I am submitting her thesis entitled "Development of Greener Coupling Reagents and Methods for the Synthesis of Esters, Amides and Peptides" to be submitted for Ph.Chemistry). I would like to thank all the faculty members of the Department of Chemistry in IIT Guwahati for their invaluable suggestions.
LIST OF ABBREVIATIONS
Amino Acids
Name 3-letter code 1-letter code
SYNOPSIS
- Introduction
- Halogen Free Synthesis of Sulphonates of Alcohol, Oxyma-O- sulphonates, and Oxime-O-sulphonates Under Microwave Irradiation
- Synthesis of Amides and Esters Using Catalytic Amount of o-NosylOXY
- Racemization Free Synthesis of Peptides and Polypeptides Using Sub-stoichiometric Amount of o-NosylOXY
- Synthesis of 1,2,4-Oxadiazoles by Using Catalytic Amount of o-NosylOXY
Therefore, we developed an environmentally friendly method for the synthesis of peptides and polypeptides using the substoichiometric amount of o-NosylOXY. However, there is no report on coupling reagents used in catalytic amounts for the synthesis of 1,2,4-oxadiazoles.
Conclusion
Chapter 4: Racemization Free Synthesis of Peptides and Polypeptide using Sub- stoichiometric Amount of ortho-NosylOXY
- Background
- Green Synthesis
- Some biologically important class of compounds
- Importance of sulphonate esters
- Importance of amides
- Importance of esters
- Importance of peptides
- Importance of 1,2,4-oxadiazole
- Existing methods for the synthesis of relevant classes of compound
- Existing methods for the synthesis of sulphonate esters
- Existing methods for the synthesis of amides, esters, and peptides
- Existing methods for the synthesis of 1,2,4-oxadiazole
- Drawbacks of existing methods
- Objectives of thesis
- References
Therefore, we have focused on the greenest approach for the synthesis of sulfonate esters, amides, esters, peptides and 1,2,4-oxadiazole. Many methods have been reported in the literature for the synthesis of sulfonate esters using halogen-based reagents or metal catalysts. PTSA−ZnCl2: An efficient catalyst for the synthesis of 1,2,4-oxadiazoles from organic amidoximes and nitriles.
Halogen Free Synthesis of Sulphonates of Alcohol, Oxyma-O-sulphonates, and Oxime-O-
A new halogen and metal free microwave assisted environment friendly method for the synthesis of sulphonate esters
To optimize the reaction conditions, the synthesis of benzenesulfonate ester (3a) from butyl alcohol (2a) was used as a model reaction. We found that in acetonitrile and dioxane the product yield was only 60% and 30%, respectively (data 1 and 2). A further increase in the yield of the reaction was not noted even when the temperature was further increased (entry 9).
Diols were selectively monosulfonylated with one equivalent of the benzenesulfonic acid (Table 2.1.1.5, entries 5a and 5b). However, by reducing the amount of silica to 30 mg, the yield of the product decreased (entry 4). These reactions proceeded efficiently despite the presence of the electron-withdrawing substituents, which were unreactive in the absence of silica.
The recovered silica gel was dried in the hot air oven and cooled under desiccator and then reused up to 5 cycles for the same reaction without decreasing the yield of the product (Figure 2.1.1.1). In this case, we could recover the rest of the starting materials by eluting with specific eluents and then recombining for the next batch.
