J. CHEM. SOC. PERKIN TRANS. i 1995
`
`1251
`
`Synthesis of all four diastereoisomers of 4-(carboxymethyl)proline,
`a conformationally constrained analogue of 2-aminoadipic acid
`Roberto Pellicciari,**" Loredana Arenare: Paolo De Caprariis," Benedetto Natalini,"
`Maura Marinozzi" and Alessandro Gallic
`a Istituto di Chimica Farmaceutica e Tecnica Farmaceutica, Universita degli Studi di Perugia, Via del Liceo I ,
`06 I23 Perugia, Italy
`Dipartimento di Chimica Farmaceutica e Tossicologica, Universita degli Studi di Napoli "Federico II",
`Via D. Montesano 49, 80131 Napoli, Italy
`Dipartimento di Farmacologia Preclinica e Clinica, Universita degli Studi di Firenze, Viale Morgagni 65, 50134 Firenze,
`Italy
`
`The dirhodium(r1) tetraacetate catalysed reaction of ethyl diazoacetate with 2,3-dihydropyrrole-2,2-dicarboxyl-
`ate 5 afforded the useful 2-azabicyclo[3.1 .O]hexane derivative 6. Its conversion into the proline-y-acetic acid
`equivalent 9 as well as into the four isomers constituting the 4-(carboxymethy1)proline 13 (16a-19a) whose
`absolute configuration was established by an alternative asymmetric synthesis of two of them is described.
`Freliminary data concerning the affinity of compounds 16a-19a for the NMDA site of the NMDA receptor
`complex are also reported.
`
`As part of on-going studies directed to the synthesis and
`biological evaluation of conformationally restricted amino acid
`analogues, we report here the preparation of compounds
`16a-l9a, the four possible isomers of the non-proteinogenic
`amino acid 4-(carboxymethy1)proline 2, which incorporate the
`(2R)- and (2s)-aminoadipic acid moieties C(2R)- and (2S)-AA].
`The (2R)-isomers 16a and 17a, in particular, are partially
`constrained analogues of (2R)-AA 1, a selective competitive
`antagonist for the NMDA receptor site of the NMDA receptor
`complex. Previous attempts to reduce the conformational
`mobility of 1 in order to achieve energetically improved inter-
`actions with the NMDA receptor site have already been re-
`ported. Interestingly, (2R,4R,5R)-2-amino-4,5-methanoadipic
`acid 3, a compound of this class, has been shown to be a selective
`
`Cbz
`
`4
`
`bbz
`
`7
`
`E'02cQ
`EtO2C
`
`I
`Cbz
`
`0
`II
`CbZ = OCCH2Ph
`
`5
`
`I I ii
`
`t
`
`Cbz
`
`6
`
`t
`
`mco2HA
`
`EtO2C
`
`N
`
`HO2C-0
`
`NMDA receptor site partial agonist, endowed with promising
`biological properties.
`For the preparation of compounds 16a-19a our attention
`was focused on the utilization of the 2-azabicycloC3.1 .O]hexane-
`3,3,6-tricarboxylate 6, prepared as outlined in Scheme 1 .4
`Condensation of diethyl N-Cbz-aminomalonate with acrolein
`in a benzene solution of sodium ethoxide gave the known
`diethyl 1 -Cbz-5-hydroxypyrrolidine-2,2-dicarboxylate 4 (82%
`yield), the dehydration of which with P 2 0 5 in refluxing benzene
`for 2 h furnished the corresponding diethyl l-Cbz-2,3-dihy-
`dropyrrole-2,2-dicarboxylate 5 in 35% yield. Dirhodium(I1)
`tetraacetate catalysed decomposition of ethyl diazoacetate in
`the presence of 5 (CH,Cl,, RT, 12 h) afforded the 2-
`azabicyclo[3.1 .O]hexane derivative 6 as a ca. 1 : 1 mixture of em
`and endo forms in 46% yield.6
`The utilization of 6 for the preparation of the synthetically
`useful lactone 9, containing a y-imino carbonyl system in
`masked form, has been the object of a preliminary communic-
`a t i ~ n . ~ This conversion was achieved by submitting 6 to partial
`hydrolysis (0.5 mol dm-3 NaOH, H,O-MeOH, RT, 5 h, 87%
`yield) followed by monodecarboxylation of the resulting
`
`I
`Cbz
`
`I
`Cbz
`
`8
`9
`Scheme 1 Reagents and conditions: i, P,O,, C,H,, reflux (35%);
`ii, EDA, RhJOAc),, CH,Cl,, RT (46%); iii, 0.5 rnol dm NaOH
`(H,O-MeOH), RT (87%); iv, PhMe, reflux (75%); v, 6 mol dm-3 HCl
`(H,O-dioxane, 5 : l), RT (87%)
`
`dicarboxylate 7 (PhMe, reflux, 12 h) and, finally, by acidic
`treatment (6 mol dm-3 HCl, RT, 72 h, 87% yield) of the result-
`ing cyclopropyl derivative 8.
