability to- wards enzymatic degradation, thus providing pseudo- peptides showing a prolonged duration of action as compared to the parent peptide hormone (1). We have also shown some years ago that chemical modifications of a particular peptide bond in a sequence can lead to peptide antagonists (2), a strategy which has since been widely followed (3). Some of the common backbone modifications are of general use, such as the “retro” [ $(NH-CO)] or the reduced peptide bond [ $(CH2- NH)] approaches. The “carba” [ $(CHrCH2)] re- placement was applied in a limited number of struc- tures, and no general methodology was published until recently, although it has been demonstrated in some instances to be an excellent mimic of the peptide bond (4), i.e. in position 28-29 of cholecystokinin.

We recently reported on a general route leading to “carba” peptide bonds replacements, which involved the reaction of a N-Boc-protected /?-substituted p- aminoaldehyde with a phosphorous ylide (5 ) . As an example, we described the unambiguous synthesis of the two diastereomeric pseudodipeptides Boc-L-Phe- I)(CH~-CH~)-L-AI~-OH and Boc-L-Phe- $(CHTCH~)- D-Ala-OH. We further improved this methodology by the means of a Horner-Emmons reaction and synthe- sized the pseudo-dipeptide units Boc-L-Leu-$(CH2- C H ~ ) - L - P ~ ~ - O H and Boc-L-Leu- $ ( C H ~ - C H ~ ) - D - P ~ ~ - OH (6), “carba” analogues of one of the HIV protease

cleavage sites. In both examples, separation of the two diastereoisomers was made possible by the presence of two chiral centers and their identification was carried out on cyclic intermediates by NOE experiments.

We now want to describe different approaches lead- ing to “carba” pseudodipeptide units such as Boc-Gly- $(CH2-CHZ)-D,L-Xaa-OH. In this particular case, the molecules bear only one chiral center. Therefore, the resolution methodology that had been applied in our earlier work was not possible.

RESULTS AND DISCUSSION

The synthesis of the “carba” analogue of the dipeptide Boc-Gly-Phe-OH (which represents the 3-4 moiety of enkephalin), i.e. Boc-GI~-$(CH~-CH~)-D,L-P~~-OH 5 as a racemic mixture was carried out according to the already described methodology (6), as depicted in Scheme 1. Commercially available Boc-palanine was converted, through its N,O-dimethyl hydroxamate 1, according to Fehrentz & Castro (7). to the correspond- ing aldehyde 2, which was reacted with the sodium salt of ethyl 2-(diethylphosphono)-3-phenylpropionate (8) (generated in 1,2-dimethoxy-ethane with sodium hy- dride at a 0.3 M concentration) to lead to ( Z ) ethyl 5- (tert.-butyloxycarbonyl)amino-2-benzyl-pent-2-enoate 3 with a 75% yield. The Z structure was assigned upon a NOE between the benzylic and vinylic protons (Scheme 1). The presence of the E isomer could not be detected either by TLC or by NMR experiments. Hy- drogenation of ester 3 at room temperature and atmo-

213

M. Rodriguez et al.

BK-tiH A O H a B K NH

B K - N H ~ O E ~ 4

Boc-NH

CH,-C6H,

SCHEME 1

spheric pressure over palladium on charcoal afforded quantitatively Boc-Gly-$(CHl-CH?)-D.L-Phe-OEt 4 which led upon saponification with sodium hydroxide in 9504 ethanol to the “carba” pseudo-peptide Boc- Gly-$(CHl-CH?)-D.L-Phe-OH 5 as a solid (method

An alternative, more straightforward, synthetic path- way is illustrated in Scheme 2. Commercially available 3-carbethoxy-2-piperidone (Aldrich) was alkylated with benzyl bromide or isobutyl bromide in refluxing abso- lute ethanol in the presence of one equivalent of sodium ethoxide to lead respectively to racemic 3-benzyl-3- carbethoxy-2-piperidone 6 (80’” yield) and 3-isobutyl- 3-carbethoxy-2-piperidone 8 (40 9 , yield). The moder- ate yield obtained in the case of the isobutyl derivative was probably due to competitive dehydrohalogenation of isobutyl bromide. Attempts to introduce a substitu- tion in position 3 of the piperidone by the means of sodium hydride in T H F led predominantly to N - alkylation.