Cycle 2 Cycle 3 Cycle 4 Cycle 5
- Plausible mechanism
- Conclusions
- Experimental section
- General consideration
- General procedure for the direct synthesis of sulphonate ester from sulphonic acids: Sulphonic acid (1 mmol), alcohol or Oxyma or oxime (1 mmol) were
- General procedure for the synthesis of sulphonate ester from sulphonic acids using silica gel: Sulphonic acid (1 mmol), alcohol or Oxyma or oxime (1 mmol)
- General procedure for the gram scale synthesis of sulphonate esters from sulphonic acids using silica gel: Sulphonic acid (1 gram, 1 equiv), alcohol or Oxyma or
- Characterization data
- References
- Selected spectra and chromatograms
- GCMS analysis of intermediates
And the improvement of the conversion rate of an underactive substrate was possible in this way. After exploring the scope of the substrate, we turned our attention to an elucidation of the reaction mechanism. However, GCMS analysis of a reaction mixture containing silica gel and sulfonic acid, free of the nucleophile, revealed two peaks with m/z values of 218 and 232 (Figures 2.2.1b and 2.2.1c, & Figures S45a and S45d) corresponding to the depicted ones respectively intermediates I and II.
Therefore, we suggest that in the first step, benzenesulfonic acid can react in a reversible manner with the SiO2 and other polycoordinated species present in the silica gel and lead to the formation of the intermediate I, II or similar species. The vibrational band at 1734 cm-1 indicates the sulfonate ester bond (O=S-O-) with silica and the band at 1639 cm-1 indicates the C=C bond of the benzene ring. This method was also investigated for the regioselective sulfonylation of diols, i.e. diols were selectively monosulfonylated with one equivalent of benzenesulfonic acid and also a higher selectivity for the primary alcohol group than for the secondary alcohol group.
Purification of the reaction products was performed by column chromatography using silica gel (60-120 mesh) using EtOAc/hexane as eluent. After completion of the reaction, the reaction mixture was filtered and the SiO 2 was dried in a hot air oven for further use. After completion of the reaction, the reaction mixture was filtered and the SiO 2 was dried in a hot air oven for further use.
Dissecting the roles of sn-1 and sn-2 carbonyls in DAG mimetics with isoform cophora substitution.
Synthesis of Amides and Esters using Catalytic Amount of o-NosylOXY
- Amide synthesis using catalytic amount of o-NosylOXY
- Ester synthesis using catalytic amount of o-NosylOXY
- Racemization study
- Plausible mechanism
- Conclusion
- Experimental Section
- Materials and methods As described in chapter 2 section 2.4.1
- General procedure for the synthesis of amide
- General procedure for the synthesis of ester
- Characterization data
Benzoic acid (1 equiv) and o-NosylOXY (varied amount) in DCM solvent were used for optimization, then DIPEA (1.5 equiv) was added to the reaction mixture. After demonstrating the synthesis of amides and esters using a catalytic amount of o-NosylOXY, we devoted ourselves to elucidating the mechanism of the catalytic effect. The evolution of water was confirmed by adding blue silica gel to the reaction mixture (Figure 3.4.1).
For the experiment, we took three sample bottles containing water, distilled DCM and the reaction mixture. This experiment proved that water is generated as a byproduct in the reaction mixture, as suggested by the drawn mechanistic pathway. Moreover, the evolution of water was also confirmed by adding anhydrous FeCl3 to the reaction mixture.
The reaction mixture was stirred for 3-5 minutes for preactivation, followed by the addition of amine (1.2 equiv). The reaction mixture was stirred for 3–5 minutes for preactivation, followed by the addition of alcohol (1.2 equiv).
- References
- Selected spectra and chromatograms
- HPLC Data for racemization test
- Time dependent GCMS studies for Intermediates (1)
- FeCl 3 test for the evolution of water as the by-product
DL-butyl 2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-3-phenylpropanoate, 3y. a) Dharm Dev, Nani Babu Palakurthy, Kishore Thalluri, Jyoti Chandra and Bhubaneswar Mandal. Benzoxazole and benzothiazole synthesis from carboxylic acid in solution and on resin by ethyl 2-cyano-2-(2-nitro-benzenesulfonyloxyimino) acetate and para-toluenesulfonic acid. An unexpected involvement of ethyl 2-cyano-2-(hydroxyimino)-acetate cleavage product in promoting the synthesis of nitriles from aldoximes: A mechanistic insight.
