Summary 6β-N(5)-methyl-5.6J,8-tetrahydro-L-biopterin·2 HC1. a new derivate of biological interest, was synthesized from 6β-5,6,7,8-tetrahydro-L-biopteril·2 HC1 and characterized. The latter compound has been used therapeutically in atypical phenylketonuria for many years. The catalytic reductive methylation with PtO2 /H2 and formaldehyde allowed selective methylation at the N(5) position. The chemical properties and stnlctures of the new compound arc compared with N(5)-methyl-tetrahydropterin and N(5)-methyl-tetrahydrofolat

Bosshard, Rene

Heizmann, Claus W.

Viscontini, Max

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Bosshard et at.: Synthesis of 6~-N(5)-Methyl-5.6.7.R-tetrahydro-L-hiopterin Pteridines Vol. 4. 1993. pp. 115-IIS Synthesis of 6P-N(5)-Methyl-5,6,7,8-tetrahydro-L-biopterin* Rene Bosshard, Claus W. Heizmann. and Max Viscontini 1I Division of Clinical Chemistry, Department of Pediatrics, University of Zurich. Steinwiesstr. 75, CH-8032 Zurich, Switzerland *'Institute of Organic Chemistry, University of Zurich. Winterthurerstr. 190. CH-8057 Zurich. Switzerland (Received January 29. 1993) Summary 6~-N(5)-methyl-5.6J,8-tetrahydro-L-biopterin· 2 HC1. a new derivate of biological interest, was synthesized from 6~-5,6,7,8-tetrahydro-L-biopteril1' 2 HCI and characterized. The latter compound has been used thera-peutically in atypical phenylketonuria for many years. The catalytic reductive methylation with PtOJ H2 and formaldehyde allowed selective methylation at the N(5) position. The chemical properties and stnlctures of the new compound arc compared with N(5)-methyl-tetrahydropterin and N(5)-methyl-tetrahydrofolate. Key words: 6~-N(5)-methyl-5,6,7.8-tetrahydro-L-biopterin, Catalytic reductive methylation, 6~-5,6,7,8-tetrahydro­L-biopterin, 6J)-5,6J,8-tetrahydro-L-biopterin, L-biopterin. N(5)-methyl-5.6.7.8-tetrahydro-pterin, Hyperphenyl-alaninemia, Phenyl ketonuria. Introduction 6~-5,6J,8-Tetrahydro-L-bioptcrin (BH4, 2) is the cofactor of aromatic amino acid monooxigenases. enzymes that control the biosynthesis of several neurotransmitters (1,2). Patients with BH4 deficiency may suffer from mental retardation and other symp-toms. Treatment of these patients requires high do-ses of this cofactor (10-20 mg/kg. per os). possibly due to its limited penetration through the blood-brain barrier. We synthesized various BH4 analogcs with the aim to obtain compounds with better penetration properties that could be applied in lower concentra-tions. In the course of these studies, we synthesized for the first time 6~-N(5)-methyl-5,6J,8-tetrahydro-L­hioptelin (3) (Scheme I). Its synthesis and chemical properties are described in this paper. In future ex-periments we will now be able to test the in vivo cofactor activities and brain penetration abilities of 3. *This work is dedicated to Prof Hans-Christoph Cunius on the occasion of hi s 70th hinhday. I'teridincs / Vol. 4 / No. 3 1 L-Bioptenn 2_ SI1- S.6 .7 .a -T etrahydfu-L-booptlltM o r, "'~ 'r' ..... ~)~!~~ >2Hel 4_ N(5)Me1I1yt-S.6.7.e-tetr'lhydrOPll!lnn 3_ 6G.