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Asymmetric Synthesis of A-Methylglutamic Acid

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Asymmetric Synthesis of A-Methylglutamic Acid Agric. Biol. Chern., 49 (6), 1761-1765, 1985 1761 Asymmetric Synthesis of a-Methylglutamic Acid and a-Methylornithine by a Chiral Isocyano Amide Reagent Yoshihiro Yamamoto, Mitsunori Kirihata, Itsuo Ichimoto and Hiroo Ueda Department of Agricultual Chemistry, College of Agriculture, University of Osaka Prefecture, Sakai, Osaka 591, Japan Received November9, 1984 An efficient approach to the asymmetric syntheses of a-methylglutamic acid and a-methyl- ornithine is described. Two chiral reagents, (2/*S)-A^-(2/-methoxymethylpyrrolidine)-2-isocyano- propionamide 4 and (2/S)-Ar-(2/-hydroxymethylpyrrolidine)-2-isocyanopropionamide 5, were employed for the asymmetric induction. a-Methylglutamic acid 7 was synthesized by the asym- metric Michael-addition of methyl acrylate to 4 and 5 as the key step. The optical yield of 7 was 10~45% (R-form). a-Methylornithine 12 was also synthesized by the reaction of 4 with acrylonitrile as the key step. The optical yield of 12 was 31.7% (i?-form). a-Alkyl amino acids are of biochemical in- In recent years, successful syntheses of the terest because of their properties as enzyme chiral ketone, aldehyde, a-alkyl amino acid inhibitors. In the previous paper,1'2) we have and dipeptide employing (£)-(-)-proline de- reported the asymmetric synthesis of a-methyl- rivatives as a chiral auxiliary reagent have been ornithine 12, a potent competitive inhibitor of reported as follows: (1) asymmetric induction ornithine decarboxylase, involving the diaster- of methyl vinyl ketone to the chiral enamine eoselective Michael-addition of acrylonitrile A,6) (2) asymmetric alkylation (benzylation) of to (+)- and (-)-menthyl a-isocyanopro- the chiral amidine ether B,7) hydrazone C8'9) pionate; however, the optical yields of (-)-12 and allylamide D,10) (3) asymmetric synthesis and (+)-12 thus obtained were only 4.7 and of L,D-dipeptide via the chiral intermediate 5.8% respectively. E.n) Sonnet et al.12) reported more recently Subsequently, although several synthetic that diastereoselective metalation and alkyla- methods for 123~5) have been reported, in- tion of the chiral amides F derived from (S)- vestigations concerning the asymmetric syn- (-)-prolinol and its methyl ether afforded a- thesis of 12 and a-methylglutamic acid 7 have alkyl amines F in 12~82%e.e. not been successful. ^^COR C^CH20CH3 C-XCH20CH, <-yCHaOCH3 <->c=Q iH iH fc CH2 / NH 0 R^ Ar R-CH-CO2CH3 R \ N-R2 B / v_^CH20R I)bqse / \^CH2' RR'=H,CH3 R R* :0R E F' Scheme 1. 1762 Y. Yamamotoet al. The S configuration induced by (£)-(-)- synthesis of 7 and 12 by using (2'S)-N-{2'- prolinol was opposite to that induced by its methoxymethylpyrrolidine)-2-isocyanopro- methyl ether. According to the new technique pionamide 4 and (2/5)-A^-(2/-hydroxy- for asymmetric induction, they13) succeeded in methylpyrrolidine)-2-isocyanopropionamide obtaining both enantiomers of 10-methyl-l- 5 as the chiral auxiliary reagents. The amide 4 dodecanol acetate, the sex pheromone com- was prepared by the reaction of (£)-(-)- plex of the smaller tea tortrix moth prolinol methyl ether lb with methyl iso- (Adoxophyes sp.), in 74% (R) or 80% (S) e.e. cyanoacetate 2a, and followed by C- The present paper deals with the asymmetric methylation with methyl iodide. R'*CH3 _R2sH . y \>CH20CH3CHal / WCH2OCH3 ^IST "H n-BuLi ^N^*H N=C 0=C-CH2-N=C O- C-CH-N« C y wCHzOR' B I I ^M/^H + R2~CH 3 4 CH3 H I CO2CH3 Ri »H ,o R'-H 2o R2aH LR!lCH3_ /-^^OH IbR'-CH3 2b R2=CH3 V H .. O-C-CH-N-C 1 5 CH3 Scheme 2. The amide 5 was prepared from (S)-(-)- ence of 1 equimolar amount of sodium hy- prolinol la and methyl 2-isocyanopropionate dride, and followed by treatment with acetic 2b. The Michael-addition of an equimolar acid to give a mixture consisting of 53%of QO-dialkyl amide 9 and 23% ofC-alkyl amide amount of 4 to methyl acrylate in the presence 8. of sodium hydride, and followed by treatment with acetic acid gave (#)-(-)-7 (Md - 1.22°, Acid hydrolysis of 9 and 8 with 2n-HC1 10% e.e.) in a 42% chemical yield. On the other gave (S>(+)-7 ([a]D +0.61°, 5%e.e.) and hand, the amide 5 was treated with 1.6 equi- (i?)