In order to improve the yield in the synthesis of com- pound 8, we alkylated 3-carbethoxy-2-piperidone with 3-chloro-2-methyl-propene to lead to compound 7 in a

A).

m S O E r a

0 0

Boc-NH 4 O H

CH2-C6H~

R = C112-C6H5 5 R = I B u 11

SCHEME 2

211

m%oEr

0 0

l c R = C H C H 5 9 R = iB:-18

65”” yield. In this way, no dehydrohalogenation could be expected as a side reaction. Compound 7 was con- verted to 3-carbethoxy-3-isobutyl-2-piperidone 8 by catalytic hydrogenation (palladium on charcoal). How- ever. since this reaction pathway implied two synthetic steps. it did not provide a significant improvement over direct introduction of the isobutyl substitution.

Saponification of compounds 6 and 8 followed by decarboxylation (neat free acid, 130” in \~uczm) afforded respectively racemic 3-benzyl-2-piperidone 9 and 3- isobutyl-2-piperidone 10 in cxcellent yields. Attempts to identify by ‘H-NMR the intermediate free acids were unsuccessful, as they quickly and extensively decarbox- ylated in DMSO at room temperature. Acid hydrolysis (2 h) of compounds 9 and 10, followed by treatment with (di-rev[.-butyl)-dicarbonate (Boc20) led to the N- protected racemic pseudo-dipeptides Boc-Gly-$(CH?- CH?)-D.L-Phe-OH 5 (identical to the one previously obtained) and Boc-GIy-$(CH,-CH*)-D,L-Leu-OH 11 (method B). Alternativcly these pseudo-dipeptides were obtained by direct treatment of compounds 6 and 8 with refluxing 6 N HCI for 48 h (decarboxylation oc- curred slowly in this medium) and subsequent N- protection (method C).

The N-protected “carba” dipeptide analogues could be obtained through much milder reaction conditions, according to the methodology introduced by Flynn er al. (9). For instance, [err.-butyloxycarbonylation of lac- tame 9 as described by Grehn et al. (lo), followed by alkaline hydrolysis led to BOC-GI~-$(CH?-CH~)-D,L- Phe-OH 5 in excellent yields (method D).

The methodologies presented in this paper allow the quick synthesis of racemic “carba” analogues of Gly- Xaa dipeptide units bearing a wide variety of side chains on the Xaa amino-acid moiety. All attempts to enzy- matically resolve (either by z-chymotrypsin or subtilisin Carlsberg in homogeneous or heterogeneous media) the Gly-$(CHz-CH,)-D,L-Xaa pseudo-dipeptides re- mained so far unsuccessful.

EXPERIMENTAL PROCEDURES

Melting points were taken on a Buchi apparatus in opcn capillary tubes. Elemental aiialyses were per- formed bq “Le Service de Microanalyses de YEN SCM” (Montpellier, France). Ascending TLC was performed on precoated plates of silica gel 60 F 254 (Merck) using the follouing solvent systems (by volume): A, AcOEti hexane, 3:7; B, AcOEt/hexane, 7:3; C, AcOEt; D, chloroform/methanol, acetic acid, 120: 10:5; E, II-

butano1,’acetic acid/water, 3 : 1: 1. The compounds were located with UV light (254 nm), charring reagcnt, nin- hydrin or chlorine/starch reagent. Column chroinatog- raphies were performed with silica gel 60,60-229 mesh, ASTM (Merck). Amino acids and derivatives were purchased from Propeptide (France) or Novabiochem (Switzerland); other chemicals were from Aldrich

