XX TrxR Thioredoxin reductase
I. Introduction
4. Results and Discussion: Synthesis of Target Benzenesulfonates
4.1. From the Bottom Up: Retrosynthesis and Initial Synthesis
“My mind rebels at stagnation. Give me work, give me problems.”
Sir Arthur Conan Doyle, The Sign of Four
While computational modeling is a vastly powerful tool in the process of drug design, the true test of the success of a designed drug lies in the results obtained through biological testing. Before a drug candidate can be subjected to this testing, the in silico molecule must be translated from the computer screen to a vial through chemical synthesis. One of the downsides to computational modeling is that a compound which shows good prospects on screen might be so synthetically challenging that the molecule might never make it to the testing phase, despite the good computational results.
After identification of suitable candidate molecules through computational modeling, retrosynthetic analysis identified two possible synthetic routes (Scheme 4.1). Pathway A involves the formation of a chalcone from a 2’-hydroxyacetophenone and a suitable benzaldehyde in a base-catalyzed condensation reaction, followed by a base-catalyzed ether formation between the phenolic oxygen atom of the acetophenone ring and an appropriate benzyl halide. Pathway B involves the same starting components;
however the order of the reactions is different. In this pathway, the ether bond is created first between the phenolic oxygen atom of the 2’-hydroxyacetophenone and a benzyl halide, followed by the base- catalyzed condensation reaction with the benzaldehyde. The final steps in these pathways are identical – addition of a sulfonyl chloride moiety to the benzyl ring, followed by conversion to the desired sulfonic acid or sulfonamide. These pathways provide access to a wide range of compounds as a variety of substituted benzaldehydes and 2’-hydroxyacetophenones are commercially available.
114
Scheme 4.1. Retrosynthetic analysis of the target molecule showing two synthetic pathways.
O O
Br
O
OH O
X O
O S O
O
R
OH O
R R
R R
R R
+ +
Pathway A
R
O O
O
O O
OH O
Br
R R
R R
R
+
+ Pathway B
Initial investigations into the formation of the ether bond were carried out using 2’-hydroxyacetophenone (60) and benzyl bromide (61) (Scheme 4.2), and the use of 1 equivalent of anhydrous potassium carbonate in refluxing acetonitrile695-697 was sufficient to effect the conversion to 62 cleanly and in almost quantitative yields in 24 hours (Table 4.1.).
115
Scheme 4.2. Synthesis of ether 62.
O O
OH O
Br
+
60 61 62
Table 4.1. Optimization of conditions for ether formation.
Reaction Equivalents K2CO3 Solvent Time at reflux
/Hours Yield /%
1 1 Me2CO696 24 66
2 1 Me2CO 48 95
3 2 Me2CO 24 82
4 1 MeCN 12 54
5 1 MeCN 24 89
1H NMR spectral analysis of 62698 showed a significant shift in the location of the methylene proton peak, from 4.60 ppm (in the spectrum of 61) to 5.23 ppm (Figure 4.1). As this region of the spectrum is clear of other signals, the course of reactions could easily be followed by observing the relative integrals of these two signals. Slow evaporation of the solvent resulted in the formation of a crystalline solid, and X-ray spectroscopic analysis of suitable crystals of 62 (CCDC deposit number pending) revealed that the two aromatic rings are essentially perpendicular to each other, rather than existing as a planar molecule (Figure 4.2). While X-ray analysis of any of the synthesized compounds is interesting, this aspect is not of great importance to the overall project, and as such the X-ray data is presented merely for the sake of interest and will not be discussed in detail (See SI for complete structural description).
116
Figure 4.1. 1H NMR spectrum of 62 showing the shift in the location of the methylene signals on formation of the ether bond.
Figure 4.2. An ORTEP view of 62 showing the perpendicular arrangement of the phenyl rings.
Displacement of the non-hydrogen atoms are shown at the 50% probability level.
62
61
117
Test reactions were also carried out to determine optimum conditions for the condensation reactions between 60 and 4-bromobenzaldehyde (63) (Scheme 4.3). For this reaction, the optimum conditions for the formation of chalcone 64699 were determined to be 2 equivalents of KOH dissolved in absolute ethanol, with the reaction stirred at room temperature for 18 hours (Table 4.2), a modification of the procedure used by Zhang and Wang.700
Scheme 4.3. Synthesis of chalcone 64.
O
Br OH
O
OH O
+ Br
60 63 64
Table 4.2. Optimization of conditions for chalcone formation.
Entry Equivalents KOH Time /Hours Yield /%
1 2 2 22
2 2 6 45
3 2 12 76
4 2 18 95
The appearance of two one-proton doublets with J-values greater than 15 Hz in the 1H NMR spectrum (Figure 4.3), along with the corresponding disappearance of the aldehyde proton (~10 ppm) and the methyl protons (~2.5 ppm) confirmed the formation of a trans double bond701 between the acetophenone and the benzaldehyde molecules as desired.
118
Figure 4.3. 1H NMR spectrum of 64 highlighting the signals corresponding to the double bond protons.
Routes for the conversion of a sulfonyl chloride moiety to a sulfonamide (Scheme 4.4) were also explored using p-toluene sulfonyl chloride (65) as the analogue molecule. The sulfonyl chloride could be easily converted to the sulfonamide 66702 using THF/aqueous ammonia at 0˚C for 1 h in high yields (over 80%) following a modification of the procedure used by Corominas and Montaña.703 Proton NMR spectral analysis of 66 showed the appearance of a two-proton singlet at 7.36 ppm in the 1H NMR spectrum, corresponding to the two amide protons (Figure 4.4). The proton-observed gHSQC 1H-{15N}
spectrum of 66 shows a signal at 7.36 ppm, indicating that the protons responsible for this signal are definitely connected to a nitrogen atom, which indicates that the conversion of the sulfonyl chloride into a sulfonamide was successful.
Scheme 4.4. Synthesis of sulfonamide 66.
S
O O
Cl
S
O O
NH2
65 66
O
OH Br
H
H a
b a
b
119
Figure 4.4. Sections of the 1H and 1D gHSQC 1H-{15N} NMR spectra of 66 showing the presence of amine protons.