-N(5)Meth)+5 ,6,7,8-te trahydto-L -bioptann The regiospecific L-biopterin synthesis from L-rhamnose or 5-deoxy-L-arabinose with 4-hydroxy-2,5, 6-triaminopyrimidine was reported earlier (3,4). For the nomenclature a,13 of the two C(6)-diastereoiso-mers of 5.6},8-tetrahydro-L-biopterin see Ref. (5). The natural form is 6~-(earlier called 6R-); according to IUPAC rules the configuation is I'R 2'S, 6R The coenzyme function of BH4 is related to the re-dox system: 613-5,6}.8-telrahydro-quinonoid-6,7-dihy-dro[8H]-L-biopterin. The title compound 3 may be 116 Bosshard cf (/1.: Synthesis' of 613-N(5)-Methyl-5.6.7.8-tetrahydro-L-biopterin as interesting as thc homologous N(S)-methyl-tetra-hydrofolate (6). The synthesis of 6~-N(S)-methyl-tctrahydro-L-bio­ptcrin is illustrated in Scheme 1. L-Bioptcrin is con-vertcd to BH4 (2) by catalytic hydrogenation in hy-drochloric acid solution. Pure 613-diastereoisomer was used for further synthesis. The catalytic rcduc-tive methylation at N(5), as the strongest basie NH-group of the molecule, is described in detail (modi-fied general method) in thc Experimental scction. In an earlier publication by Matsuura and Sugimoto (7) the methylation method developed by Rylander (8) was applied to tetrahydropterin (4) and its 6-alkyl derivatives. In a later electrochemical oxidation study 7.8-dihydropterin was formed from 4 under release of the 5-methyl group (9). Results and Discussion N(5)-methyl-tetrahydro-L-biopterin (3) is more sta-ble to oxidation than the starting material 2 and has different physicochemical properties, i.e., rever-sed optical rotation. The appearance of thc blue t1uorescence on the TLC-plate after several hours of exposure to air requires the release of the methyl group. The UV-spectrum (see Table I) is similar to that of N(5)-methyl-tetrahydropterin (7) and also pH-dependent (amphoteric character), with pKs de-termination possibilities. Chirale HPLC control analysis as for the diastereoisomers 6a/6~-separation of tetrahydro-L-biopterin has so far not been done. The spectral data of the new compound are shown in Figures 1 and 2 CHNMR), and in Table I and Figure 3 (IN). followed by mass spectrometry data. 'H-NMR (0.1 N DCI/D20) (Fig. I): 4.05-3.99 del, J=5, H-C (7 eq); 3.98-3.92 m, H-C (6) and H-C (2' = ; 3.84-3.78 dJ • J = S. H-C (7 ax); 3.43 broad s. H-C (l '); 3.22 s. CHrN (5); 1.3 1 d. J = 2, CHrC (2'). After irradia-tion at 3.43 (double resonance) (Fig. 2), the peak 3.43 disappears and the m changes in the new signals at 3.98-3.96 d. J = 2, H-C (2') and 3.92 t. H-C (6). For the assignment of the IH-NMR peaks of 3 we use the results we have already obtained with other tetrahydropterin derivatives (10-12). GC/MS Ana(vsi~ of 613-N(5)-methyl-tetrahydro-L-biopte-rin' 2 HCI The sample was derivatized by a mixture of Ptcridines " Vol. 4 / No. 3 Table I. UV spectrum of 613-N(S)methyl-S.6.7.8-tetrahydro-L-biopterin' 2 Hel. A, nm A2 nm Solution pH (10, = molar (10, = molar Ion state extinction) extinction) IN HCI 0.1 N Hel o 212 (103IS) 220 (16500) 1/15 M KH,P04 3.S* 222 (17815) H,O 4 222 (17770) 1/ 15 M KH ,POJ 8.2 220 Na,HP04 (17262) (Soercnsen) 0.1 N NaOH 13 222 (12540) 262 ( 1502S) 264 ( 127(0) 264 (IISOO) 264 (10055) 286 ( 11129) 278 (9035) Di-cation (mono-) cation zwitterion (neutral molecule) anion (stability?) *pH adjusted with I N HC!. The largest pH-dependent batho-chromic shift (see A,) occurs betwee n cation ion state and neutral molecule. The max. molecular extinction (MW: 328.20) is in the same pH range (see £,) in accordance with Fig. 3. , ~.O , 3. 5 , 3 .0 2.5 y 2.0 y 1. 5 i 1.0 ~ Figure 1. 'H-NMR spectrum of 613-N(5)mcthyl-5.6.7JHetrahy-dro-L-biopterin· 2 HCl in 0.1 DCI / D20 . BSTFA/acetonitrile (1 :3. v/v) for 60 min at 100°C to the TMS ethers, as described (13). GC: Carlo Erba. to m fused silica SE-S4 column, 0.8 bar He: injector 320t , temperature program 80t / min to 200 °c within 12 min. then increased to 250t at a rate Bosshard el at.: Synthesis of 6f3-N(5)-Methyl-5.6.7.S-tetrahydro-L-biopterin 11 7 ;.:rc :'. IH-NMR spectrum of 3. irradiated at H-C (I ' ). 