Synthesis and Structural Determination of 6-O -prop-2-ynyl-1,2:3,4-di-O - Isopropylidene-α-D-Galactose

: In this work, an important sugar alkynyl ether has been synthesized in two subsequent steps starting from commercially available D -galactose ( 3 ). This kind of compounds is highly significant in the synthesis of biologically active molecules such as 1,2,3-triazole and isoxazoles. In the first step, galactose ( 3 ) was reacted with acetone in the presence of anhydrous copper (II) sulfate to produce 1,2:3,4-di-O-isopropylidene-α-D - galactose ( 4 ) in good yield. The latter was reacted with excess of 3-bromoprop-1-yne in DMF in the presence of NaOH pellets to afford the target molecule 5 in a very good yield. The temperature of this step is crucial in determining the reaction yield. The exact structure of compound 5 is identified using NMR technique and DFT calculations.


Figure 1. Naturally occurring acetylenes isolated from plant source; dehydromatricaria ester (1) and tariric acid (2)
Various alkynes possess remarkable biological activity and they have been utilised as drugs, 7,8 for example, the birth control medicine norethynodrel. 9 In addition, it was reported that some of the natural acetylenes i.e. falcarinol has been used as a pesticide. 10 Furthermore, both terminal and internal alkynes have essential role in the synthesis of many organic compounds like; aldehydes, 11 ketones, 12 carboxylic acids, 13 alkenes, 14 triazoles, [15][16][17][18][19] isoxazoles 20,22 and polymer. 23,24 Their conjugated polymers have great conductivity, and they have many applications in organic photonics. 25 The simplest way to prepare with variable alkyne chain is the alkylation of acetylene. 26 Alternatively, they can be synthesized by the elimination of X 2 or HX from alkenes 27 or propargylation of phenols and alcohols. 28 However, the latter requires harsher conditions due to their low acidity. 29 Alkynyl monosaccharides can be synthesized through the glycosylation reaction of propargyl alcohol with sugar peracetates. 30 Herein, we prepare alkynesugar ether by the reaction of propargyl bromide and galactose diacetonide using mild conditions and determine the 3D shape of the synthesized molecule.

Materials and Methods: General
Chemicals, reagents, and solvents were purchased from commercial providers; Ajax, Alfa Aesar and Sigma-Aldrich Chemical. Flash column chromatography was performed with silica gel (40-60 mesh). Aluminium-supported coated with silica (0.2 mm, 60 F 254 ) TLC plates were used. The progression of reactions was observed by TLC using freshly prepared alkaline KMnO 4 stain to develop spots on TLC plates. Melting points were determined on an Electrothermal Stuart SMP 30 capillary melting point apparatus. Specific rotation was detected using Automatic digital polarimeter KRUSS P3001 RS. Infrared spectra were recorded using prestige 21, SHIMADZU 2001, FT-IR. Proton, carbon and 2D NMR spectra were collected on Avance III 300, FT-NMR spectrometer, Bruker 2010 spectrometer at 300 MHz and 75 MHz respectively with TMS as an internal standard. The NMR spectra were recorded at the University of New South Wales, Australia. Synthesis Synthesis of 1,2:3,4-diacetonide-α-D-galactose (4) 31 Zinc chloride (10.0 g, 0.029 mol) was portions-wise added to acetone (50 mL) and concentrated sulfuric acid (0.1 mL) was added at r.t. to give a clear solution. Galactose (3.5 g, 0.018 mol) was then added, and the resulting white suspension stirred for 5 h at r.t. Afterward, a suspension of Na 2 CO 3 (6.5 g, 0.063 mol) in H 2 O (10 mL) was added to the yellow reaction mixture at 0 °C. The suspension was allowed to stir for 30 min then filtered and the solid discarded. The volatile solvent was removed in vacuo below 30 °C to give a residue. The resulting yellow oil and aqueous layer were separated, and aqueous layer further extracted with Et 2 O (3 × 10 mL). The combined organic layers were dried over MgSO 4 , and the solvent removed under reduced pressure to yield 1,2:3,4-diacetonide-α-D-galactose (4) as a paleyellow oil (3. Protected sugar 4 (0.52 g, 2 mmol) was dissolved in DMF (10 mL) in a dry flask and sodium hydroxide pellets (0.32 g, 8 mmol) were added. The flask was cooled in an NaBr 60 g-ice 100 g bath at -20 °C and the contents stirred for 10 min before 3-bromoprop-1-yne (2.2 mmol) was added dropwise. The reaction mixture was then allowed to stir for 24 h, gradually warming to r.t. The reaction mixture was partitioned between EtOAc (50 mL) and water (100 mL), the layers separated, and the aqueous layer extracted with more EtOAc (3 × 50 mL). The combined extracts were dried over Na 2 SO 4 and evaporated to dryness under reduced pressure. The residue was flash chromatographed (silica gel, Et 2 O / n-hexane 1:9) to

