Protecting groups play significant role in modern organic synthesis, since they allow hiding the rea — Organic Chemistry Chemistry Question
Acetone as a protecting agent
Protecting groups play significant role in modern organic synthesis, since they allow hiding the reactive X-H groups (X = O, N, S) from interaction with, mainly, nucleophilic and oxidizing reagents. At the same time, protecting groups are further easily removed by applying specific reagents under mild conditions. Acetone, commonly known as an organic solvent, is also widely used in organic synthesis as a protecting agent. Acetone reveals a broad spectrum of the reaction ability towards hydroxyl, amino and thiol groups forming either hemiketals or ketals (and their N- and S-analogues) depending on the number and location on nucleophilic X-H groups. In the form of its (hetero)ketal, the acetone residue can be considered in the protected molecule as part of a five-membered saturated 1,3-diheterocycle.
In this task you will prepare acetone derivatives of carbohydrate D-mannose (I) and α-amino acid L-cysteine (II).
[VISUAL]
Chemicals and Reagents
• D-Mannose, C6H12O6
• Iodine, crystalline
• Anhydrous acetone
• Na2S2O3 solution, dilute
• Chloroform
• Na2SO4, calcined
• L-Cysteine hydrochloride
• Ninhydrine reagent (0.3 % sol-n of ninhydrine in n-butanol cont. 3% of sodium acetate)
• C4H9OH, n-butanol
• CH3COONa, solution
Equipment and Glassware
• Magnetic stirrer with heating
• Magnetic bar
• Glass beaker, 50 or 100 cm3 (2 ea.)
• Round-bottom flask, 50 cm3
• Reflux condenser
• Laboratory stand with metal rings and clamps
• Thermometer
• Adding funnel
• Separating funnel
• Filter flask
• Porous Shott’s glass filter (2 ea.)
• Rotary evaporator
• Water- or vacuum pump
• Analytical balance (± 0.001 g)
• Pipette pump
• Capillary for melting point determination (2-3 ea.)
• Glass tube for capillary filling
• Melting point apparatus
• Filter paper
• Glass rod
• Ice bath
Procedure
A. D-Mannose protection with acetone
Fix a beaker on a magnetic stirrer with a metal ring attached to a stand. Place 200 mg of mannose, 60 mg of crystalline iodine and 12 cm3 of anhydrous acetone in the beaker. Attach to stand a thermometer with its bulb in the reaction mixture. Heat the reaction mixture for ca. 30 min at 35°С with stirring. After all the mannose is dissolved, turn the heater off and cool the mixture down to the room temperature. Then fix an adding funnel above the beaker using a metal ring attached to the stand (take care the stopcock is closed!). Pour the dilute Na2S2O3 solution into the funnel and add it dropwise to the brown reaction mixture until the color disappearance. Add 10 cm3 of water and transfer the reaction mixture from the beaker into a separating funnel (take care the stopcock is closed!) fixed on the stand using a metal ring. Add 10 cm3 of chloroform and close the funnel by placing the stopper at its top. Take the funnel in your hands so that its narrow end is directed upwards and away from yourself. Carefully turn the stopcock, release the air and close the funnel back. Shake the funnel several times with agitation and release the air as described above. Repeat shaking and air release three times. Then hang the funnel back on the metal ring and wait until the aqueous and organic layers are clearly separated. Remove the stopper from the top of the funnel. Carefully open the stopcock and let the lower organic layer to flow into a beaker. Leave the upper aqueous layer in the funnel. Add another 10 cm3 of chloroform to the funnel and repeat the extraction procedure using the same beaker. Wash the combined organic layers with 10 cm3 of water using a clean separation funnel. Place calcined Na2SO4 into the beaker with combined organic layers. Fix the beaker on the magnetic stirrer, add the magnetic bar and stir the mixture for 15 min. Filter the drying agent off. Remove the solvent from the filtrate using a rotary evaporator. Weigh the obtained white product and calculate the yield. Pick out a few crystals of the product for further determination of its melting point.