`We report now a new synthetic transformation of 6 which
`allows the entry into the title compounds. Indeed, treatment of 6
`with hydrogen in the presence of 10% Pd-C in MeOH at room
`temperature for 1.5 h leads to reductive cleavage of the
`cyclopropyl moiety with the consequent formation of 10 (98%
`yield), which was sequentially submitted to Cbz protection,
`partial saponification (0.5 mol dm-3 NaOH, H20-MeOH, RT,
`5 h) and thermal monodecarboxylation (PhMe, 15 h) of the
`dicarboxylic acid 12 to give the desired 4-(carboxymethy1)-
`proline derivative 13 with an overall yield of 62.5% starting from
`
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`
`1252
`
`J. CHEM. SOC. PERKIN TRANS. I 1995
`
`Cbz
`
`13
`
`vii, vii
`
`i X . X l _ c
`
`16 R=Et,R’=Mnt
`16a R=R’=H
`
`*
`
`Cbz
`
`14
`
`13 -
`v, v
`
`H
`
`i X , X l
`
`17 R=Et,R’=Mnt
`
`17a R = R ’ = H
`
`-
`
`#
`
`
`
`vii, viii
`
`.:hMe
`
`Cbz
`
`15
`
`Mnt=
`
`i X , X l
`
`18 R=Et,R’=Mnt
`
`18a R = R ’ = H
`
`H
`
`i X J L ,
`
`19 R = Et, R’ = Mnt
`19a R = R ’ = H
`
`I
`
`M e
`Scheme 2 Reagents and conditions: i, H,, 10% Pd-C, MeOH (98%); ii,
`CbzCI, NaHCO, (9704); iii, 0.5 mol dm NaOH (H,&MeOH), RT
`(90%); iv, PhMe, reflux (73%); v, (+)-menthol, DCC, DMAP, CH,Cl,-
`DMF (1 : l), 76 h; vi, MPC (14: 63%, 15: 6.7%); vii, H,, 10% Pd-C,
`MeOH; viii, MPC (16: 42%, 17: 32%, 18: 38%, 19: 24%); ix, 6 mol dm
`HCl, reflux, 14 h; x, Dowex, 50 x 2-200 (16a: 86%, 17a: 98%, 18a: 84%,
`19a: 83%)
`
`Table 1
`16a-19a
`
`Selected physicochemical properties of the four isomers
`
`16a
`17a
`18a
`19a
`
`224-225
`23 2-2 3 3
`2 2 8-229
`234-235
`
`+ 66.8
`+ 34.2
`- 66.5
`- 33.4
`
`6. HPLC analysis of 13 showed that it was a mixture of all the
`four possible isomers, whose resolution required the preparation
`of the corresponding (+)-menthy1 e ~ t e r s , ~ as shown in.Scheme
`2. Thus, esterification of 13 with (+)-menthol in CH,CI,-DMF
`(1 : 1) in the presence of DCC and DMAP for 76 h afforded the
`corresponding (2R) and (2s) diastereoisomeric compounds 14
`and 15, respectively, which were separated by medium pressure
`chromatography (MPC) in 63 and 7% yield, respectively. The
`(2R) derivative 14 was then submitted to catalytic hydro-
`
`genolysis (10% Pd-C, MeOH, RT, 2.5 h) followed by MPC to
`afford the two esters (2R,4R)-16 and (2R,4S)-17 (30 and 24%
`yield, respectively). Analogously, the (2s) derivative 15 afforded
`the two esters (2S,4S)-18 and (2S,4R)-19 in 38 and 25% yield,
`respectively.
`Finally, each of the four isomeric esters 16-19 was submitted
`to acidic hydrolysis (6 mol dm-3, HCl, reflux, 12 h) followed
`by cation exchange resin chromatography eluting with 10%
`pyridine in water. In this way, ( + )-(2R,4R)-4-(carboxymethyl)-
`( + )-(2R,4S)-4-(carboxymethyl)proline 17a,
`proline 16a,
`( - )-(2S,4S)-4-(carboxymethyl)proline 18a and ( -)-(2S,4R)-
`4-(carboxymethy1)proline 19a were obtained in 86, 96, 87 and
`9 1 % yield, respectively.8 Relevant physicochemical properties
`and I3C NMR data of the four isomers 16a-19a are reported
`in Tables 1 and 2, respectively.
`The assignment of the absolute configurations to the four
`isomers 16a-19a was made possible by the enantiomerically
`pure compound (EPC) synthesis of two of them (Scheme 3).
`
`ii, iii -
`
`19a
`
`H
`
`21
`t
`
`20
`
`V
`
`I
`
`22 R = H , R ’ = B ~
`23 R = Cbz, R’ = Bu‘
`vi L,
`
`24 R=Cbz,R’=H
`vii
`25 R = Cbz, R’ = Mnt
`
`viii. ix
`
`R ,.-CO2R‘
`
`1
`s6c02R‘
`
`R02C”-
`
`1
`H
`
`RQC‘*- sQ
`
`I
`H
`
`ii
`
`ii
`
`27 R = Me, R’ = Mut
`26 R = Me, R‘ = Mnt
`19a R = R ‘ = H
`18a R = R ’ = H
`Scheme 3 Reagents and conditions: i, H,, PtO,, AcOH, 50 psi (55%);
`ii, 6 rnol dm-3 HCl, 14 h, reflux; iii, Dowex 50 x 2-200 (19a: 70%, 18a:
`88%); iv, H,, 10% Pd-C, AcOH, 40 psi (98%); v, CbzCl, NaHCO,
`(99%); vi, CF,CO,H, CH,CI,, 6 h, RT (72.5%); vii, (+)-menthol,
`(1 : l), 76 h (78%); viii, H,, 10% Pd-C,
`DCC, DMAP, CH,CI,-DMF
`MeOH; ix, MPC (26: 18%, 27: 26.5%)
`
`Thus, Peterson condensation of the protected S-4-oxoproline
`with tert-butyl a-trimethylsilylacetate afforded the correspond-
`ing (2S)-diester 209 (34% yield) which was submitted to
`catalytic hydrogenation in the presence of PtO, in acetic acid
`(50 psi,? 10 h) to give stereospecifically the known (2S,4R)-
`diester 21 in 55% yield.g Acidic hydrolysis of 21 followed
`by cation exchange resin chromatography eluting with 10%
`pyridine in water afforded (-)-(2S,4R)-4-(carboxymethyl)-
`proline 19a (70% yield), identical with that already reported
`in the literature.*
`t 1 psi = 6.89 kPa.
