Synthesis of 6-azido-1-(phenylsulfonyl)hexanol from 6-bromohexanol
Abstract
6-bromohexanol was reacted with a series of reagents through three reactions in order to
yield 6-azido-1-(phenylsulfonyl), an azide-functionalized linker used to attach a TAK-242
prodrug to tissue surfaces for sustained release.1 The first reaction utilizes an Sn2 mechanism to
remove the halogen and add an azido group and had an 85.6% yield. The trichloroisocyanuric
acid (TCCA) deprotonates the alcohol in the second reaction and allows TEMPO to deprotonate
the alpha carbon in order to create an aldehyde with a 42.6% yield. In the final reaction n-BuLi
deprotonates the methyl group of the phenyl methyl sulfone (PMS) allowing it to attack the
carbonyl carbon of the aldehyde. This creates the final product in a 12.2% yield (4.45% overall).
Scheme I: 6-bromohexanol to 6-azidohexanol under microwave
Scheme II: 6-azidohexanol to 6-azidohexanal using trichloroisocyanuric acid
Scheme III: 6-azidohexanal to 6-azido-1-(phenylsulfonyl)hexanol using n-BuLi
The FTIR and NMR data used to track the progress of the reaction show that the products
at each stage are fairly pure, except for the crude NMR taken before the column separation.
Introduction
Diabetes is an important area of research in modern medicinal chemistry. In diabetics, the
pancreas either no longer makes insulin due to the destruction of beta cells due to an autoimmune
condition (Type I) or progressive insulin resistance (Type II). Now, pancreatic islet cell
transplantation gives those with diabetes a chance to naturally produce insulin through new cells.
A recurring issue with transplantation is the rejection or damage of the transplanted organ or
cells, especially through inflammation. A new method of reducing inflammation is modifying the
surface of the islet cells to slowly “bleed” anti-inflammatory drugs, like TAK-242. In order to
attach TAK-242 to the islet cells, a linker molecule must be used that can bind to both the
prodrug and the cell surface.
Figure I: Scheme of the attachment of linker to TAK-242, the cell, and subsequent release1
This experiment was done in order to create a linker that can attach a prodrug to the islet
cell surfaces in order to be hydrolyzed to release the prodrug to reduce inflammation.1 When
producing molecules to be used for pharmaceutical purposes, it is important to have high purity,
yield, and, ideally, low cost.
Results and Discussion
The FTIR data (included at end) for this experiment showed the reactions were
progressing as expected. There were a few important peaks that were expected at every step in
the reaction. The most obvious peak was the wide alcohol peak at ~3300cm-1. This peak was
seen in the starting material and the 6-azidohexanol, but disappeared when the hydroxide group
was converted to a aldehyde. At this point in time three new peaks appeared. The carbon oxygen
double bond stretch showed sharply at ~1700cm-1 while the carbon hydrogen bond had two
peaks at ~2730/2830cm-1. These peaks disappeared, and the alcohol peak reappeared after the
addition of PMS. Another bond that was tracked was the azido group’s. There was initially no
peak because the starting material contained a halide, but when the azido group was added, a
new peak showed at ~2070cm-1 and remained for the rest of the experiment. It was more difficult
to tell which peaks were both new and important after the addition of PMS. From available FTIR
data it would be expected that two new peaks would appear in the 1225-980cm-1 stretch for the
sulfur/oxygen double bonds and in 710-570cm-1 for the S-C stretch.2
NMR data (included at end) also showed a good progression of the reactions. The initial
reactant NMR was very clean, showing the expected triplets at 3.6 and 3.4ppm for the carbons
adjacent to the alcohol and halide, respectively. The alcohol hydrogen showed lower than
expected at 2.6ppm. The rest of the alkane hydrogens showed at 1.4, 1.55, and 1.85 as expected.
The next NMR for 6-azidohexanol showed similar peaks, but the azido group is not nearly as
polar so the hydrogens on the adjacent carbon were more shielded than those next to the halide.
This resulted in a triplet showing at 3.25ppm. A multiplet appeared around 3.4ppm and could be
the alcohol peak that is not seen anywhere else, but it should, theoretically, just be a rough
singlet. After the conversion to 6-azidohexanal, another NMR was taken. The most important,
and distinct, change in this graph was the addition of the triplet aldehyde peak at 9.8ppm. This
also further deshielded the carbons adjacent to the aldehyde and moved their peak up to 2.5ppm.