Direct synthesis of alcohol sulfonates, Oxyma-O-sulfonates and Oxime-O-sulfonates under microwave irradiation, ChemistrySelect.
Racemization Free Synthesis of Peptides and Polypeptides using Sub-Stoichiometric Amount of
- Reaction optimization and substrate scope for peptides and polypeptides synthesis using sub-stoichiometric amount of o-NosylOXY
- Racemization suppression efficiency of I for peptide synthesis
Reaction optimization and substrate scope for peptide and polypeptide synthesis using substoichiometric amounts of o-NosylOXY polypeptide synthesis using substoichiometric amounts of o-NosylOXY. We further performed the same model reaction with other coupling reagents such as DCC, EDC, HBTU, BenzylOXY and TosylOXY with 0.2 equiv. Therefore, we found that only o-NosylOXY acted with a catalytic amount, while the others did not show any catalytic behavior.
With those optimized reaction conditions in hand, we investigated the applicability of this method for peptide synthesis from various N-protected amino acids with C-protected amino acid. The reaction worked well with the standard N-protecting groups of amino acids such as Fmoc (Scheme 4.1.1, entries 3a-3m), Cbz (entries 3n-3o) and Bz (entry 3p), as well as with the bulky amino acid side chain in good yield . For racemization studies, we synthesized Fmoc-DL-Ala-L-Ala-OMe and Fmoc-L-Ala-L-Ala-OMe dipeptides using the catalytic amount I and compared their HPLC profiles.
The appearance of a twin peak in the HPLC profile of Fmoc-DL-Ala-L-Ala-OMe corresponds to the two diastereomeric products (Figure 4.2.1, right panel) while the presence of a single peak of Fmoc-L-Ala-L-Ala -OMe shows a single stereoisomeric product (Figure 4.2.1, left panel). Examination of racemization by comparing the HPLC, 1H NMR, and 13C NMR profiles of Fmoc-L-Ala-L-Ala-OMe (left panel) and Fmoc-DL-Ala-L-Ala-OMe (right panel).
HPLC
1 H NMR
13 C NMR
Synthesis of polypeptides using sub-stiochiometric amount of o-NosylOXY
First, we synthesized Boc-Leu-Val-Phe-Phe-OMe (Figure 4.3.1a) with 0.2 equiv of I, this is the fragment of the amyloid β peptide which is responsible for Alzheimer's disease. A stepwise coupling based on Boc chemistry was performed in DCM following solution phase methodology in 70% yield. Next, we synthesized the peptide IAPP (22-27):Asn-Phe-Gly-Ala-Ile-Leu-NH2 (Figure 4.3.2b), which is known to be the key sequence responsible for initiating aggregation of the amylin peptide that leads to type 2 diabetes.
A stepwise Fmoc chemistry-based coupling of amino acids was performed in DMF on MBHA square amide resin according to the SPPS (solid phase peptide synthesis) protocol. In each step, 1.5 equiv of Fmoc amino acids, 0.2 equiv of I and 4 equiv of DIPEA were used and vortexed gently. When we performed the reaction between Fmoc-Gly-OH and the C-protected amino acid using 0.2 equiv of I, the reaction did not work.
Synthesized peptide sequences: a, Boc-LVFF-OMe in solution b, NFGAIL c, ATQRLANFLVHSSNNFGAILSSTNVGSNTYG-NH2 d, ATQRLANFLVHSSNNFGA-Ant-LSSTNVGSNTY- NH2 using the SPSPPS strategy. We synthesized the peptide IAPP (8-37) (IAPP, islet amyloid polypeptide:. 8-37) with a β-switch element: ATQRLANFLVHSSNNFGA-Ant- LSSTNVGSNTY-NH2 (Figure 4.3.4d) with amino acids in a stepwise manner Resin square amide following the orthogonal Fmoc/tBu protection strategy.
Plausible mechanism
Conclusion
Experimental Section
- Materials and methods As described in chapter 2 section 2.4.1
- General procedure for the synthesis of dipeptide
- Solution Phase Synthesis of Boc-LVFF-OMe
- Solid Phase Synthesis of NFGAIL-NH 2
- Solid Phase Synthesis of IAPP (8-37) peptide and IAPP (8-37) peptide with one β-breaker element
Then TFA was evaporated with a rotary vacuum evaporator, the solution was washed 3-4 times with diethyl ether and finally a white solid (H2N-FF-OMe) was obtained. The hexapeptide was manually assembled stepwise on Fmoc Rink Amide MBHA resin using an Fmoc/tBu orthogonal protection strategy. Peptide purification was performed by preparative HPLC and a linear gradient of 0 to 5%, 0-18.
Amino acids were manually assembled stepwise on Fmoc Rink Amide MBHA resin using Fmoc/tBu protection strategy.
Characterization data
Selected spectra and chromatograms
- NMR ( 1 H and 13 C) and Mass spectra of peptides
- HPLC Data for racemization test
- HPLC chromatogram and mass spectra of Boc-LVFF-OMe via solution phase synthesis
- HPLC chromatogram and mass spectra of NFGAIL-NH 2 via solid phase synthesis
- HPLC chromatogram and mass spectra of
- HPLC chromatogram and mass spectra of ATQRLANFLVHSSNNFGA-Ant- LSSTNVGSNTYG-NH 2 via solid phase synthesis
HPLC chromatogram and mass spectra of Boc-LVFF-OMe via solution phase synthesis. HPLC chromatogram and mass spectra of ATQRLANFLVHSSNNFGA-Ant-LSSTNVGSNTYG-NH2 via solid phase synthesis LSSTNVGSNTYG-NH2 via solid phase synthesis.
Synthesis of 1,2,4-Oxadiazoles by Using the Catalytic Amount of o-NosylOXY
- Reaction optimization and substrate scope for the synthesis of 1,2,4- oxadiazoles
The main disadvantages of these coupling reagents (Chapter 1, Section 1.5) were the use of an excessive amount of coupling reagents, which involves harsh reaction conditions, the generation of unwanted byproducts, chemical waste and racemization. To address some of these drawbacks, we have developed a catalytic synthesis of 1,2,4-oxadiazoles with a broad substrate range. Initially, we optimized the reaction conditions using the synthesis of 3,5-diphenyl-1,2,4-oxadiazole as a model product.
To this I (1 eq) and Hunig's base (DIPEA, 3 eq) were added and the reaction mixture was stirred for about 2-3 min. Then benzamidoxime (1 equiv.) was added and the reaction mixture was allowed to stir at room temperature for 2 hours. When we performed the reaction under microwave irradiation, a higher conversion of 4 was surprisingly obtained in a much shorter reaction time with 1 equiv of I.
We now wanted to discretely understand the effect of I in the first and second step of the reaction.
Optimizing the amount of reagent I under microwave irradiation for the synthesis of O-acylamidoxime intermediate.a. Then we wanted to confirm that the first step in the above condition also works with a catalytic amount of I. Thus, we took a varied amount of I and performed the first step of the reaction in mild condition (40 °C and 20 watts).
Next, we wanted to investigate whether there is any effect of I on the second step, i.e. the dehydrating cyclization step, because we observed a clear and consistent change in the yield and amount of the existing intermediate in the results summarized in Table 5.1.2. Later we wanted to check the conversion from 3 to 4 under microwave radiation with a varied amount of I (Figure 5.1.2). For this we used 0.5 equiv of reagent under the same conditions. We observed that the yield of the reaction was increased to 81% and the starting material was only 18%.
HPLC data for conversion of 3 to 4 under microwave irradiation with a varied amount of I, time of reaction is 5 min (temp 100 °C, 50 Watt). Time-dependent HPLC data for conversion of 3 to 4 under microwave irradiation with 1 equiv. of I (temp 100 °C, 50 Watt) and 3 equiv. DIPEA.
