The synthetic scheme developed by Paul Brear was used and adapted to improve on the overall synthetic yield over six steps. The retrosynthetic approach to Optactin is shown in scheme 1.
Scheme 1. Retrosynthesis to Optactin from starting material CHES.
CHES was used as the starting material in the synthetic approach to Optactin. Although CHES is a cheap starting material, generating aminoalkylsulfonic acid intermediates en route to Optactin could pose a synthetic challenge. Aminoalkylsulfonic acids in solution can exist in multiple ionization states (Long et al., 2010). CHES, an aminoalkylsulfonic acid is zwitterionic possessing both an anionic sulfonic acid (deprotonated in aqueous solution) and a cationic secondary amine (pKa of 9.5) (Fuguet et al., 2008). CHES and generated aminoalkylsulfonic acid molecules will be hydrophilic molecules, limiting the use of hydrophobic solvents in the synthetic scheme. It was decided therefore that the sulfonic acid of CHES would be masked by conversion into a β-sultam. This β-sultam could then be ring-opened generating the sulfonic acid in the last step.
In the first step of the synthesis, CHES was reacted with PCl5 to form the sulfonyl chloride in
excellent yield. The sulfonyl chloride was then cyclized in the presence of a base to form the β- sultam. Deprotonation on the most acidic carbon occurs with LDA and this is functionalised upon addition of allyl bromide to produce a diallylated species. Ring closing metathesis was then performed on 25 to produce a spiro product, 26 in moderate yield. Dihydroxylation of 26 in the presence of osmium tetroxide formed two meso compounds 27 and 28. Separation of27 and 28 was
104
achieved by flash column chromatography in moderate (60%) to low (8%) yields respectively. Hydrolysis and ring opening of the β-sultam compound 27 was achieved by microwave irradiation with water. Recrystallisation afforded Optactin in moderate yield (45%). Successful synthesis of 16 mg of Optactin in an overall yield of 8.5% over 6 steps prompted us to optimize the synthetic route (previously reported overall yield by Dr Brear for Optactin using this route was 2.9% over the 6- steps).
Scheme 2. Synthesis of Optactin: Reagents and conditions (a) PCl5, DCE, reflux, 2hrs, 98%; (b) Na2CO3, EtOAc,
RT, 48hrs, 84%; (c) LDA (2.5 eq), allyl bromide (4.0 eq), THF, 2hrs, -78°C, 80%; (d) Grubbs 1st generation catalyst (0.05%mol), DCM, RT, 3hrs, 51%; (e) cat. OsO4, NMO, THF:H2O (9:1), RT, 16hrs ((27:60%), (28:8%)); (f) H2O,
microwave irradiation, 140°C, 8 minutes, 42%; (g) H2O, microwave irradiation, 140°C, 8 minutes, 45%.
The use of 1,2-dichloroethane (DCE) in reaction step 1 seemed unnecessary and was the first step identified for modification/optimization. DCE is toxic and can cause acute effects on the human nervous system, liver and kidneys resulting in cardiac arrhythmia, pulmonary edema, respiratory depression, nausea and narcosis.
Classified as a group B2 probable carcinogen, DCE has a long-term exposure limit (LTEL) of 5ppm (Service, 2001). Considerations for solvent replacement in a reflux experiment include solubility and boiling point. Dichloromethane (DCM), a commonly used solvent in chemistry laboratories is less toxic than DCE with a LTEL of 100ppm (England, 2016). Similar polarity and dipole moments of DCM (0.309, 1.60 D) and DCE (0.327, 1.80 D) would indicate that the solubility of CHES within these two solvents would be similar (PubChem, PubChem). The boiling point of DCM is slightly lower than DCE (57.3°C) at 39.6°C which might impact on reaction time and/or yield (PubChem, PubChem). Reflux of
105
CHES with PCl5 in DCM for the same duration resulted in a lower yield (90%). However, increasing
the reaction time by an hour produced a similar yield to the original conditions of 95% (Scheme 3).
Scheme 3. Step 1 of Optactin synthesis: a,i) PCl5, DCE, reflux, 2hrs, 98% (original conditions); a,ii) PCl5, DCM,
reflux, 3hrs, 95% (optimised conditions).
Reaction steps 2 and 3 required no optimization as they proceeded with excellent yield. Step 4 was the next step identified for optimization (Scheme 4).
Scheme 4. Step 4 of Optactin synthesis: (d,i) Grubbs 1st generation catalyst (Figure 35) (0.05 mol%), DCM, RT, 3hrs, 51% (original conditions); d,ii) Grubbs 1st generation catalyst (0.05 mol%), toluene, 40°C, 5hrs, 71% (optimised conditions).
Under the original conditions a moderate yield (51%) of spiro compound 26 was obtained. Ring closing metathesis is a carbon-carbon bond formation from two alkenes generating a cycloalkene (Grubbs et al., 1995). A number of ruthenium carbine complexes have been developed to catalyze olefin metathesis (Grubbs, 2006). The first air stable and water-soluble metathesis catalyst to be used widely in organic synthesis was the Grubbs 1st generation catalyst (Schrodi and Pederson, 2007, Schwab et al., 1995). Since then a variety of catalysts have been generated for olefin metathesis (Vougioukalakis and Grubbs, 2009). The Grubbs 2nd generation catalyst is an example of another catalyst developed for improved olefin catalysis; it is reported as more active than the first generation catalyst with a broader substrate scope (Schrodi and Pederson, 2007, Trnka and Grubbs, 2001, Scholl et al., 1999)(Figure 35).
106
Figure 35. Grubbs 1st generation and Grubbs 2nd generation catalysts.
Different temperatures, reaction time and catalyst were investigated (Table 3.). Increase in reaction time resulted in only a slight increase in yield from 51% to 54%. Further improvement in yield (64%) was obtained by increasing the temperature from 20°C to 40°C. Increase in temperature to 40°C combined with an increased reaction time improved the yield further to 71%. Changing the catalyst from the first generation to the second generation catalyst at room temperature (RT) resulted in a yield similar to the best condition identified, but due to cost this was not selected for use.
Table 3. Reaction conditions screened to optimize the ring closing metathesis reaction.
A total of 487mg of Optactin was synthesized when incorporating both optimized conditions into the synthetic scheme. This resulted in a total overall yield of 11.8%, which is a 4-fold improvement from the overall yield reported by Dr Brear (2.9%).