"Carba" peptide bond surrogates

the mixture was stirred at room temperature until ab- sence of gas evolution (ca. 1 h). Aldehyde 2 (2.9g, 16.74 nimol) was then added and the mixture was stirred for 15 min at room temperature. The solvent was concentrated under reduced pressure and the res- idue dissolved in ethyl acetate. Work-up as described for compound 1 afforded an oil which was purified by silica gel column chromatography (eluent: solvent A). Yield 4.2 g (757,); Rf(A) 0.75; 'H-NMR ( D M s 0 - d ~ ) 6 ppm 7.30-7.10 (m, 5H, Ar), 6.91 (t, IH, 3J = 6.0 Hz, NH), 6.82 (t, 1H. 'J = 7.5 Hz, vinylic CH), 4.06 (q, 2H, 3J = 7.1 Hz, Et CH2), 3.62 (s, 2H, benzylic CH?), 3.06 (in, 2H, CH25), 2.40 (in, 2H, CH24), 1.37 (s. 9H, Boc), 1.15 (t, 3H, 3J = 7.1 Hz, Et CH3), a NOE between vinylic CH and benzylic CH2 suggests the Z structure.

(France). All reagents and solvents were of analytical grade. 'H-NMR experiments were performed on a Briiker WM 360 WB spectrometer at 293 K. Chemical shifts are given relative to the residual signal of DMSO- d6 (2.5 ppm). Resonance assignments are made by decoupling experiments and 2D spectra (COSY). NOES were measured in the difference mode. The following abbreviations were used: BOP, benzotriazolyloxytris- (dimethylamino) phosphonium hexafluorophosphate; DMF, dimethylformamide; DIEA, N,N-diisopropyl- ethylamine; NMM, N-methylmorpholine; THF, tet- rahydrofuran; TFA, trifluoroacetic acid. Other abbre- viations used were those recommended by the IUPAC- IUB Commission (European J . Biochem. 1984, 138, 9- 37).

Boc-PAla N,O-dimethvlhydroxumu~e (1) To a solution of Boc-PAlanine (4.0 g. 21.1 mmol) in DMF (30 mL) were successively added N,O-dime- thylhydroxylamine hydrochloride (2.44 g, 25 mmol), BOP (9.35 g, 21.1 mmol) and NMM (7.47 mL, 67.3 mmol) and the mixture was stirred for 3 h at room temperature. Ethyl acetate was then added (250 mL) and the solution washed with a saturated aqueous so- dium bicarbonate solution (3 x 80 mL), brine (100 mL), a 1 M aqueous potassium hydrogen sulphate solution (3 x 80 mL), brine (100 mL), dried over sodium sul- phate and concentrated in vuczio to leave the title com- pound as pale yellow oil. Yield 4.5 g (92",); Rf(B) 0.45; IH-NMR (DMSO-d6) 6 ppm 6.69 (t, lH, 3J = 5.6 Hz, NH), 3.65 (s, 3H, OCH3), 3.14 (m, 2H, CH?), 3.08 (s,

Boc). 3H, CH3), 2.52 (t, 2H, 3J = 7.0 Hz, CH?), 1.37 (s, 9H.

Boc-pulaninal(2) To a cold (0') solution of hydroxaniate 1 (4.3 g, 18.5 mmol) in anhydrous ether (50 mL) was added por- tionwise lithium aluminium hydride (1.05 g, 27.7 mmol) over a period of 15 min. After an additional 15 min, ethyl acetate (50 mL) was added cautiously, followed by a 1 M aqueous potassium hydrogen sulphate (KHSO4) solution (200 mL). After vigorous stirring for 15 min at room temperature, the organic layer was sep- arated. washed with brine (100 mL), dried over sodium sulphate, and concentrated under reduced pressure to afford compound 2 as an oil which was used in the next step without further purification. Yield 2.78 g (92%); Rf(B) 0.55; 'H-NMR (DMSO-d6) 6 ppm 9.62 (t, lH, 'J = 1.7 Hz, CHO), 6.84 (t. lH, 'J = 5.4 Hz, NH), 3.21 (m, 2H, CH2), 2.52 (td, 2H, 3J = 6.6 Hz and 1.7 Hz, CH?), 1.37 (s, 9H, Boc).