3.43 360 isosbestic pt. Figure 3. UV spectrum of 6f3-N(S)-mcthyl-S.6,7.8-tetrahydro-L-biopterin 2 HC\. Curve numher represents pH of solution (C=SO mg 3/ L). of 4"C Imin. MS: VG-7070F. El f with 40 EV, ion source temperature 220°C. emission current 50 flA, scan range mle 100-750 at a rate of 1.5 sec. Result: The title compound showed a weak signal for the molecular ion with four trimethylsilyl groups (m/e 543. 0.5% relative intensity), as common for pteri-dines; the spectrum showed peaks for silyl derivatives with fewer (three) TMS groups (m/e 471); the base peak (m/e 324) is formed by loss of the enti re side chain at C-6 and two remaining TMS groups. TLC Silica gel K60F254 plates '(Merck); solvent: isopro-panol-H,BOJ (O.S M, in H 20)-H20 (4:1:0.5). 3: R, 0.59 (fluorescence appeared only after 24 h exposure to air): 4: Rr 0.34. HVE Electrophoresis on paper (Schleicher-Schull No. 2043B) with buffer, pH 1.9: acetic acid-formic acid-water (120:26:8S0 v/v). 3: Rx 203: 4: R, 248 in rela-tion to L-biopterin: R = 100. Experimental Part Apparatus The optical rotation was measured with a Perkin Elmer Polarimeter Type 241; the 'HNMR spectrum was recorded on a Brucker 300 MHz NMR spectro-meter, and the UV spectrum was registered with a Diode Array H P845 I A spectrophotometer. Chemicals Fonnaldehydc. 36% in H20. stabilized with 10% methanol, platinum IV-oxide (Adams catalyst), me-thanol puriss. and diethylether abs. (Fluka, CH-9470 Buchs, Switzerland), 6-[3-S,6,7,8-tetrahydro-L-biopte-rin' 2 HCI (Schircks Laboratories, CH-8645 Jona. Switzerland). 6[3-N(S)-Methyl-S.6,7,8-tetrahydro-L-biopterin· 2 HCI (3): In a hydrogenation apparatus (round bot-tom 3-neck flask 2S0 ml) with magnetic stirrer, ni-trogen flow device, and Hrballoon (connected with a hydrogen bomb for temporary reloading), acetic acid (3 ml , 2 N) and methanol (40 ml) were added to the Pt02-catalyst (90 mg maximum). After N2 flow the catalyst was prehydrogenated for approx. 20 min under HTballoon pressure, stirring, ;"IlU cooling (ice water bath). 6[3-S,6,7,8-Tetrahydro-L-biopterin· 2 HCl (315 mg, 1.0 mmol) was added (solid) under Nrflow in the reaction bulb. A mixture of acetic acid (3 ml, 2 N) and methanol (20 ml), followed by formal-dehyde 36% (0.48 ml. 6.2 111mol) was added and the hydrogenation (i.e. the reductive methylation) was started under stirring and H 2-balloon pressure for 2 h at ISoC. After N 2-flow a second portion of fOlm-aldehyde 36% (OJ ml. 3.8 mmol) was added and again hydrogenated for 2 h (ca. 1 SoC ) under stirring. The catalyst was filtered oft: protected under Nr flow over a glass filter D4. covered with Celite 53S (argon protection of the filter to avoid self ignition of catal-yst). and rinsed with methanol 90% (15 ml). The filtrate was evaporated (Rotavapor) to ca. 10 ml. then HCI (100 1..11. 6 N) and ethanol abs. (20 ml) were added and the filtrate was reconcentrated to S ml. Ethanol (abs., 30 ml) was added again and evapora-Pteridincs / Vol. 4 / No.3  Il lI Bosshard el al .: Synthesis o f 6~-N(5 )-Methyl-5,6),8-tetra hydro-L-biopterin ),;1 ted to a slurry (ca. 5 mI). On addition of ethcr (abs., 30 ml) under ice bath cooling, the white dihydro-chlo ride was left to cri stallize ovemight at 4°C. The product was collected (04 glass filter), washed with a few ml of a mixture of ethanol-ether (abs., \:1), followed by ether and dried in a vacuum desiccator over P20 , and NaOH pellets separately at