Polyesterification of Vitamin C under Basic Anhydrous Conditions


Synthesis of 3-Substituted Esters of Ascorbic Acid Using 5,6-O-Cyclohexylidene-L-Ascorbic Acid 

I would like to thank San Diego Mesa's Bridges to Baccalaureate Program for allowing me to conduct research while attending community college.  A special thank you to Dr. Alexander for making this available to us students.  Countless hours were spent running reactions that would require us to spend extra time in the laboratory, and for helping all of us with our abstracts, presentations, and teaching us valuable laboratory experience that would follow us.

The scope Dr. Alexander's research was investigating the properties and synthesis of ascorbic acid's fatty acid derivatives.  Under his supervision, we conducted novel synthetic pathways to creating new derivatives that would be substituted at the 2,3,5, and 6 positions on the molecule with fatty acids of differing lengths.  From here, additional diagnostic testing was conducted in order to properly classify each derivative.  The results from this laboratory were sent to Dr. Russel's laboratory at UC San Diego where experimentation was conducted to test the inhibition of PFK-1 enzyme, a key contributor to glycolysis.

The passion behind this research was searching for an alternative way to stopping the growth of a tumor.  Since cancer cells require more energy input than normal healthy cells, the idea is to limit their ability to convert energy through glycolysis.  The rationale behind attaching fatty acids to ascorbic acid is to make the molecule more membrane soluble in hopes of optimizing Vitamin C's ability to inhibit PFK-1 enzyme.

Summer 2016     Polyesterification of Vitamin C under Basic Anhydrous Conditions

My first summer spent in this laboratory was geared towards honing the basic laboratory skills needed to conduct research.  Due to the course planning at community college, I was only able to participate in the lab part time in order to transfer on-time while taking summer classes.  Having never taken an organic chemistry course, I was mentored by my peers in addition to studying the material during my free time.  It was during this time that I shadowed Emmanuel Gutierrez as sort of an introduction into the field.  It was during this time that I learned the importance of preparation and how to classify the results at every time interval of the procedure.

The scope of this summer's research was to investigate the reaction mechanism and reactivity of the molecule.  We found that the 6- position was most reactive under basic conditions because it contained a primary hydroxyl group.  Due to steric hinderance, the 2- position was second most reactive but was rarely mono-substituted, making the 2,6- derivative the second most favored, followed by the 2,5,6-trisubstituted, and 2,3,5,6-tetrasubstituted.  In order to qualitatively analyze this, we utilized IR and NMR spectroscopy in addition to thin-layer and column chromatography.

The purpose of this research was pivotal in order to create the different derivatives.  Without knowing the reaction mechanism, synthesizing one of the 15 different derivatives would be a matter of guessing.  

Summer 2017     Synthesis of 3-Substituted Esters of Ascorbic Acid Using 5,6-O-Cyclohexylidene-L-Ascorbic Acid 

Utilizing the findings from the previous summer's research, 3 new research projects were proposed in order to create novel synthetic pathways; all of them being mono-substituted derivatives at the 2, 3, and 5 positions.  Mine being at the 3-.  The rationale behind creating a mono-substituted derivative stemmed from the findings from the pharmacology lab in 2016.  They found that in theory, a tetra-substituted derivative would yield the greatest membrane solubility; however, the steric hinderance from the fatty acids could limit ascorbic acid's ability to inhibit PFK-1.  In addition to our findings in 2016, another year had passed and I had finally received a formal education in organic chemistry that I could apply to my research.

Creating a derivative 3- position posed a challenge for myself and Bryant Moss due to it being energetically unfavorable.  In order to bypass the reactivity of the 6- position we utilized an acetal protection group before proceeding with our reaction, which went as follows below:

In conducting this reaction, we encountered many issues with the yields after every time step.  Recrystallizations were vital to this synthesis in order to limit the side product yield.  In order to harvest the ascorbic acid with protecting group (CHAA) , sodium bicarbonate and methanol were added in order to neutralize before removal of excess cyclohexanone, methane and sodium bicarbonate.  The extraction of the CHAA required 20mL acetone, 500mL of hexane per 1g of CHAA which seemed excessive, but was the most efficient method to date.  Due to this inefficiency, the yield of this reaction was very low.


Another error was encountered via esterification of CHAA with Lauryol Chloride under basic conditions due to the hygroscopic nature of this new molecule.  In order to bypass this effect, the reaction was conducted under inert conditions using a Schlenk Line.  Trimethylamine was chosen as the base due its basic carbonyl-activating properties, which would lead to the synthesis of a 3- substituted product of higher yield, in addition to disubstituted side products.  Again, the crystallization and extraction would account for a large amount of error in yield.  After the conclusion of this research, we were able to recover some IR spectroscopy images that suggest the synthesis of a 3-substituted ascorbic acid derivative in addition to a 3-substituted CHAA.