-(-)-7 ([a]D - 5.35°, 44% e.e.) respectively. molar amounts of methyl acrylate in the pres- y S.CH20CH3 O=C-CH-N=C 1) CH2=CHC02CH32)AcOH O=C-C-NHCHO . CH3 CH2CH2CO2CH3H+ HO2C-C-NH2CH3 CH2CH2CO2H7 Scheme 3. y \>CHz0H NqH ^N^' H 2) AcOH O=C-CH-NSC y s.CH20CH2CH2C0zCH3CH3 I CH3 O=C-C-NH CHO 9 CH2CH2CO2CH3 I > S.CH2OH 1 9H3 O=C-C-NH CHO 8 CH2CH2CO2CH3 Scheme 4. > wCH20CH3 I)CH2*CHCNr > y^CHzOCHs H2/Ni ^ > v^CH2OCH3 Sr ""H 2)AcOH ^N^"H Ac20 ^N^^H O-C-CH-N=C I ?H3 I ?H3 CH3 O-C-C-NHCHO O-C-C-NH CHO 4 10 CH2CH2CM II (CH2)3-NHAc H+ ?H3 ^ HO2C-C-NH2 (CH2)3C02H 12 Scheme 5. Asymmetric Synthesis of a-Methyl Amino Acid 1763 In agreement with the investigation result CH2C12 (50ml), washed with water (10 ml) and dried over described by Sonnet et al.,12) the configuration Na2SO4. Evaporation of the solvent gave the crude of 7 induced by the (S)-(-)-prolinol alkyl product 4, which was purified by silica gel column chro- matography (eluting with an increasing amount of ether was reversed from R to S as the size of AcOEt in benzene) to give 1.7g of pure 4 as an oil (85% the O-alkyl substituent becomes larger yield). IR v££ cm"1: 2140 (N=C:), 1650 (C=O). (H->CH3 ->CH2CH2CO2CH3). ^-NMR (CDC13) 5: 1.33-1.52 (3H, d, /=8Hz, CH3), Finally, 12 was synthesized in a manner 3.12 (3H, s, O-CH3). Anal. Found: C, 61.09; H, 8.16; N, similar to that already described. In this ex- 14.20. Calcd. for C10H16O2N2: C, 61.20; H, 8.22; N, 14.28%. periment, the amide 4 was employed as a chiral auxiliary reagent, because it gave a more (2''S )-N-(2''-HydroxymethyIpyrrolidine)-2-iso- favorable result for the Michael reaction than cyanopropionamide 5. To a solution of the methyl 2- did 5. The amide 4 was treated with a 1.2 isocyanopropionate 2b (1 g, 8.8mmol) in MeOH(5ml) equimolar amount of acrylonitrile, and fol- was added a solution of the prolinole la (2 g, 19.8 mmol) in MeOH(3 ml), the mixture being stirred for 2 hr at ambient lowed by treatment with acetic acid in a similar temperature. manner to Scheme 3. The obtained 10 was After removing the solvent, the residue was purified by hydrogenated over Raney Nickel catalyst in silica gel column chromatography (eluting with an increas- acetic anhydride to yield ll. The acid hy- ing amount of AcOEt in benzene) to afford 5 as an oil (1.5g, 93%). IRv^cm"1: 3350(OH), 2130(N=C:), 1640 drolysis of ll with 6N-HC1 gave (R)-(-)-12- (C=O). ^-NMR(CDCI3) S: 1.24~1.68 (3H, d, J=1Hz, HC1 ([a]D -3.33°, 32%e.e.) in a 28% overall CH3), 4.0-4.34 (1H, br.s, OH). Anal Found: C, 59.35; H, yield from 4. 7.76; N, 15.32. Calcd. for C9H14O2N2: C, 59.32; H, 7.74; N, 15.37%. EXPERIMENTAL a-Methylglutamic acid 7. Infrared spectra were taken with a JASCO IRA-2 MethodA: A solution of4 (1.96g, 10mmol) in dry THF spectrometer, and NMRspectra were recorded with JEOL (10ml) was added to a stirred suspension ofNaH (240mg, JNM-MN60 spectrometer. Melting points were deter- 10mmol) in dry THF (50ml), and the mixture was stirred mined on Yanaco MP-S3melting point apparatus and for lOmin at -78°C. To the reaction mixture was added a are uncorrected. Optical rotations weremeasuredon a solution of methyl acrylate (1.03g, 12mmol) in dry THF JASCO DIP-SL polarimeter. (10ml), the mixture being stirred for 3hr at -78°C and then kept for 3hr at between -30 and -35°C. The (S) -N- {2-Me thoxyme thylpyrro lidine) - isocyanoace tam ide reaction mixture was then acidified with acetic acid (5 ml) 3. To a solution of the methyl isocyanoacetate 2a (3g, and the solvent evaporated in vacuo (40 ~50°C) to afford 30.3mmol) in MeOH(10ml) was added a solution of the the adduct 6. This product was purified by silica gel prolinole methyl ether lb (6.2g, 53.9mmol) in MeOH column chromatography (eluting with an increasing (5ml), the mixture being stirred for 24hr at ambient amount ofAcOEt in benzene) to give 2.1 g of pure 6 as a temperature. After removingthe solvent, the residue was colorless oil (70% from 3). IR v^ cm"1: 3300 (NH- purified by silica gel column chromatography (eluting with CHO), 1760 (C=O). ^-NMR S: 1.75 (3H, s, CH3), 3.22 an increasing amount of AcOEt in benzene) to afford 3 (3H, s, O-CH3), 3.56 (3H, s, O-CH3). Anal. Found: C, (4.69g, 85% from 2a), mp 52°C. [a£5 -79.56° (c=0.675, 55.91; H, 8.ll; N, 9.28. Calcd. for C14H24O5N2: C, 55.98; EtOH). IR vSl cm"1: 2150 (N=C:), 1660 (C=O). *H- H, 8.05; N, 9.32%. NMR (CDC13) 6: 1.68-2.34 (4H, m), 3.14 (3H, s, O- A solution of6 (1.06g, 3.51mmol) in 3n-HC1 (30ml) CH3).
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