Chemoenzymatic Protocol for the Synthesis of Enantiopure β-Blocker (S)-Bisoprolol

Round 1

Reviewer 1 Report

In this manuscript, Jacobsen and coworkers disclosed an efficient synthetic route to (S)-Bisoprolol by enzyme-catalyzed kinetic resolution of (R)-4. However, the authors should addressed all the issues below before publication.

1) In table 1, the data make the  audience confused. The author should reorganize the data. In addition, why not present the data of compound 6?

2) In introduction part, the authors should review the synthetic methods for (S)-Bisoprolol including the chemical catalysis. There are too many methods, such as asymmetric epoxide, asymmetric reduction.

3) In scheme, the author showed the pathway by dashed arrows. Why the pathway was unsuccessful?

4) Why the ee of (S)-7 was different with (R)-4?

5) In Supplementary Materials, the authors should present all the data of the compound in maintext. I can not find out the HPLC spectra of 4, 5, 6.  In addition the racemic HPLC spectra of 4, 5 and 7 were impure. Please purify these compounds.

Author Response

In this manuscript, Jacobsen and coworkers disclosed an efficient synthetic route to (S)-Bisoprolol by enzyme-catalyzed kinetic resolution of (R)-4. However, the authors should addressed all the issues below before publication.

• In table 1, the data make the  audience confused. The author should reorganize the data. In addition, why not present the data of compound 6?

Authors: The data are reorganized. Compound 7 was directly synthesized from compound 6 in one pot, and the total yield was measured only for compound 6. It is therefore not included in Table 1.

• In introduction part, the authors should review the synthetic methods for (S)-Bisoprolol including the chemical catalysis. There are too many methods, such as asymmetric epoxide, asymmetric reduction.

Authors: The use of asymmetric epoxide in stoichiometric amounts has been discussed line 35. A sentence was added about chemical catalysis. Synthesis of (S)-bisoprolol by use of chiral chemical catalysis, for instance by asymmetric reduction, is not reported.

• In scheme 1, the author showed the pathway by dashed arrows. Why the pathway was unsuccessful?

Authors: As explained from line 113 to 121, attempts to synthesize key intermediates 8a and 8b from 4-((2-isopropoxyethoxy)methyl)phenol (2) and 1,3-dichloropropan-2-one or 1,3-dibromopropan-2-one were not successful because some of the reactants were not stable under basic conditions.

4) Why the ee of (S)-7 was different with (R)-4?

Authors: We agree that this must be discussed more clearly, and we have included the following discussion: As the remaining alcohol in a transesterification reaction always will be 100% ee, if some yield may be sacrificed, we here report the obtained (R)-4 in >99% ee. When converting an enantiopure compound to the amine, and further to the fumarate as we have done, the enantiopurity of the building block should be retained. However, one reason for the lowering in the ee-value for (S)-7 (96% ee) compared to (R)-4 (99% ee) could be that small amounts of the ester (S)-5 was present in the purified product after the column separation of (R)-4 from (S)-5 in the batch reaction, and that this ester probably was hydrolysed to (S)-4 during the next  purification steps (including NaCl-solution (water)).  If so, (S)-4 would also be converted to (S)-6 and (S)-7, thus lowering the ee of the final drug product (S)-7. We have however not seen (S)-4 in any of the spectra (see Supplementary Materials figures S6 to S25), but concentration differences may be the reason, since we observe the R-enantiomer of S-7 in the chromatogram, figure S28.

5) In Supplementary Materials, the authors should present all the data of the compound in maintext. I cannot find out the HPLC spectra of 4, 5, 6.  In addition the racemic HPLC spectra of 4, 5 and 7 were impure. Please purify these compounds.

Authors: We have presented ¹H and ¹³C NMR spectra and included also COSY, HMBC and HSQC NMR spectra for the pure compounds 4 (99% purity), 5 (after derivatisation reaction of 4 with butanoic anhydride and pyridine-which is now included in the Materials and Methods) and also the spectra for 7, all in the Supplementary Materials. The chiral HPLC-analyses of the racemic compounds 4, 5 and 7 were only performed in order to separate the respective enantiomers for calculation of the ee values from the transesterifications of 4 giving ester 5, and for determining the ee of the final enantiopure drug (S)-7 ((S)-bisoprolol hemifumarate).  The “impurities” on the HPLC chromatograms were determined as impurities on the column and as they do not interfere with the compounds of interest, we did not run these compounds again after conditioning the column. However, we analysed all the samples from the enzyme catalysed transesterification reaction of 4 after the column was conditioned, so we hereby confirm that these analyses are correct. The chromatogram of (S)-7 was run after conditioning of the column and shows no interfering impurities (see figure S28 in Supplementary Materials).

 

Reviewer 2 Report

In this manuscript, the authors reported a new method for the synthesis of (S)-bisoprolol hemifumarate via a transesterification reaction of the racemic chlorohydrin 1-chloro-3-(4-((2-isopropoxyethoxy)methyl)phenoxy)propan-2-ol catalysed by lipase B. The newly developed approach could provide the R-chlorohydrin in high enantiomeric purity and its reaction with isopropylamine and fumaric acid gave (S)-bisoprolol fumarate in 96% ee. This work should be of the general interest of medicinal chemists working in the field. This reviewer suggests its acceptance after the following issues being addressed.

 

1.       The authors should provide the most competitive industrial synthetic route and have it compared with this chemo-enzymatic protocol, enabling the readers to have a clear understanding of the strength of the current approach.

2.       In Scheme, (R)-4 should be (S)-4.

3.       In ref. 9, ‘Tetrahedron Letters’ should be ‘Tetrahedron Lett.’.

Author Response

In this manuscript, the authors reported a new method for the synthesis of (S)-bisoprolol hemifumarate via a transesterification reaction of the racemic chlorohydrin 1-chloro-3-(4-((2-isopropoxyethoxy)methyl)phenoxy)propan-2-ol catalysed by lipase B. The newly developed approach could provide the R-chlorohydrin in high enantiomeric purity and its reaction with isopropylamine and fumaric acid gave (S)-bisoprolol fumarate in 96% ee. This work should be of the general interest of medicinal chemists working in the field. This reviewer suggests its acceptance after the following issues being addressed.

1. The authors should provide the most competitive industrial synthetic route and have it compared with this chemo-enzymatic protocol, enabling the readers to have a clear understanding of the strength of the current approach.

Authors: We have not been able to find any industrial process for production of (S)-bisoprolol published on Scifinder. This may be due to industrial secrets, but it can also mean that the drug is not produced as a single enantiomer. All the previously reported syntheses for this enantiopure drug involve the use of stoichiometric amounts of enantiopure reactants, which are very expensive, this is explained in the manuscript.

2. In Scheme, (R)-4 should be (S)-4.

Authors: We have corrected the error in Scheme 1: (R)-5 was represented instead of (S)-5. This has been corrected. No mistake was made for (R)-4.

3. In ref. 9, ