Calculate the theoretical yield of virstatin ethyl ester and virstatin.
Procedure – Step 1 Synthesis of virstatin ethyl ester (with notes and musings about procedural steps in italics)
1,8-naphthalimide (0.25 g) and potassium carbonate (0.26g) are added to a 10 ml round bottom flask containing a stir bar.
1.3 ml DMF (N,N-dimethylformamide) is added to the flask. The DMF will be in a “sure seal” bottle with a septum. You will obtain the DMF using a syringe and needle. Wear gloves when obtaining and dispensing the DMF. Add ethyl 4-bromobutanoate (0.27 ml) (it might say ethyl 4-bromobutyrate on the bottle) to the reaction flask, using an automatic pipet.
Heat under reflux, with stirring, for 1 hour. What will the sand temperature have to be to get DMF to boil?? When would you start heating the sand!?
After cooling to room temperature add 5 ml water to the reaction solution and cool the resulting mixture in an ice bath.
Using a Buchner funnel, vacuum filter the ice cold reaction mixture, using water to assist in transfer of the solid crude product.
Place the crude product in a 50 ml Erlenmeyer flask along with a boiling stone and 10 ml methanol. Heat the mixture to boiling . (Make sure sand bath is at a proper temperature to boil methanol.
Note 3: The 10 ml methanol can first be added to the 10 ml round bottom flask and used to coax any remaining virstatin ethyl ester off the sides of the glassware. Squirting the solvent up and down through a pipet to “blast off” the solid clinging to the sides of the flask can be helpful. Goal: get all virstatin ethyl ester into the Erlenmeyer flask.)
Heat the methanolic solution to boiling again ( or it still is boiling if no hot filtration was necessary) and then add water, approximately a milliliter at a time, to the boiling mixture until a slight cloudiness develops. The paper says approximately 10 ml water will be necessary for this. (The product is soluble in methanol not but as soluble in water, so dissolving the substance in methanol and then adding water should create the optimal recrystallization solvent.) Try to not force the material out of solution with too much water, because something you do not want may be forced out as well!!
Cool the mixture to room temperature and then in an ice bath. Collect the resulting crystals by vacuum filtration, (Buchner funnel) washing with an ice cold mixture of a few milliliters of 1:1 methanol and water. Transfer the crystals to a watch glass and leave them in your locker.
In the following lab period you will weigh your virstatin ethyl ester, and prepare a sample for NMR analysis before beginning the synthesis of virstatin from this precursor. SEE NMR preparation note below.
Instructors’ notes from the JChemEd article:
- Minimize reagents exposure to air.
- Student results: light tan powder, 0.24 to 0.32g.
Week 2 - Procedure – Step 2 –Synthesis of Virstatin
Add 0.10g of your virstatin ethyl ester (synthesized last week/ characterized today) to a 10 ml round bottom flask containing a stir bar.
Add 1.5 ml methanol to the flask, followed by 0.15 ml 10% NaOH (aq). Heat the mixture under reflux for 30 minutes. (Proper sand temperature is???)
Cool the reaction mixture and place it in an ice bath. Add sufficient 3M HCl (aq), a milliter at a time, with stirring, to the reaction mixture to adjust the pH to strongly acidic to pH paper – red.
Add 3 ml water to the mixture and leave the reaction mixture in an ice bath for a few more minutes.
Vacuum filter the resulting solid, washing with water.
Recrystallize the solid from a minimum amount of hot methanol. Cool in ice, scratch if necessary.
Vacuum filter the resulting crystals and wash them with 2 ml icy cold methanol.
Store the virstatin sample in your locker. Pass in any remaining virstatin ethyl ester now. You will weigh the virstatin product, and prepare an NMR sample next week, passing in any remaining virstatin at that time.
1,8-naphthalimide would not appear in the NMR because that same structure is part of the product, and far enough away from the structural differences between the ester and the imide that no differences would be apparent between the peaks for 1,8-naphthalimide and the same hydrogens from the ester.
Instructor notes from the paper:
- A significant amount of precipitate should appear when the reaction mixture is acidified.
- Typical student yield 0.045 to 0.077g, 50-80% yield, of tan small needles.
![Virstatin can be synthesized in a two step process as shreyn below:
Step 1:
3'
H
K,CO3
DMF
Reflux 1 hour
Br.
ethyl 4-bromobutanoate
|1,8-naphthalimide
Step 2:
virstatin ethyl ester
3'
3'
1'
HO.
4
1'
2'
2'
1. NaOH (aq)
Methanol
Heat 30 min
1
1
2. HСІ
2
3
5
4
4'-[N-(1,8-naphthalimide)]-n-butanoic acid
Virstatin ethyl ester
(virstatin)](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Fdc99cc4f-9438-44d8-9b43-49724a6b39cf%2F85f2eba4-5120-43f5-beda-a1e1c57842b2%2Fb65b1es_processed.png&w=3840&q=75)
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