( Z Ethl'l 5-tert.-but~~loxycarbonvl-anzino)-2-benzyl-pent- 2-enoute (3) To a cold (0') solution of ethyl 2-(diethy1phosphono)- 3-phenylpropionate (8) (5.26 g, 16.74 mol) in dry 1.2- dimethoxyethane (50 mL), was added at once sodium hydride (60% in mineral oil) (670 mg, 16.74 mmol) and

Boc- Gly- gCH2- CH2)- D, L-Phe- OEt (4) Compound 3 (4.1 g, 12.3 mmol) was hydrogenated overnight in 95:, ethanol at room temperature and atmospheric pressure in the presence of IO", palladium over charcoal catalyst. The catalyst was removed by filtration, and the residue concentrated under reduced pressure to afford the title compound as a colourless oil. Yield 4.1 g (977,); Rf(A) 0.74; 'H-NMR (DMSO-d6) 6ppm7.30-7.11 (m,5H,Ar),6.74(t , 1H,3J=5.8Hz, NH), 3.97 (4, 2H, '5 = 7.1 Hz, Et, CH.), 2.88 (ni, 2H, HcrGly), 2.76 (dd, IH, 'J = 8.8, *J = 13.4 Hz, HPPhe), 2.72 (dd, lH, 3J = 6.1. 2J = 13.4 Hz, HP'Phe), 2.63 (m, lH, HrPhe), 1.49 (m, 2H, CH?), 1.37 (m, 2H, CH?), 1.37 (s, 9H, Boc), 1.05 (t. 'J = 7.1 Hz, Et CHI).

Boc- Gly- $(CH~-CH~)-D,L-P~~-OH (5)

Method A . Ester 4 (1.0 g, 2.98 mmol) was dissolved in 959, ethanol (5 mL) and treated with 1 N sodium hy- droxide (3.5 mL, 3.5 mmol). After 2 h stirring at room temperature, the mixture was diluted with water (50 mL) and extracted with ether (2 x 25 mL). The aqueous phase was acidified to pH 2 with 1 N potassium hqdro- gen sulphate and extracted with ethyl acetate (3 x 30 mL). The combined organic extracts were washed with brine (50 mL), dried over magnesium sulphate and concentrated under reduced pressure to leave an oil which solidified. It was recrystallized in a mixture of ether and hexane. Yield 890 mg (97",): m.p. 75-77"; Rf(D)0.55; 'H-NMR (DMSO-dh) 6ppin 12.06 (s, lH , COOH),7.30-7.13(m,5H,Ar).6.74(t,lH,'J=5.8Hz, NH), 2.88 (m, 2H, HxGly), 2.80 (dd, l H , 'J =8.8, ' J = 13.4Hz, HpPhe), 2.67 (dd, l H , 'J =6. l , .J = 13.4 Hz, HpPhe), 2.54 (m, lH, HcrPhe), 1.44 (ni, 2H, CHr), 1.39 (in, 2H, CH?), 1.36 (s, 9H, Boc).

Method B. 3-Benzyl-2-piperidone 9 (2.50 g, 13.2 mmol) was suspended in 6 N HC1 (40 mL), and the mixture was heated to reflux. After 2 h heating, the mixture was brought to pH 10 with 5 N sodium hydroxide, diluted with tert.-butanol (50 mL) and treated with (di-(err.- buty1)-dicarbonate (Boc.0) (4.3 g, 20 mmol). The pH

215

M. Rodriguez er cil.

of the reaction was maintained around 10 by addition of 1 N sodium hydroxide up to completion of the reac- tion. The mixture was then diluted with water (200 mL). washed with ether (2 x 50 m L ) and hexane (50 mL.1. the aqueous phase was acidified with 1 N potassium hy- drogen sulphate and extracted \vith ethyl acetate (3 x 80 mL). The combined organic layers were washed with 1 N potassium hydrogen sulphatc (50 i d , ) , brine. dried over sodium sulphate and concentrated under reduced pressure to afford a white solid that \ \ as re- crystallized from a mixture of ether and hexane. The compound was identical to the one previously obtained. Yield 3.42 (84";).

Method C. The title compound was obtained accor- ding to the procedure described above (method B) from 3-carbethoxy-3-benzyI-2-piperidone 6 (4.12 g. 15.8 mmol). 48 h reflux was necessary to bring lactanie hydrolysis to completion. Yield 3.24 g (87",,).

Method D . 3-Benzyl-2-piperidone 9 (1.89 g, 10.0 mmol) was dissolved in dry acetonitrile (10 m L ) and treated with 4-dimethylamino pyridine (122 mg, 1 nimol) and (di-rert.-butyl)-dicarbonate (Boc.0) (2.40 g, I 1 mmol). After 2 h stirring at room temperature the mixture was concentrated to dryness and the residue quickly filtered through a silica gel column (eluent ethyl acetate 'hexane 3:2). Pure fractions (Rf 0.92, solvent B) were pooled. concentrated under reduced pressure. and the oily rcs- idue treated in T H F (20 m L ) \vith 1 N aqueous sodium hydroxide ( I 5 mL, 15 mmol). After overnight stirring at room temperature. the title conipound was precipitated upon acidification with 1 N potassium hydrogen sul- phate. It was collected. washed \vith water and dried iti w c t m over phosphorous pentoxide. Yield 2.36 g (77",,).

3- Betizyl-3 -carhethox).-2 -piperid(itw (6) To a solution of sodium ethoxide. generated from so- dium (1.15 g, 50 mmol) in absolute ethanol (80 mL) was added at room temperature 3-carbethoxy-2- piperidone (Aldrich) (8.56 g. 50 mmol). After 5 min stirring, benzyl bromide (5.94 mL, 50 mmol) was added and the mixture was heated at 65" for 2 h. Afler cooling to room temperature, the milky suspension was poured into water (600 mL). The mixture was extracted with ethyl acetate (3 x 100 mL). the combined organic ex- tracts were washed with water (2 x 80 mL), dried over sodium sulphate and concentrated under reduced pres- sure to afford a residue which was crystallized in a mixture of ether and hexane. Yield 10.45 g ( S O ' , ) ; m.p. 51-52'; Rf(F3) 0.35: 'H-NMR (DMSO-d6) 3 ppm 7.70 (broad s. 1H. NH). 7.30-7.10 (m, 5H. Ar). 4.12 (m. 2H. Et CH?) , 3.33 and 2.96 (d. 1H each. 'J = 13.4 Hz, benzylic CH?), 3.02 and 2.77 (m, IH each. H6. H6'), 1.94 and 1.67 (m. 111 each, H4. H4') . 1.61 and 1.39(m. 1H each, H5. H5') . 1.18(t. 311. 'J = 7.1 Hz. Et CH2).

276

3~2-Me~li~~l-pn~y-2-eti~~~1~-3-c.~1~betho.~y-2-piperidotie (7) To a solution of sodium ethoxide. generated from so- dium (0.575 g. 25 mmol) in absolute ethanol (40 mL) \vas added at room temperature 3-carbethoxy-2- piperidone (Aldrich) (4.28 g, 25 mmol). After 5 niin stirring. 3-chloro-2-methyl-propenc (4.94 mL, 50 nimol) and sodium iodide (100 mg) were added and the mix- ture was heated at 70' for 2 h. After cooling to room temperature. the milky suspension was poured into \vater (600 mL). The mixture was extracted with ethyl acetate (3 x 100 mL). the combined organic extracts were Lvashed with water (2 x 80 mL). dried over sodium sulphate and concentrated under reduced pressure to afford a residue which was crystallized upon drying. Yield 3.23 g(57",,); n1.p. 47-49"; Rf(B)0.35; 'H-NMR (DMSO-d(,) b ppm 7.69 (broad s, 1H. NH), 4.84 and 4.76 (In, 1 H each, vinylic CH?) , 4.09 (m, 2H. Et CH.). 3.13 (m. 2H. H6. €16'). 1.93 and 1.74 (m, 1H each. H5, 11.5'). 1.81-1.60 (m, 211. H4, 114'), 2.72 and 2.49 (d. 1H each. 'J = 13.7 Hz. allylic CH?), 1.63 (s, 3H, CH?), 1.17 (t. 3H. 'J = 7.1 Hz, Et CH?) .

kfetliorl A . Synthesized as described above from 3- carbethoxy-2-piperidone (4.28 g. 25 mmol) and isobu- tylbromide (5.43 niL. 50 mmol) in the presence of so- dium iodide traces. Silica gel column chromatography (eluent: solvent €3) afforded the title compound as an oil. Yield 2.2 g (40",,); Rf(B) 0.35; 'H-NMR (DMSO- dt,) b ppni 7.56 (broad s. IH, NH), 4.06 (m, 2H, Et CII.), 3.14 ( i n , 2H. H6. H6'). 2.04 and 1.73 (ni, I H each. H.5. H.5'), 1.82-1.62(m.2H, H4.H4') , 1.82-1.62 (m. 2H. C H ? iBu). 1.71 (in. IH, C H iBu), 1.16 (t. 3H. ?J = 7.1 Hz. Et CHj), 0.87 and 0.84 (d, 3H each, ?J =6.3 H z . C H ? iRu).

Method B . Compound 7 (3.00 g, 13.3 mmol) was hy- drogenated in ethanol (100 niL) overnight at room tem- perature under a pressure of 5 atm, in the presence of a lo", Pd,X catalyst. The catalyst was removed by filtration and the solvent was removed under reduced pressure t o afford the title compound in a 890; yield, identical to the one obtained through method A.

3- Bet i~~, l -~-piperif lot ie I 9) Compound 6 (4.3 g, 16.45 mmol) was dissolved in 950;) ethanol (20 mL) and treated with 2 N aqueous sodium hydroxide (10 mL. 20 mmol). When no starting mate- r i d could be detected by TLC, the mixture was poured into water (200 mL) containing concentrated hydro- chloric acid (3 mL). The precipitate which formed was collected by filtration. thoroughly washed with water, hexanc and dried it1 inc'uo.

The solid was heated it? i u w o without solvent at 130" for 5 min. At that time, no evolution of carbon dioxide was any longer detectable. Crystallisation occurred upon cooling at rooni temperature. The solid was col-

“Carba” peptide bond surrogates

2. Martinez, J., Bali, J.P.. Magous, R.. Law. J . , Lignon. M.F., Rodriguez. M. & Castro. B. (1985) C R Acurf. Sc;. P0ri.c. 300 (SCrie 11). 437-440; Martinez, J., Bali. J.P., Rodriguez. M.. Cas- tro. B., Magous. R., Laur, J . & Lignon, M.F. (1985) J . Med. Cheni. 28, 1874-1879; Mendre, C., Rodriguez, M.. Gueudct. C., Lignon, M.F., Galas, M.C., Laur, J.. Worms, P. & Martinez, J. (1988) J. Bid. Chern. 263, 10641-10645

3. Coy, D.H., Heinz-Erian. P., Jiang, N.Y., Sasaki, Y.. Taylor. J.. Moreau, J.P., Wolfrey, W.T., Gardner. J.D. & Jensen. R.T. (1988) J . B id . Chern. 263, 5056-5060; Haffar, B.M., Hocart, S.J., Coy, D.H.. Mantey, S., Chiang, H.C.V. & Jcnsen, R.T. (1991)J. Bid. C!iern. 266, 316-322: Hocart. S.J.. Murphy. W . A . &Coy, D.H. (1990)J. Med. Chern. 33. 1954-1958; Quian,J.M.. Coy, D.H. Jiang. N.Y., Gardner, J.D. & Jensen, R.T. (1989) J . Biol. Chem. 264, 16667-16671