This NMR showed two unexpected peaks at 3.3 and 3.4 along with heavy interference peaks
from acetone and chloroform. The crude NMR taken before the column separation was, as
expected, messy, but surprisingly similar to the purified product NMR. The crude NMR had an
additional significant peak at 1.9ppm. The final NMR had the expected benzene peaks in the 7.58.0ppm range, with the hydrogens closer to the S=O bonds being more deshielded. The alcohol
peak showed at 3.4ppm as expected and the adjacent carbon also showed the expected quartet at
4.1ppm. This is more deshielded because it sits next to the alcohol but is also adjacent to the
sulfur which is double bonded to two oxygens. The alkane peaks remained the same as prior.
There was still interference from acetone at 2.17ppm and chloroform at 7.26ppm along with an
unexpected, strong singlet at 2ppm. This could be the beta carbon from the alcohol, as the strong
deshielding from the alcohol may have further deshield the next carbon, but it would still be
expected to be a quartet. Unknown multiplets also appeared at 1.5ppm. These appeared on the
crude NMR and are most likely leftover contaminants that made it through the column.
1H NMR (300 MHz, Chloroform-d) δ 3.61 (t, J = 6.6 Hz, 2H), 3.39 (t, J = 6.8 Hz, 2H), 2.63 –
2.55 (m, 1H), 1.85 (dq, J = 8.1, 6.7 Hz, 2H), 1.64 – 1.27 (m, 4H).
Reported coupling constants for 6-bromohexanol
1H NMR (300 MHz, Chloroform-d) δ 3.66 (t, J = 6.5 Hz, 1H), 3.28 (t, J = 6.9 Hz, 1H), 2.30 –
2.11 (m, 1H), 1.61 (dt, J = 9.9, 6.9 Hz, 1H), 1.45 – 1.33 (m, 0H).
Reported coupling constants for 6-azidohexanol
1H NMR (300 MHz, Chloroform-d) δ 9.79 (t, J = 1.6 Hz, 1H), 3.48 – 3.35 (m, 1H), 3.30 (t, J =
6.8 Hz, 2H), 2.55 – 2.30 (m, 2H), 2.00 – 1.70 (m, 1H), 1.74 – 1.55 (m, 2H), 1.54 – 1.34 (m, 1H).
Reported coupling constants for 6-azidohexanal
1H NMR (300 MHz, Chloroform-d) δ 8.02 – 7.83 (m, 3H), 7.77 – 7.53 (m, 4H), 4.13 (q, J = 7.2
Hz, 3H), 3.42 (q, J = 3.6, 2.2 Hz, 0H), 3.39 (s, 2H), 3.41 – 3.12 (m, 4H), 3.08 (d, J = 0.6 Hz, 1H),
2.06 (d, J = 0.6 Hz, 3H), 1.93 – 1.81 (m, 1H), 1.27 (td, J = 7.1, 0.6 Hz, 4H), 0.87 (s, 0H).
Reported coupling constants for 6-azido-1-(phenylsulfonyl)hexanol
The only UV active compounds used in the experiment were PMS and the final product,
so they were TLC’d after the final purification and to track the progress of the column. All
TLC’s were run in 25/75 ethyl acetate/hexanes.
Molecule
PMS
Final Product
Rf value
0.20
0.55
Table I: TLC data
The reactions had good yields throughout the experiment until the column
chromatography was done. The 85.6% yield for the first reaction was fair, but considering the
simplicity of the reaction and its mechanism would indicate that a quantitative yield could
possibly be achieved. The next reaction progressed with a 42.6% yield, but was a considerably
more complicated reaction. The final reaction was similarly complicated, using n-BuLi to
deprotonate PMS which would act as a nucleophile to attack the aldehyde. A similar yield would
be expected, but product was most likely lost in the column due to user error leading to the
12.2% yield.