`
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`

`
`J. CHEM. SOC. PERKIN TRANS. 1 1995
`3C NMR data of the four isomers Ma-19a
`
`Table 2
`
`a
`
`16a
`
`17a
`
`18a
`
`19a
`
`C-1
`C-2
`c- 3
`c-4
`c- 5
`C-6
`C-7
`
`176.44
`60.79
`34.39
`33.39
`50.14
`36.43
`174.24
`
`177.17
`61.97
`35.46
`35.34
`50.78
`37.26
`174.82
`
`177.03
`61.55
`35.15
`34.13
`50.90
`37.06
`174.93
`
`177.84
`62.06
`35.71
`35.46
`50.85
`37.83
`174.97
`
`Table 3
`Displacement of NMDA specific ~-[~H]glutamate binding to
`rat cortical membranes by the four isomers 16a-19a
`
`Compound
`
`IC,, (pmol dm-')"
`
`L-Glu
`NMDA
`I6a
`17a
`18a
`19a
`
`0.05 2 0.005
`0.31 2 0.04
`202 k 43
`8.0 k 2.0
`8.1 k 1.4
`1.7 2 0.65
`
`" IC,, values were calculated from inhibition curves based on 4-6
`different concentrations of the compounds using the ALLFIT com-
`puter program. Values are the mean k SEM of 3-4 separate determin-
`ations.
`
`Conversely, catalytic hydrogenation of 20 with 10% Pd-C
`in acetic acid (40 psi, 12 h) afforded 22 as an inseparable
`mixture of the corresponding (2S,4R)- and (2S,4S)-diesters
`which was resolved via the formation of the corresponding ( +)-
`menthyl esters. Thus, Cbz protection of 22 under mildly
`basic conditions followed by acidic hydrolysis (CF,CO,H,
`CH2Cl,, RT, 6 h) of the N-Cbz ester 23 afforded the
`corresponding acid 24 (72.5% yield) which was esterified with
`(+)-menthol
`to give a mixture of
`the corresponding
`diastereoisomeric esters 25 (56% yield). Removal of the N-Cbz
`group from 25 followed by MPC afforded the two diesters
`(2S,4S)-26 and (2S,4R)-27 in 18 and 26.5% yield, respectively,
`which were then submitted to acidic hydrolysis (6 mol dm-3
`HC1) and cation exchange resin chromatography (10%
`pyridine in water) to yield the enantiomerically pure (-)- (2S,
`4S)-4-(carboxymethyl)proline 18a {[a]k2 - 66.5 (c 0.5, H20))
`and ( -)-(2S,4R)-4-(carboxymethyl)proline 19a ([a]k2 - 33.4
`(c. 0.35, H,O)) in 88 and 91% yield, respectively. Since the two
`remaining isomers 16a and 17a are enantiomers of 18a and
`19a, respectively, the examination of their optical rotations (see
`Table 1) made the assignment of their absolute configuration
`straightforward.
`
`Biological results
`A preliminary evaluation of the affinity of the four isomers of
`4-(carboxymethy1)proline 16a-19a at the NMDA site of the
`NMDA receptor complex is reported in Table 3. As previously
`shown,",'
`the NMDA-sensitive L-[~H]GIu binding to rat
`cortical membranes is almost exclusively due to the interaction
`of the ligand with the NMDA receptor site. The results clearly
`indicate that none of these compounds met the goal of a potent
`NMDA ligand. However, the higher binding potency of the two
`cis-4-(carboxymethyl)prolines 17a and 19a in comparison with
`
`1253
`
`the trans compounds 16a and 18a, can be explained with the
`suitable geometry of the former compounds which fits the
`pharmacophoric requirements for the interaction with the
`NMDA site of the NMDA receptor complex.'" In particular,
`the cis-S-proline derivative 19a exhibited an IC,o value
`approximately five times lower than that reported for the cis-R-
`proline derivative 17a (see Table 3).
`In conclusion, we report here the preparation of the 2-
`azabicyclo[3.1 .O]hexane derivative 6, a versatile synthetic
`intermediate, and its exploitation for the preparation of the
`proline-y-acetic acid equivalent 9 and for the synthesis of the
`four possible stereoisomers of 4-(carboxymethyl)proline, a
`conformationally constrained 2-aminoadipic acid analogue.
`The compounds 16a-19a have been submitted to binding
`studies in order to test their affinity to the NMDA site of the
`NMDA receptor complex; full biological characterization is in
`progress and the results will be reported in due time.
`
`Experimental
`Mps were determined on a Kofler micro-hot-stage apparatus
`and are uncorrected. IR spectra were recorded on a Perkin-
`Elmer 1320 spectrometer. ' H and 3C NMR spectra were taken
`on a Bruker AC 200 spectrometer and the chemical shifts are
`reported on the 6 scale relative to tetramethylsilane. Specific
`rotations were recorded on a Jasco Dip-360 digital polarimeter
`and are given in units of lo-' deg cm2 g-'. HPLC was carried
`out on a Waters Delta Prep 3000 system equipped with a 484
`UV-Visible detector and a computerized acquisition data
`system (Baseline 8 1 OTM). Analytical reverse-phase HPLC was
`performed on a Beckman RP-C18 Ultrasphere (25 cm, 4.6 mm,
`5 pm spherical silica, 80 A pore). Gas chromatographic analyses
`were performed on a Hewlett-Packard HP 5890-11 system
`[column and conditions: Supelco SPTM - 2250, 30 m, 0.25 mm
`ID, 0.20 pm f.t., 19O(5')/29O0C, 10"C/min] equipped with a
`flame ionization detector and a HP 3394A recorder-integrator.
