Novel Furanoid α-Substitued α-Amino Acid as a Potent Turn Mimic in Peptide Synthesis

A stereoselective approach has been developed to the new sugar amino acid and potential potent turn mimic 5-O-(tert-butyldimethylsilyl)-3-deoxy-1,2-O-isopropylidene-3-methoxycarbonylamino-alpha-D-xylofuranose 3-C-carboxylic acid (12), via the [3,3]-sigmatropic rearrangement of allylic thiocyanates (Z)-6 and (E)-7, prepared from D-xylose. The synthesis of a new dipeptide 13 is also described.


Introduction
Sugar amino acids (SAAs) can be found in nature largely as a molecules that combine the structural features of simple amino acids with those of simple carbohydrates [1].The resulting hybrid is a highly substitued polyfunctionalized synthon which can be used for the preparation of modified analogues of biologically active peptides and/or oligosaccharides.Sugar amino acids represent an important class of conformationally constrained templates that have been used extensively in recent years in many peptidomimetic studies [1,2] and have emerged as attractive building blocks for the incorporation of a sugar moiety into short peptide sequences using standard peptide coupling techniques, thus opening the door to novel peptidomimetics.They are also known as β-turn mimics and introduction of conformationally restricted nonpeptide isosteres into the peptide backbone to achieve desirable secondary structures is a great interest of many synthetic chemists [3].

Results and Discussion
We report herein a synthetic approach to 5-O-(tert-butyldimethylsilyl)-3-deoxy-1,2-Oisopropylidene-3-methoxycarbonylamino-α-D-xylofuranose 3-C-carboxylic acid (12) as a building block for peptide scaffold and conformationally restrained peptidomimetics.The Wittig reaction of known 5-O-TBDMS-1,2-O-isopropylidene-α-D-erytro-pentofuranos-3-ulose (1) [4] with ethoxycarbonylmethyl-enetriphenylphosphorane in dry dichloromethane gave, after chromatographic separation, pure (Z)-α,β-unsaturated ester 2 and its (E)-isomer 3 (9.6:1,90.5%) (Scheme 1).The Z and E configurations of the exocyclic double bond in 2 and 3 were determined by 1 H-NMR spectral analysis, including NOE data.The irradiation of the H-4 proton in 2 led to a 3.4 % enhancement of the intensity of H-6, while the irradiation of the H-6 proton resulted in a 3.9 % enhancement of the intensity of the H-4 signal, along with a small 1.3 % enhancement of the H-2 proton.On the other hand, when the H-2 proton in 3 was irradiated, a strong (5.0 %) NOE enhancement of the H-6 signal was observed, while the irradiation of the H-6 proton resulted in a 4.2 % enhancement of the intensity of the H-2 signal.Finally, the irradiation of the H-4 proton in 3 resulted in a 1.2 % enhancement of the intensity of the H-6 proton signal.Reduction of esters with LiAlH 4 in dry diethyl ether afforded the allylic alcohols (Z)-4 and (E)-5 in 96 and 72% yields, respectively, after silica-gel chromatography.The thiocyanates (Z)-6 and (E)-7 were prepared by S N 2 substitution of the O-mesyl group in the corresponding mesylates, derived from allylic alcohols (Z)-4 and (E)-5, by the thiocyanate group (KSCN/CH 3 CN) (Scheme 1).The thermal rearrangements of thiocyanates (Z)-6 and (E)-7 was carried out in heptane under a N 2 atmosphere at 90 o C for 16 h and gave isothiocyanate 8 as the sole reaction product in 82 and 70% yields, respectively, after silica-gel chromatography.The epimeric isothiocyanate 8a was not detected in the reaction mixtures.The stereochemistry of the new quarternary carbon center (C-3) introduced in 8 was determined as (S) by 1 H-NMR spectral analysis, including NOE data.Thus, when the H-4 proton was irradiated, a strong (8.2%)enhancement of the intensity of the H-6 signal was noted and the irradiation of the H-6 proton resulted in a 5.4 % enhancement of the intensity of the H-4 signal, indicating a cis relationship between the vinyl group and the H-4 proton on the furanoside ring (Figure 1).In our subsequent strategy, the isolated isothiocyanate 8 was converted into the desired αsubstitued α-amino acid 12.In the first step, the isothiocyanate group was transformed into the thiourethane 9 in 95% yield after silica-gel chromatography by its reaction with CH 3 ONa in dry methanol at room temperature for 4 h.The thus prepared thiocarbamate 9 was converted into the corresponding oxygen derivative 10 by the action of mesitylnitrile oxide in acetonitrile (23 h, 93%) (Scheme 2).The oxidation of carbamate 10 was accomplished with a catalytic amount of ruthenium (III) chloride and NaIO 4 in 2:2:3 CCl 4 /CH 3 CN/H 2 O to give aldehyde 11 in 70% yield.This aldehyde was then oxidized to the protected amino acid 12 in 74% yield (after flash chromatography) by treatment with sodium chlorite/2-methyl-2-butene (Scheme 2).
Among the methods available for the linkage of amino acids and peptides coupling with carbodiimides is one of the most frequently used.Our coupling reaction with DCC [5] and glycine methyl ester hydrochloride was performed in dry dichloromethane in the presence of Et 3 N at 0 o C for 1 h and then for 18 h at room temperature to afford dipeptide 13 in 78% yield after silica-gel chromatography (Scheme 2).