Results and Discussion:
The target compound 5 has been synthesized from D-galactose in two subsequent steps (Scheme 1):

Scheme 1. Synthesis of alkyne 5 from D-galactose (3)
Acetone is widely utilized in carbohydrate chemistry as a protecting group particularly for the syn vicinal diols. 37 The reaction of the hexose 3 with excess of acetone in the assistance of zinc chloride and sulfuric acid at room temperature for five hours afforded 1,2:3,4-di-O-isopropylidene-α-D-galactose (4) in a very good yield 81%.  The formation of compounds 4 has also been evidenced by 1 H NMR (300 MHz, CDCl 3 ) spectrum (Fig. 3)  Moreover, the structure of alcohol 4 is assigned by 13   First, the addition of 3-bromoprop-1-yne to the reaction mixture is crucial in the determination of the yield of reaction. The effect of temperature, at the addition of the propargyl bromide, on the reaction outcome is illustrated in Table 1.   The lower the temperature at the addition of propargyl bromide to an alcohol, the higher the reaction yield. This could be attributed to the formation of allenes during the reaction. 38 It is suggested that the formation of compound 5 follows the S N 2 mechanism in its initial step. Then, proton is subtracted due the base effect (Scheme 2).

Scheme 2. Suggested mechanism of the formation of compound 5
The disappearance of the (O-H) stretching band at 3483 cm -1 and the appearance of the bands at 3253 cm -1 and 2109 cm -1 due to the stretching of (≡C-H) and (C≡C) respectively in the FTIR spectrum (Fig. 5) are excellent evidence of the formation of alkyne 5.   140 The 13 C NMR spectrum of compound 5 (Fig. 8) shows three new signals at 79.8, 74.7 ppm and 58.7 ppm belonging to C2, C1 and C3 respectively. It is known that the preferred 3D shape of the pyranose six-membered ring in the saccharides is chair conformations 4 C 1 and 4 C 1 (Fig. 9). 40,41

Figure 9. Chair conformations of pyranoses
The computational study of compound 5 demonstrated that the lowest energy galactopyranose ring adopted chair 3 C O conformation (Fig. 10). According to Karplus equation, the magnitude of coupling constant decreases as the dihedral angle between the vicinal hydrogen atoms raises from 0° to 90° and then starts to increase with the raise of the angle. 42 In the suggested 3D shape of the six-membered ring, the dihedral angle between H2' and H3' is approximately 90° and this is confirmed by 1 H NMR as the coupling constant 3 J H,H between H2' and H3' is 2.4 Hz.

Conclusion:
Alkynyl sugar ether was produced by reacting protected sugar with propargyl bromide in the presence of a base. It is found that the reaction temperature is crucial in the determination of the final yield of the propargyl derivative. The 1D and 2D NMR spectra were consistent with the computational study, which revealed that the sugar pyran ring adopted chair conformation. However, the oxygen atom occupied the bottom edge of the chair not C-1 as expected.