Reaction
Reactant mass (g)
Halide -> Azide
0.601
Alcohol -> Aldehyde
0.407
Aldehyde -> Sulfonyl
0.171
Reactant moles
0.00332
0.00284
0.00121
Mol Ratio
1 to 1
1 to 1
1 to 1
Moles Product
0.00284
0.00121
0.000148
Theoretical Yield (g)
0.475
0.401
0.343
Actual Yield (g)
0.407
0.171
0.042
Percent Yield
85.6
42.6
12.2
Table II: Stoichiometric ratio of reaction and yields
Conclusions
From the experimental data, it would appear that the prodrug linker 6-azido-1(phenylsulfonyl)hexanol can be produced with fair yields and high purity. With more practice
high yields could probably be achieved. The main area of loss with this experiment was in the
column. The step-wise eluent setup may not have been ideal for running the molecule in question
and might need to be adjusted for the future. In addition, the time spent in the column needs to be
reduced so the diffusion of the molecule has less of an effect, although this is a user error and
will improve with time. Another probable error occurred upon the addition of NH4Cl used to
quench the reaction. The protocol said to wait until the solution was clear and then add the
quenching agent. Due to time constraints, the NH4Cl was added while there was still color in the
solution. Upon addition, the solution immediately turned clear, indicating the reaction may have
been cut short. This drug is meant to act as a mediator of slow drug release in-vivo for islet cell
transplantation. A 2018 paper by the Kane lab at Baylor University concluded that this linker
effectively releases TAK-242 over several hours after transplantation, effectively providing
protection against inflammatory damage. Furthermore, it was found that the conjugation
chemistry this linker is used for can be applied to other tissues and organs, indicating a possible,
broader application.1
Experimental Section
0.6g of 6-bromohexanol and 0.43g of sodium azide were heated in 4ml of water in the
microwave for 1 hour at 100 degrees C. The mixture was diluted with 25ml of water and
extracted with three 25ml aliquots of DCM. The organic layer was separated with a separatory
funnel, dried with magnesium sulfate, and rotovapped to give a colorless oil. NMR and FTIR
characterizations were taken at this point. Two days later the 6-azidohexanol (0.407g) was
vigorously stirred with TEMPO (0.01g) and sodium bicarbonate (0.284g) in 5.7ml of
dichloromethane and 0.59ml of water. After this mixture is thoroughly stirred, 0.264g of solid
trichloroisocyanuric acid was added in small portions and allowed to stir for an additional 30
minutes. The organic phase was separated and washed successively with saturated aqueous
NaHCO3 and brine. This mixture was again dried with magnesium sulfate and rotovapped to
provide the 6-azidohexanal. Additional NMR and FTIR characterizations were done at this time.
Five days later, 0.172g phenyl methyl sulfone was stirred with 13ml anhydrous THF (should
have been 3.3ml) and cooled to -78 degrees C. At this point, 0.688ml of 1.6M n-BuLi was added
dropwise and the solution was allowed to warm to 0 degrees C over 30 minutes. The mixture was
then re-cooled to -78 degrees C and 0.172g 6-azidohexanal was added. The mixture was stirred
for 15 minutes and removed from the cooling bath to warm to room temperature. When the
mixture became clear, 5ml of saturated aqueous NH4Cl was added to quench the reaction. This
mixture was left to sit for two days. The mixture was finally diluted with ethyl acetate and
washed with water and brine, dried with magnesium sulfate, and rotovapped to yield 0.299g of
crude product. A crude NMR was taken at this point. A column was set up with silica gel and run
with a step gradient of 0, 25, and 50% ethyl acetate in hexanes. The progress of the column was
tracked with TLC run in 25/75 ethyl acetate/hexanes. After purification, 0.042g of 6-azido-1(phenylsulfonyl)hexanol, a pale, yellow oil was yielded. Final TLC, NMR, and FTIR
characterization was done at this time.
References
1. Chang, Charles A. Et Al. Ex-vivo generation of drug-eluting islets improves transplant
outcomes by inhibiting TLR4-Mediated NFkB upregulation. Biomaterials. 2018. 159, 13-24.
2. Hampton, Demoin. Et Al. (2010). Vibrational spectroscopy tutorial: sulfur and phosphorus.
Available from http://faculty.missouri.edu/~glaserr/8160f10/A03_Silver.pdf.