Conclusions
In summary, a stereocontrolled synthesis of the nonproteinogenic α-substitued α-amino acid 12 employing thiocyanates (Z)-6 and (E)-7 as the educts has been reported.The coupling reaction between this sugar amino acid 12 and glycine methyl ester provided dipeptide 13 as a potent peptidomimetic.

General
The melting points were determined on the Kofler block and are uncorrected.Optical rotations were measured in chloroform with a P3002 Krűss polarimeter and reported as follows: [α] D 25 (c in g per 100 mL).NMR spectra were recorded at room temperature on a Varian Mercury Plus 400 FT NMR spectrometer ( 1 H at 400.13 MHz and 13 C at 100.6 MHz).Chemical shifts are referenced either to tetramethylsilane used as internal standard for 1 H or to the solvent signal ( 13 C-NMR, δ CDCl 3 =77.0). 13C-NMR multiplicities were determined by using a DEPT pulse sequence.Reactions were routinely monitored by TLC (Merck 60 F 254 ) and the products were visualized by UV light absorption at 254 nm or by spraying with Mo-reagent or KMnO 4 -reagent.All reactions were performed under an atmosphere of nitrogen.Solvents were purified by standard procedures and distilled before use.
Column chromatography was carried out on the glass columns using Kieselgel (0.035-0.070 mm) silica gel.
was prepared according to a published procedure [4].

5-O-(tert-Butyldimethylsilyl)-3-deoxy-1,2-O-isopropylidene-3-C-(E)-(2-thiocyanatoethylidene)-α-Dxylofuranose (7)
To a solution of (E)-5 (0.23 g, 0.70 mmol) in dry CH 2 Cl 2 (1.7 mL) were added Et 3 N (0.15 mL, 1.04 mmol) and CH 3 SO 2 Cl (0.07 mL, 0.83 mmol) at 0 °C.The reaction mixture was stirred at 0 °C for 15 min and then at room temperature for 45 min.The solvent was evaporated under reduced pressure.The residue was diluted with diethyl ether (3 mL) and the solid was removed by filtration.Evaporation of the solvent under reduced pressure afforded crude mesylate which was used directly in the next reaction without any further purification.To a solution of crude mesylate (0.27 g, 0.66 mmol) in CH 3 CN (3 mL), KSCN (0.08 g, 0.82 mmol) was added.After stirring for 3 h at room temperature under a nitrogen atmosphere, the solvent was evaporated.The residue was diluted with diethyl ether (3 mL) and the solid was removed by filtration.The evaporation of the solvent under reduced pressure and chromatography of the residue (hexane-ethyl acetate, 7:1) afforded 0.18 g (70% from 5) of pure thiocyanate 7 as a colorless oil; [α] D 25 = +164.

5-O-(tert-Butyldimethylsilyl)-3-deoxy-1,2-O-isopropylidene-3-C-(Z)-(2-thiocyanatoethylidene)-α-Dxylofuranose (6)
Et 3 N (3.18 mL, 22.9 mmol) and CH 3 SO 2 Cl (1.42 mL, 18.3 mmol) were added at 0 °C to a solution of (Z)-4 (5.05 g, 15.3 mmol) in dry CH 2 Cl 2 (36 mL).The reaction mixture was stirred for 15 min at 0 °C and then for 45 min at room temperature.The solvent was evaporated under reduced pressure.The residue was diluted with diethyl ether (60 mL) and the solid was removed by filtration.Evaporation of the solvent under reduced pressure afforded crude mesylate which was used directly in the next reaction without further purification.To a solution of crude mesylate (5.99 g, 14.7 mmol) in CH 3 CN (55 mL), KSCN (1.78 g, 18.3 mmol) was added.After stirring for 5 h at room temperature the solvent was evaporated.The residue was diluted with diethyl ether (60 mL) and the solid was removed by fitration.The evaporation of the solvent at reduced pressure and chromatography of the residue (hexane-ethyl acetate, 7:1) gave 4.13 g (73% from 4) of pure thiocyanate 6 as a white solid; m.