RATIO DE CAPITAL GLOBAL DE LAS IMF Y LA BANCA MÚLTIPLE 2006-2010

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Although the methodology had been developed for the stereoselective synthesis of serine, a method was required for the introduction of deuterium at C-6. The

dideuteriated dihydropyrazine (117) could be synthesised from dideuteriated glycine | and valine. Initially (3R)-2,5-dimethoxy-3-/sopropyl-3,6-dihydropyrazine (1 1 3 ) was

prepared from glycine and (2R)-valine in up to a maximum overall yield of 62% (from glycine) (Scheme 2.18). Glycine was converted to the methyl ester (118) by reflux in methanol with thionyl chloride in dry methanol In 100% yield, the proton NMR spectrum contained a singlet for the methyl ester at 3.76 ppm and a singlet for the methylene protons at 3.86 ppm. (2R)-valine was N-protected with benzyl chloroformate in 8 8% yield, and the resulting N-carbobenzoxy-(2R)-valine (119) was reacted with glycine methyl ester (118) to form the dipeptide N-carbobenzoxy-(2R)-valine-glycine methyl ester (120) in 87% yield. This compound exhibited the required proton NMR spectrum and had Infrared absorptions at 1645 and 1535 cm'^ indicative of the presence of an amide bond. The mass spectrum of the compound gave the [M+Hf at 323. Removal of the nitrogen protecting group was effected by hydrogenation with 10% Pd/C in dry

methanol:dichloromethane 1:3, this caused partial cyclisation to cyc/o-((2R)-valine- glycine) (1 2 1) (a diketopiperazine). The cyclisation was completed by refluxing the mixture of products from the hydrogenation step for 12 hours in dry toluene, the precipitated product was obtained by filtration and dried thoroughly. The proton NMR spectrum showed a doublet at 0.87 and 0.96 ppm with J 7 Hz for the valine methyl groups, a double septet at 2.2 ppm for the proton adjacent to the valine methyl groups,

a doublet at 3.88 ppm, J 3 Hz for the ring C-3 proton and a double doublet at 3.9 and 4.09 ppm for the C-6 protons. The conversion of the diketopiperazine (1 2 1) to the bis-lactim ether (113) with trimethyloxonium tetrafluoroborate in dry dichloromethane gave the most variable yields (66-89%) and was wasteful of trimethyloxonium tetrafluoroborate. It was impractical to synthesise large quantities of the bis-lactim ethers and so they were purchased commercially. Synthesis of the deuteriated analogues in this manner would require large quantities of deuteriated glycine, and would be very costly.

Several methods for introducing deuterium at C-6 of the commercially purchased bis-

lactim ethers were considered using the known kinetic preference for C-6 proton

a b s t r a c t i o n . M e t h o d s based upon repeatedly quenching the nBuLI generated anion of the dihydropyrazine (113) with ^HaOor CHaO^H were quickly abandoned due to the formation of an array of by-products after the first cycle. This method also suffered from the potential problem of low selectivity for removing protium from the singly deuteriated compound in the second cycle of anion formation.

Stirring compound (113) in ^HzO in the presence of KOH under a variety of conditions also proved to be of little or no utility. At room temperature, the exchange of the C-6

protons was almost undetectable after 3 hours, and at higher temperatures, several

by-products were formed. However, under optimised conditions, CH3 0^H:^H2 0 (10:1,

v/v), refluxing in the presence of 1 equivalent of KO^H, the C-6 deuteriatlon of a 1 M solution of compound (113) proceeded smoothly, without the formation of side products or 0-3 deuteriated material, and was complete within 3 h, (Scheme 2.19, Figure 2.3). PhCH Hb COoH _COpM© NHCOpCHpPh (113, Ha=Hb=H) (117, Ha=Hb=^H) (121)

Reagents: i) SOCI2 (1.2 eq), MeOH, 0°O, then reflux 30 min, 100%; ii) PhCH2 0COCI (1.1 eq), NaHC0 3(3.5 eq), HgO, rt, 18 hours, 88%; iii) isobutylchloroformate (1 eq), N-methylmorpholine (2 eq),

EtOAc/DMF, stir, rt, 16 h, 87%; iv) H2, Pd/C, DGM/MeOH (3:1); v) PhMe, reflux, 16 h, 45% over 2 steps; vi) [MesO] BF^^ (3.5 eq), DOM, 66%.

Scheme 2.18. The synthesis of (3R)-2,5-dimethoxy-3-/sopropyl-3,6-dihydropyrazine .

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i)

X J L A

V— ik

i PPM

Figure 2.3. i)The 200 MHz proton NMR spectra in CD3OD ot (3R)-2,5-dimethoxy-3-/sopropyl-

3,6-dihydropyrazine (113). The signais at 4.02 ppm are due to the C-3 proton (1H. d, J3.8 Hz) and the 6-0 protons (2H, s), at 3.7 for the 2 and 5 methyloxy protons (6H, 2s), 2.15 for the

/sopropyl C-H proton and at 1.0 and 0.75 for the /sopropyl CHa’s (6H, 2d, J6.8 Hz), ii) The 200 MHz proton NMR spectra in C D3OD of (3R)-|"6-2H2]-2,5-dimethoxy-3-/sopropyl-3,6- dihydropyrazine (117). The signal at 4.02 ppm Is due to the C-3 proton (1H, d, J3.8 Hz).

OCHa T J L h --- >- T H a C O ^ N ^ ^ H H a C O ^ N ^ ^ H

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Reagents: i) CH30^H-^H20 (10:1 v/v). KOH (1 eq), reflux, 3 h, 80%. Scheme 2.19. The synthesis of C6 dideuteriated

dihydropyrazines.

This pleasing selectivity for C-6 anion formation over C-3 anion formation was further probed by refluxing compound (113) under the optimised exchange conditions for 6 h, twice as long as that required for exchange at C-6. Very little deuterium (<10 atom %)

was incorporated at 0-3 as judged by proton NMR spectroscopy and mass spectrometry. It was noted that 0-6 alkylated derivatives of the dihydropyrazine

(113)

To confirm that the chiral centre at 0-3 of the dideuteriated material

(117)

I

the dihydropyrazine ring was cleaved (0.1 M HOI, 16 h) to give the valine and glycine methyl esters. These were separated, the valine methyl ester was hydrolysed (5 M HOI, reflux, 3 h) and converted to its free base form with propylene oxide. The optical rotation of the recrystallised valine was -24.2° (c. 1.01, 5 M HOI) which compared favourably to that of an authentic sample of (2R)-va!ine [-24.95° (c. 1.09, 5 M HOI)], indicative of an enantiomeric excess of >97%. Thus, the deuteriation had proceeded highly selectively and without disturbing the stereogenic centre at 0-3.