room temperature: 0.31 g dihydrochloride (3), yield ca. 95 %. For recrista llization the dihydrochloride (0.3 g) was dissolved in warm methanol (20 ml) under ad-dition of He] (50 Ill, 6 N) and watcr (600 fJ.I). Aftcr addition of ca rbon (10 mg) the solution was filtered (glass filter 04, covered with Celite 535), and th e filtrate was chilled. HC I (800 Ill, 6 N) and, gradually, diethylethcr (25 ml) were added under cristallization of the product. After 2 h in the ice hath the cristals were filtered, washed with up to 10 ml cold ether Imethanol (2: I), then wash cd with ether and dried in a vacuum desiccato r; yield: 0.25 g 6S-N(5)-methyl-5,6.7,8-tetrahydro-L-hioptelin· 2 HCl. AnalysL\': calc. for CoH I7NsO} 2 HCI (328.20): C 36. 60, H 5.84, CI 21.60, N 21.34%; found C 36.40, H 5.68, CI 21 .32, N 21 .08%. Specific optical rotation : [ u]21"[)( ')x'! nl11)+44.8°(C = 1.056; 0.1 N HCI). (In comparison 6S-tetrahydro-L-bioptc-rin : [ U]2I[)-65.2° (C = 1.135; 0.1 N HC I). Acknowledgement This work was suppo rted by the Swiss Natio nal Science Foundation, grant no. 3 1-33897.92. We th ank H. Frohofer for the elementa ry analys is and for measuring the specific optical rotation and Dr. R. Hollenstein fo r the "HNMR spectroscopy data, hoth at the Institute of Organic C hemistry, University of Zurich; Dr. T. Kuster and E. Wetzel for the mass spectrometry: and M . Killen lo r preparation of the Pteridines / Vol. 4 / No.3 manuscript. References I. Scrivcr C R. Ka ufma n S. Woo SLC. The hyperphenylalan-inemias. In: Scriver C W, cd. T he metabolic basis o r inheri-ted di sease I. 6th ed. New York: McGraw Hill , 1989: 495-562. 2. Curtius H-C h. Rl au N. Kuster T. Prerins. In: Ho mm es FA. ed. Techniques in diagnostic huma n biochemica l gen-etics, a laborato ry manual. New York: Wiley-Liss, 1991; 377-96. 3. Schircks B. Visco ntini M. A new regiospccific synthesis of L-bioptcrin. Helv. C him. Acta 1985: 68: 1639-42. 4. Taylor EC, 1acobi R. A total synthesis of L-crythrobiopte-rin. 1. Am. Chem. Soc. 1976; 98: 2301 -06. 5. Viscontini M. Designation a nd representation of Ictrahy-dropterins. In : Levi ne RA Mil stien S, Kuhn D. Curti us H-Ch. cds. Pteridines and bi~genic amines in neuropsy-chi atry. pedi a trics, and immunology. Grosse Pointe: la ke-shore Publishing Co., 1989: 33-50. 6. Benkovic S1. On the mech a nism of action of fo late and biopteri n requiring enzymes. Ann. Rev. Biochem. 1980; 49: 227-5. 7. Matsuura S. Sugimoto T. Heterocyclic chemistry, Pa rt VIII: Synthesis of 5-alkyl-tetrahydropteridines. Res. Bull. Dcpt. Gen. Educ. Nagoya Univ. 1969: IJ: 9- 17. 8. Rylander PN. Catalytic hydrogenations over platinum mc-tals. New York: Academic P ress. 1967: 29 1-3. 9. Ka rber LG, Dryhurst G. Electrochemica l oxida tion of 5-methyl-5.6.7.8-tetra hydrorterin . 1. Elcctroa naL Chem .• 1984: 160: 141-57. 10. Weher R. Viscontini M. Zur Konform atio n von 6-Methyl-und 5,6-Dimethyl-5.6,?,lI-tetrahydropterin. Helv. C him Acta 1975: 58: 1772-1 780. II. Weber R. Viscontini M. Zur Konfiguration and Ko nfor-mation von 6,?-Di methyl- und 5.6,7-Trimethyl-5,6,?,lI-tetra-hydropterin. Helv. C him. Acta 1976: 59: 2379-23l12. \2. Weber R Vi scontini M. Ko nfo rm ationsa nalyse von acylier-tem 5,6'?8-Tetrahydroptelin. Helv. Chim. Acta 1977; 60: 16 1-165. 13. Kuster T. Niederwicser A Gas ch romatography-mass spec-trometry of trimethylsilylpterid ines. 1. C hromatogr. 1983; 278: 245-54. 

Synthesis of 6β-N(5)-Methyl-5,6,7,8-tetrahydro-L-biopterin

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