`Combustion analyses were performed on a 1102 Automatic
`Analyzer, Carlo Erba (Italy). Flash chromatography was
`performed on Merck silica gel (0.040-0.063 mm). Medium
`pressure chromatography (MPC) was performed on Merck
`LiChroprep Si 60 (0.040-0.063 mm, lobar columns). Ether refers
`to diethyl ether. 10% Pyridine refers to 10% pyridine in water.
`
`Diethyl l-benzyloxycarbonyl-5-hydroxypyrrolidine-2,2-
`dicarboxylate 4
`Sodium ethoxide (10.2 g, 0.15 mol) was added to a stirred
`solution of diethyl N-benzyloxycarbonylaminomalonate (30.0
`g, 97 mmol) in anhydrous benzene (200 cm3). After 15 min,
`acrylaldehyde (5.6 g, 100 mmol) was added dropwise to this
`mixture over 20 min and stirring was then continued for 12 h.
`Evaporation of the solvent gave a residue (33 g) which was
`submitted
`to flash chromatography, eluting with
`light
`petroleum-ether (1 : 1) to afford the title compound as a pale
`yellow oil (29.0 g, 82%); 6,(200 MHz; CDCI,) 1 .OO-1.40 (6 H, 2
`m, 2 x CH,CH,), 1.80-2.10 (2 H, m, 4-H,), 2.35-2.75 (2 H, m,
`3-H,), 3.40(1 H, brs,OH), 3.90-4.30(4H,2m, 2 x CH2CH3),
`5.05 (2 H, s, CH2Ph), 5.55-5.70 (1 H, br s, 5-H) and 7.20 (5 H,
`m, ArH).
`
`Diethyl1-benzyloxycarbonyl-2,3-dihydropyrrole-2,2-
`dicarboxylate 5
`Phosphorus pentoxide (9.0 g, 8 1 mmol) was added to a solution
`of the pyrrolidine 4 (29.0 g, 79 mmol) in anhydrous benzene
`(200 cm3) and the resulting mixture was refluxed for 2 h.
`Filtration of the reaction mixture and evaporation of the
`solvent gave a residue (1 1 g) which was submitted to flash
`chromatography, eluting with light petroleum%ther (4 : 6), to
`give the title compound (9.62 g, 35%); 6,(200 MHz; CDCI,)
`
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`

`
`1254
`
`J. CHEM. SOC. PERKIN TRANS. i 1995
`
`Triethyl2-benzyloxycarbonyl-2-azabicyclo [ 3.1 .O] hexane-
`3,3,6-tricarboxylate 6
`A solution of ethyl diazoacetate (13.12 g, 115 mmol) in
`anhydrous dichloromethane (250 cm3) was added dropwise over
`12 h to a magnetically stirred solution of the olefin 5 (9.62 g, 28
`mmol) in anhydrous dichloromethane (1 00 cm3) containing
`Rh,(OAc), (1.34 g, 3 mmol) under argon at room temperature.
`Evaporation of the solvent gave a residue (6.9 g) which was
`submitted
`to flash chromatography, eluting with
`light
`petroleum-ether (1 : l), to give the title compound as a pale
`yellow oil (5.6 g, 46%); dH(200 MHz; CDCl,) 1.30 (9 H, m,
`3 x CH,CH,), 1.90 and 2.20 (2 H, 2 m, 3-H,), 2.75 (1 H, m, 4-H),
`2.95 (1 H, 2 d, J 7 , CHCO,Et), 3.954.35 (7 H, m, 3 x CH,CH,
`and SH), 5.15 (2 H, 2 s, CH,Ph) and 7.30 ( 5 H, m, ArH).
`
`l.lOand1.25(6H,2t,J7.5,2 x CH2CH,),3.20(2H,d,J15,
`Diethyl4- [ (ethox ycarbony1)methyll pyrrolidine-2,2dicarboxyl-
`3-H,), 4.00 and 4.25 (4 H, 2 q, J 7.5, 2 x CH,CH,), 4.90 (1 H,
`ate 10
`d,J15,4-H),5.12(2H,2~,CH,Ph),6.60(1H,d,J30,5-H)
`Hydrogen was bubbled for 1.5 h into a magnetically stirred
`and 7.30 ( 5 H, m, ArH).
`suspension of compound 6 (5.6 g, 13 mmol) in methanol (100
`cm3) containing 10% Pd-C (1.7 g) at room temperature.
`Filtration of the mixture through Celite and evaporation of the
`filtrate gave the title compound as a yellow solid (3.86 g, 98%),
`mp 80-81 "C; purity was 99% by GC analysis; dH(200 MHz;
`CDCl,) 1.25 (3 H, t, J 7, CH,CO,CH,CH,), 1.33 (6 H, t,
`2 x CH,CH,), 2.30 and 2.90 (2 H, 2 m, 3-H,), 2.55 (2 H, m,
`CH,CO,Et), 2.85 ( I H, m, 4-H), 3.32 and 3.88 (2 H, 2 m, 5-H,),
`4.12 (2 H, q, J 7, CH,CO,CH,CH,),
`4.35 (4 H, m, 2 x
`CH,CH,).