4. Mendre, C., Rodriguez, M., Lignon, M.F., Galas, M.C.. Worms, P. & Martinez, J . (1990) Eitropeari J . Phrrrriiocol. 186, 213-222

5. Rodriguez, M., Aumelas, A. & Martinez, J. (1990) Terrrrhedrori Lett. 31. 5153-5156

6. Rodriguez, M., Heitz. .4. &Martinez. J . (1990) Terrtrlredrori Lerr. 31. 7319-7322

7. Fehrentz, J.A. & Castro, B. (1983) Synthesis, 676-678 8. Villeras. J . & Rambaud, M. (1983) SFnthesis. 406-408, and ref-

9. Flynn, D.L., Zelle, R.E. & Grieco, P A . (1983)J. Org. Chern. 48,

10. Grehn. L.F., Gunnarsson, K. & Ragnarsson, U. (1986) Acrrr

erences therein

2124-2426

Chem. Scund. B40, 745-750

lected, washed with hexane to afford the title com- pound. Yield 2.65 g(85’4) ; m.p. 115-1 16”; Rf(C)0.30; ‘H-NMR (DMSO-d6) 6 ppm 7.40 (broad s, IH, NH), 7.31-7.14 (m, 5H, Ar), 3.16 and 2.59 (dd, 1H each, ‘J = 3.9 and 9.8 Hz respectively, ’J = 13.4 Hz, benzylic CH’), 3.15-2.99 (m, 2H, H6, H6‘), 2.38 (m, lH, H3), 1.69 and 1.51 (m, 1H each, H5, H5’), 1.58 and 1.30 (m, 1H each, H4, H4’).

3 - Is0 hi rtjd-2 -piperidone (1 0) Synthesized from compound 8 (2.10 g, 9.24 mmol) as described for compound 9. Yield 1.25 g (87 ”/); m.p. 97- 99‘; Rf(C) 0.25; ‘H-NMR (DMSO-d6) 6 ppm 7.25 (broad s, lH, NH), 3.10 (m, 2H, 3J = 4.9 Hz, H6, H6’), 2.09(m, lH, H3), 1.85 and 1.34(m, 1H each, H4, H4’), 1.70 and 1.58 (in, 1H each, 3J = 3.0 Hz and 3.2 Hz respectively, H5, H5’), 1.67 (m, lH, C H iBu), 1.63 and 1.16 (m. 1H each, CH2 iBu), 0.88 and 0.83 (d, 3H each, 3J = 6.4 Hz, CH3 iBu).

Boc-Glj~-$(CH2-CH~)-D,L-Leu-OH (11) The title compound was obtained from 3-isobutyl-2- piperidone 10 (1.24 g, 7.99 mmol) as a pale yellow oil according to method C described above. Yield 2.13 g (98”,); Rf(D) 0.57; ’H-NMR(DMSO-d6) 6ppm 11.97 (s, lH,COOH),6.70(t, 1H,3J=5.7Hz,NH),2.89(m, 2H, HcrGly), 2.54 (m, lH, HaLeu), 1.51 (m, lH, HyLeu), 1.46 (m, lH, HPLeu), 1.36 (2m, 4H, 2 CH:!), 1.36 (s, 9H, Boc), 1.15 (m, lH, 3J=8.1 Hz, 2J= 13.0Hz, Hp’Leu), 0.86 and 0.84 (d, 3H each, 3J = 6.3 Hz, H6Leu).

REFERENCES

1. Spatola, A.F. (1983) in Chernistrj and Biorheniistry of Amino acid^. Peptides arid Proreiris (Weinstein B. ed.), Vol. 7, pp. 267- 357, M. Dekker, New York

Address:

Dr. Jeatr Martinez CCIPE, Faculte dc Pharmacie 15, Avenue Charles Flahaut 34060 Montpellier Ccdex France Tel. (33)67040183 Fax (33)67 5475 33

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