B. Modification of L-Cysteine with acetone
Fix a round-bottom flask on a stand. Place 100 mg of L-cysteine hydrochloride in 2 cm3 of anhydrous acetone in the flask. Attach the reflux condenser and heat the mixture to boiling. The starting amino acid hydrochloride readily dissolves, which is shortly followed by the product precipitation. Keep refluxing for about 30 min, then remove the condenser and cool down the reaction mixture using an ice bath. Knead the content of the flask and transfer it onto the glass Shott filter. Turn on the vacuum or water-pump, connect it to the filtration flask and filter the precipitate off. After the mother liquor stops dropping down, disconnect the flask and take the glass filter off. Rinse the reaction flask with the mother liquor, place the glass filter back, pour the content of the reaction flask onto the filter, and connect to the vacuum line. After the mother liquor stops dropping down, disconnect the flask. Add 1 cm3 of anhydrous acetone to the precipitate, knead with a glass rod, and connect the flask to the vacuum line again. To provide for effective drying, keep the precipitate pressing with the glass rod from time to time. Keep drying the product under vacuum for at least 10 min. Pick out a few crystals of the product for further determination of its melting point.
Test reaction
Do the following test to check whether the reaction of cysteine protection with acetone is complete.
Ninhydrine reaction. Dissolve several milligrams of the product in aqueous acetone, and immediately apply a drop of the resulting solution to filter paper. Cover the spot with a drop of ninhydrine reagent. Gently heat up the filter paper. Perform the same test with the starting amino acid. Compare the results and explain the difference.
Expected Test Result and Solution: Ninhydrine test. The spot with the product will show no color change, whilst that with the starting amino acid will become colored (blue-violet to brown-violet).
Determination of melting point
Determine the melting points of the products according to the directions in Problem 31.
Melting points and yields of the products:
- Acetone derivative of mannose: Melting point 118-120 °C, Yield 79%
- Acetone derivative of cysteine: Melting point 148-150 °C, Yield 68%
Draw the mechanism of formation of 1,3-dioxolane ring from acetone and 1,2-diol. Which catalyst acid or base, will you apply? Why?
Model Answer
[VISUAL]
Transformation of hemiketal into full ketal needs the acid catalysis to protonate hydroxyl group, which is further removed in the form of water molecule. The resulting positively charged carbocation-type intermediate is stabilized by electron donation from oxygen lone pair.
Draw the products of acetone reaction with trans- and cis-cyclohexane-1,2-diols. Which of the products is thermodynamically more favorable?
Model Answer
[VISUAL]
cis-Fused six- and five-membered rings in the resulting product of cis-cyclohexane-1,2-diol are more stable than trans-fused rings. The reason is the higher bond and angles distortion in trans-fused bicycles.
Based on the answer to Question 2, explain the nature and stereochemistry of the product of the D-mannose reaction with acetone paying attention to the mutual stereochemical relationships between vicinal hydroxyl groups in the starting sugar. Why the initial six-membered pyranose transforms into five-membered furanose? What is the way of such transformation in carbohydrate chemistry?
Model Answer
In the furanose form of D-mannose, there is a possibility to form two rather than one (in the pyranose from) 1,3-dioxolane rings, which is more thermodynamically favorable. Pyranose – furanose transformation proceeds via the open aldehyde form of the carbohydrate.
What conditions and reagents would you apply to remove acetone protecting groups from diacetonemannose?
Model Answer
Aqueous hydrochloric acid.
Draw the mechanism of product formation in the reaction of cysteine with acetone. Explain the role of hydrochloric acid.
Model Answer
[VISUAL]
Acid catalysis enhances the electrophilicity of carbonyl carbon atom (enhancing carbonyl activity). Thiol group reacts first due to higher nucleophilicity compared to that of amino group.
Draw the mechanism and products of the reaction between cysteine and ninhydrine. Show the product which is responsible for the color of the reaction mixture.
Model Answer
[VISUAL]
The reaction mechanism shows cysteine reacting with ninhydrin, undergoing decarboxylation, and subsequently condensing with another ninhydrin molecule to produce Ruhemann's purple (the colored product).