`Diethyl 1-benzyloxycarbonyl-4-[ (ethoxycarbonyl)methyl] -
`pyrrolidine-2,2-dicarboxylate 1 1
`Benzyl chloromethanoate (1.9 cm', 13.4 mmol) was added
`dropwise in 10 min under vigorous magnetic stirring to cold
`(0°C) saturated aqueous NaHCO, (70 cm3) containing the
`pyrrolidine 10 (3.86 g, 12.8 mmol). Stirring was continued for 12
`h at room temperature after which the reaction mixture was
`extracted with ethyl acetate ( 3 x 20 cm3). The combined
`organic phases were dried (Na,SO,) and evaporated and the
`residue upon flash filtration on silica gel afforded the title
`compound as an oil (5.43 g, 97%); dH(200 MHz; CDCI,) 1 .OO-
`1.30 (9 H, m, 3 x CH,CH,), 2.15 and 2.78 (2 H, 2 m, 3-H,), 2.40
`(2 H, m. CH,CO,Et), 2.58 (1 H, m, 4-H), 3.23 and 3.90 (2 H, 2 m,
`5-H,), 4.00-4.30 (6 H, m, 3 x CH,CH,), 5.05-5.15 (2 H, m,
`CH,Ph) and 7.20-7.40 ( 5 H, m, ArH).
`
`2-Benzyloxycarbonyl-3-ethoxycarbonyl-2-azabicyclo-
`[3.1 .O] hexane-3,6dicarboxylic acid 7
`A solution of the triester 6 (0.320 g, 0.74 mmol) in aqueous
`sodium hydroxide (0.5 mol dm-,; water-methanol 1.4: 1) was
`magnetically stirred for 5 h at room temperature. After
`controlled ( t d 30°C) evaporation of the methanol, the
`reaction mixture was diluted with water (1 5 cm3), acidified with
`6 rnol dm-3 HCI to pH 4 5 and extracted with chloroform
`(3 x 10 cm3). The combined extracts were dried (Na2S0,) and
`evaporated and flash filtration of the residue on silica gel
`yielded the title compound (0.244 g, 87%); 6,(200 MHz; CDCI,)
`1.20-1.35 (3 H, m, CH,CH,), 2.2@-2.55 (3 H, m, 3-H, and 4-
`H), 3.45 (1 H, m, CHCO,H), 3.95 (I H, m, 5-H), 4.25 (2 H, m,
`CH,CH,), 5.30 (2 H, s, PhCH,), 7.35 (5 H, m, ArH) and 9.70 (2
`H , b r s , 2 x C0,H).
`
`1 -Benzyloxycarbonyl-~carboxymethyl-2-ethoxycarbonyl-
`pyrrolidine-2-carboxylic acid 12
`The triester 11 (5.43 g, 12.5 mmol) was added to a solution of
`NaOH (4.57 g, 114 mmol) in water-methanol (200 cm3; 6:4)
`and the resulting mixture was stirred for 5 h at room
`temperature. The methanol was carefully removed ( t < 30 "C)
`and the resulting aqueous solution was acidified (to pH 4-5)
`with 6 mol dm-, HCl and then extracted with chloroform
`(3 x 60 cm3). The combined organic phases were dried
`(Na,SO,) and evaporated and flash filtration of the residue on
`silica gel afforded the title compound as an oil (4.25 g, 90%);
`dH(200 MHz; CDCl,) 1.20 (3 H, t, J 6, CH,CH,), 2.20-2.80
`( 5 H, m, 3-H,, 4-H and CH,CO,H), 3.20 and 3.90 (2 H, 2 m,
`5-H,), 4.20 (2 H, q, J 6 , CHZCH,), 5.10 (2 H, S, CH,Ph), 7.20
`( 5 H, m, ArH) and 9.10 (2 H, br s, 2 x C0,H).
`
`2-Benzyloxycarbonyl-3-ethoxycarbonyl-2-azabicyclo-
`[3.1 .O]hexane-6-carboxylic acid 8
`A solution of the dicarboxylic acid 7 (0.244 g, 0.64 mmol) in
`toluene (20 cm3) was heated at reflux for 12 h. Evaporation of
`the solvent followed by flash filtration of the residue on silica gel
`yielded the title compound (0.160 g, 75%); 6,(200 MHz; CDC1,)
`1.20 (3 H, m, CH,CH,), 2.10-2.60 (3 H, m, 3-H, and 4-H), 2.70
`(1 H, m, CHCO,H), 4.10(3 H, m, CH,CH3 and 5-H), 4.20 and
`4.60 (1 H, 2 m, 2-H), 5.15 (2 H, m, PhCH,), 7.30 ( 5 H, m, ArH)
`and 8.00-9.50 ( I H, br s, C0,H); 6,(50.32 MHz; CDCI,)
`24.53,25.58,26.79,29.62,30.16,31.90,45.53,46.11,59.63,60.58,
`61.46, 67.66, 128.07, 128.45, 136.14, 154.42, 155.21, 170.97,
`172.34, 174.94 and 175.55.
`
`Ethyl l-benzyloxycarbonyl-4-(carboxymethyl)pyrrolidine-2-
`carboxylate 13
`A solution of compound 12 (4.25 g, 11.2 mmol) in toluene
`(120 cm3) was refluxed for 15 h. After evaporation of the
`solvent, the residue (3 g) was submitted to flash chromatography,
`eluting with chloroform-methanol (99 : I), to afford the title
`compound (2.76 g, 73%); dH(200 MHz; CDCI,) 1.10-1.40 (3 H,
`m, CH,CH,), 1.60-2.30 (2 H, m, 3-H,), 2.30-2.50 (2 H, m,
`CH,CO,H), 2.70 (1 H, m, 4-H), 3.20 and 3.90 (2 H, 2 m, 5-H,),
`4.004.50 (3 H, m, 2-H and CH,CH,), 5.15 (2 H, m, CH,Ph)
`and 7.30 ( 5 H, m, ArH).
`
`8-Benzyloxycarbonyl-3-oxo-8-aza-2-oxabicyclo [ 3.3.01 octane-
`7-carboxylic acid 9
`A solution of compound 8 (0,160 g, 0.48 mmol) in 6 mol dm
`HCl(l8 cm3, water-dioxane 5 : 1) was magnetically stirred for 72
`h at room temperature. The reaction mixture was then extracted
`with dichloromethane (3 x 15 cm3) and the combined organic
`phases were dried (Na,SO,). Evaporation of the solvent gave a
`solid which was recrystallized from dichloromethane-hexane to
`Ethyl (2R)- 1 -benzyloxycarbonyl4 [(men thyloxycarbony1)-
`give the title compound (0.127 g, 87%), mp 97-99 "C; purity was
`99% by HPLC (Found: C, 59.15; H, 5.0; N, 4.55. Cl5HI5NO6
`methyl] pyrrolidine-2-carboxylate 14 and ethyl (229-1-
`benzyloxycarbonyl-4- [ (menthyloxycarbonyl)methyl] pyrrolidine-
`requires C, 59.01; H, 4.95; N, 4.59%); v , , , ( C H C ~ ~ ) / C ~ ~ '
`1781
`2-carboxylate 15
`and 1717;6,(500 MHz; CDCl,) 1.90-2.10 and 2.30-2.50 (2 H, 2
`m, 6-H2),2.55(1 H,m, 5-H),2.75and3.1O(2H,2m,4-H2),4.55
`( +)-Menthol (1.53 g, 9.8 mmol) was added to a solution of the
`(1 H, m, 7-H), 5.20 (2 H, s, CH,Ph), 6.10 (1 H, 4 d, J 5.3, 1-H),
`carboxylic acid 13 (2.76 g, 8.2 mmol) in dichloromethane-DMF
`6.90 (1 H, br s, C0,H) and 7.30 ( 5 H, br s, ArH); 6,(50.32 MHz;
`(300 cm3, 1 : 1) containing DCC (2.0 g, 9.7 mmol) and 4-(N,N-
`dimethy1amino)pyridine (DMAP) (0.1 g, 0.82 mmol) and the
`CDCl,) 33.61, 34.14, 34.78, 34.93, 36.58, 37.69, 59.67, 60.29,
`68.10,91.68,92.00,92.58, 127.75, 127.88,128.00,128.26,128.55,
`resulting mixture was stirred for 76 h at room temperature. After
`135.47, 154.00, 174.39 and 175.54.
`evaporation of the solvent, the residue was taken up in ethyl
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`J. CHEM. SOC. PERKIN TRANS. I 1995
`
`acetate (150 cm,) and washed with water (2 x 50 cm3). The
`organic layer was dried (Na,SO,) and evaporated to give a
`residue (3.5 g) which was submitted to MPC, eluting with light
`petroleum-ethyl acetate (6 : 4), to afford the (2R) derivative 14
`(2.45 g, 63%); dH(200 MHz; CDCl,) 1.25 and 1.70 (21 H, 2 m,
`CH,CH, and menthyl H), 1.95 (2 H, m, 3-H,), 2.50 (2 H, m,
`CH,CO,), 2.70 (1 H, m, 4-H), 3.10 and 3.85 (2 H, 2 m, 5-H,),
`3.65 (1 H. m, CO,CH), 4.00-4.40 (3 H, m, 2-H and CH,CH,),
`5.05-5.15 (2 H, m, CH2Ph) and 7.30 (5 H, m, ArH). Further
`elution with the same solvents gave a mixture of 14 and 15
`(0.3 g). Continued elution with the same solvents (7 : 3) gave the
`pure (2s) derivative 15 (0.26 g, 6.7%); dH(200 MHz; CDC1,)
`1.30and 1.70(21 H,2m,CH2CH,andmenthylH), 1.95(2H,m,
`3-H2), 2.55 (2 H, m, CH,C02), 2.70 (1 H, m, 4-H), 3.15 and 3.90
`(3 H, 2 m, 5-H, and 2-H), 3.70 (1 H, m, CO,CH), 4.40 (2 H, m,
`CH2CH,), 5.10-5.20 (2 H, m, CH2Ph) and 7.30 (5 H, m, ArH).
`
`Ethyl (2R,4R)-4-[(menthyloxycarbonyl)methyl]pyrrolidine-2-
`carboxylate 16 and (2R,4S)4[ (menthyloxycarbonyl)methyl]-
`pyrrolidine-2-carboxylate 17
`Hydrogen was bubbled for 2.5 h into a magnetically stirred
`suspension of compound 14 (2.45 g, 5.16 mmol) in methanol
`(200 cm3) containing 10% Pd-C (0.250 g) at room temperature.
`Filtration of the mixture through Celite and evaporation of the
`filtrate gave a residue (1.8 g) which was submitted to MPC
`eluting with chloroform-methanol (99: 1) to afford the (2R,4R)
`derivative 16 (0.74 g, 42%), mp 65-66 "C; purity was 99% by
`GC analysis; 6,(200 MHz; CDCl,) 1.20 and 1.75 (21 H, 2 m,
`CH,CH, and menthylH), 1.90 and 2.10 (2 H, 2 m, 3-H,), 2.50 (3
`H, m, 4-H and CH,CO,), 3.00 (1 H, m, NH), 3.25 and 3.60 (2 H,
`2 m, 5-H2), 3.75 (1 H, m, CO,CH), 3.85 (1 H, m, 2-H) and 4.1 5
`(2 H, q, J 7.5, CH,CH,). Further elution with the same solvents
`gave a mixture of 16 and 17 (0.43 8). Continued elution with the
`same solvents gave the pure (2R,4S) derivative 17 (0.57 g, 32%),
`mp 122-1 23 "C; purity was 99% by GC analysis; dH(200 MHz;
`CDC1,) 1.25 and 1.75 (21 H, 2 m, CH,CH3 and menthylH),
`1.95 and 2.50 (2 H, 2 m, 3-H,), 2.55 (2 H, m, CH,CO,), 2.70 (1
`H, m, 4-H), 3.20 and 3.65 (2 H, 2 m, 5-H,), 3.70 (I H, m, NH),
`3.80-4.00 (2 H, m, C0,CH and 2-H) and 4.20 (2 H, q, J 7.5,
`CH,CH ,).
`
`Ethyl (2S,4S)4[ (menthyloxycarbonyl)methyl]pyrrolidine-
`2-carboxylate 18 and (2S,4R)-4-[(menthyloxycarbonyl)methyl]-
`pyrrolidine-2-carboxylate 19
`Hydrogen was bubbled for 1 h into a magnetically stirred
`suspension of compound 15 (0.26 g, 0.55 mmol) in methanol (50
`cm3) containing 10% Pd-C (0.026 g) at room temperature.
`Filtration of the mixture through Celite and evaporation of the
`filtrate gave a residue (0.2 g) which was submitted to MPC
`eluting with chloroform-methanol (99 : 1) to afford the (2S,4S)
`derivative 18 (0.070 g, 38%), mp 195-196 "C; purity was
`controlled by GC analysis (99%); 6,(200 MHz; CDCl,) 1.15 and
`1.70 (21 H, 2 m, CH,CH3 and menthylH), 1.90 and 2.15 (2 H, 2
`m, 3-H,), 2.50 (3 H, m, 4-H and CH,CO,), 3.15 (1 H, m, NH),
`3.25 and 3.55 (2 H, 2 m, 5-H,), 3.65 (1 H, m, C0,CH) and 3.85 (3
`H, m, 2-H and CH2CH,). Further elution with the same
`solvents gave a mixture of 18 and 19 (0.065 g). Continued
`elution with the same solvents gave the title compound 19 (0.045
`g, 24%), mp 100-101 "C; purity was controlled by GC analysis
`(99%); 6,6200 MHz; CDCl,) 1.20 and 1.70 (21 H, 2 m,
`CH,CH, and menthylH), 1.95 and 2.35 (2 H, 2 m, 3-H,), 2.50
`(3 H, m, 4-H and CH,CO,), 2.80 (1 H, m, NH), 3.20 and 3.55
`(2 H, 2 m, 5-H,), 3.70 (1 H, m, C0,CH) and 3.85 (3 H, m, 2-H
`and CH,CH,).
`
`(2R,4R)-4-(Carboxymethyl)pyrrolidine-2-carboxylic acid 16a
`A magnetically stirred suspension of 16 (0.160 g, 0.47 mmol) in 6
`mol dm HCI (8 cm3) was refluxed for 12 h. After cooling, the
`
`1255
`
`reaction mixture was extracted with chloroform (5 cm3) and
`then neutralized with 10% aqueous NH,OH and concentrated
`under reduced pressure. The residue was diluted with water (5
`cm3) and submitted to cation exchange resin chromatography,
`eluting with 10% pyridine to afford the title compound (0.070 g,
`8673, mp 224-225 "C (Found: C, 48.35; H, 6.6; N, 8.15.
`C7H1 ,NO4 requires C, 48.55; H, 6.40; N, 8.09%); Cali2 + 66.8
`(c 0.5, H,0);6,(200 MHz; D20) 1.80-2.00 (1 H, m, 3-Ha), 2.12-
`2.30 (1 H, m, 3-Hb), 2.32-2.60 (3 H, m, 4-H and CH,CO,H),
`2.80-2.95(lH,m,5-Ha),3.50-3.60(1H,m,5-Hb)and4.00-4.10
`(1 H, m, 2-H); 6,(50.32 MHz; D,O + MeOH) 176.44, 174.24,
`60.79, 50.14, 36.43, 34.39 and 33.39.
`
`(2R,4S)4(Carboxymethyl)pyrrolidine-2-carboxylic acid 17a
`A magnetically stirred suspension of 17 (0.100 g, 0.29 mmol) in 6
`rnol dm-, HCl (5 cm3) was refluxed for 12 h. After cooling, the
`reaction mixture was extracted with chloroform (5 cm3) and
`then neutralized with 10% aqueous NH40H and concentrated
`under reduced pressure. The residue was diluted with water (5
`cm3) and submitted to cation exchange resin chromatography,
`eluting with 10% pyridine to afford the title compound (0.050 g,
`98%), mp 232-233 "C (Found: C, 48.6; H, 6.55; N, 8.1.
`+ 34.2
`C,H, lNO, requires C, 48.55; H, 6.40; N, 8.09%);
`(C 0.5, HZO); 6H(200 MHz; D2O) 1.50-1.70 ( I H, m, 3-Ha),
`2.38-2.70 (4 H, m, CH,CO,H, 3-Hb, 4-H), 2.85-3.00 (1 H, m,
`5-Ha), 3.40-3.60 (1 H, m, 5-Hb) and 3.95-4.08 (1 H, m, 2-H);
`d,(50.32 MHz; D 2 0 + MeOH 99:l) 177.17, 174.82, 61.97,
`50.78, 37.26, 35.46 and 35.34.
`
`(2S,4S)4(Carboxymethyl)pyrrolidine-2-carboxylic acid 18a
`A magnetically stirred suspension of 18 (0.070 g, 0.20 mmol) in 6
`rnol dm-, HCl(3.5 cm3) was refluxed for 12 h. After cooling, the
`reaction mixture was extracted with chloroform ( 5 cm3) and
`then neutralized with 10% aqueous NH40H and concentrated
`under reduced pressure. The residue was diluted with water (5
`cm3) and submitted to cation exchange resin chromatography,
`eluting with 10% pyridine to afford the title compound (0.030 g,
`84%), mp 228-229 "C (Found: C, 48.45; H, 6.6; N, 8.15.
`C7H,,N0, requires C, 48.55; H, 6.40; N, 8.09%); [a]i2 -66.5
`(C 0.5, H2O) (lit.,' [a]i4 -69
`(C 0.5, H20)); dH(200 MHZ;
`D20) 1.80-2.00 (1 H, m, 3-Ha), 2.12-2.30 (1 H, m, 3-Hb), 2.32-
`2.60 (3 H, m, CH,CO,H and 4-H), 2.80-2.95 (1 H, m, 5-Ha),
`3.50-3.60 (1 H, m, 5-Hb) and 4.00-4.10 (1 H, m, 2-H); 6,(50.32
`MHz; D,O + MeOH 99: 1) 177.03, 174.93,61.55, 50.90,37.06,
`35.15 and 34.13.
`
`(2S,4R)-4-(Carboxymethyl)pyrrolidine-2-carboxylic acid 19a
`A magnetically stirred suspension of 19 (0.045 g, 0.132 mmol) in
`6 mol dm-, HCl(3 cm3) was refluxed for 12 h. After cooling, the
`reaction mixture was extracted with chloroform ( 5 cm3) and
`then neutralized with 10% NH40H and concentrated under
`reduced pressure. The residue was diluted with water (5 cm3)
`and submitted to cation exchange resin chromatography,
`eluting with 10% pyridine to afford the title compound (0.019 g,
`83%), mp 234-235 "C (Found: C, 48.5; H, 6.4; N, 8.1.
`C,H, ,NO4 requires C, 48.55; H, 6.40; N, 8.09%):
`- 33.4
`(c 0.35, H20); &(200 MHz; D20) 1.50-1.70 (1 H, m, 3-Ha),
`2.20-2.68 (4 H, m, CH,CO,H, 3-Hb and 4-H), 2.85-3.00 (1 H,
`m, 5-Ha), 3.40-3.50 (1 H, m, 5-Hb) and 3.95-4.08 ( 1 H, m, 2-H);
`6,(50.32 MHz; D,O + MeOH 99: 1) 177.84, 174.97, 62.06,
`50.85, 37.83, 35.71 and 35.46.
`
`Methyl (2S)-l-benzyloxycarbonyl-4-[ (tevt-butoxycarbony1)-
`met h y lidene ] pyrr olidine-2-carboxyla te 20
`tert-Butyl (trimethylsily1)acetate (1.634 g, 8.7 mmol) was added
`dropwise in 5 min via a syringe pump to a cold (-78 "C)
`solution of lithium diisopropylamide [prepared from addition
`of butyllithium in hexane (2.5 mol dm-3 solution, 3.5 cm3) to a
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`1256
`
`solution of diisopropylamine (1.22 an3) in THF (36 cm3)].
`A solution of methyl (2S)-N-benzyloxycarbonyl-4-oxopyrro-
`lidine-Zcarboxylate (2.0 g, 7.2 mmol) in THF (1 8 cm3) was then
`added in 15 min in the same way and the resulting mixture was
`magnetically stirred for 2.5 h at -78 "C. After addition of 5%
`citric acid (22 cm3) the reaction mixture was allowed to warm to
`room temperature and concentrated under reduced pressure.
`The aqueous phase was then extracted with ethyl acetate
`(3 x 30 cm3) and the combined extracts were washed with water
`(40 cm3) and brine (40 cm3), dried (Na,SO,) and evaporated.
`The residue (2 g) was submitted to flash chromatography,
`eluting with light petroleum-ether (1 : l), to afford a mixture of
`the cis- and trans-olefin 20 (0.90 g, 34%); 6,(200 MHz; CDCl,)
`1.45 (9 H, m, Bu'), 2.76 and 3.08 (2 H, 2 m, 3-H,), 3.55 and 3.70
`(3 H, 2 s, CO,Me), 4.60 (3 H, m, 2-H and 5-H2), 5.00-5.25 (2 H,
`m, PhCH,), 5.70 (1 H, m, CHC0,Bu') and 7.30 (5 H, m, ArH).
`
`Methyl (2S,4R)4 [ (tert-butoxycarbonyl)methyl] pyrrolidine-
`2-carboxylate 21
`A solution of 20 (0.42 g, 1.31 mmol) in acetic acid (3 cm3) was
`added to a suspension of PtO, (0.065 g) in acetic acid (3.5 cm3)
`and the resulting mixture was placed in a Parr apparatus under
`a hydrogen atmosphere at a pressure of 50 psi for 10 h.
`Filtration of the mixture through Celite and evaporation of the
`solvent gave a residue (0.2 g) which was submitted to flash
`chromatography, eluting with chloroform-methanol (98 : 2), to
`afford the title compound (0.150 g, 55%); dH(200 MHz; CDCI,)
`1.45 (9 H, m, But), 1.55 and 2.40 (2 H, 2 m, 3-H,), 2.28 (2 H, m,
`CH,CO,), 2.55 (1 H, m, 4-H), 2.70 and 3.15 (2 H, 2 m, 5-H,),
`2.75 (1 H, m, NH), 3.70 (3 H, s, C0,Me) and 3.85 (1 H, m, 2-H).
`
`(2S,4R)-4-(Carboxymethyl)pyrrolidine-2-carboxylic acid 19a
`A magnetically stirred suspension of 21 (0.050 g, 0.20 mmol) in
`6 mol dm-, HCl(4 cm3) was refluxed for 2 h. After cooling, the
`reaction mixture was extracted with CHCI, (1 x 5 cm3),
`neutralized with 10% aqueous NH,OH and concentrated under
`reduced pressure. The residue was diluted with water (5 cm3)
`and submitted to cation exchange resin chromatography,
`eluting with 10% pyridine, to afford the title compound
`(0.025 g, 70%).
`
`Methyl (2S)-4-[ (tert-butoxycarbo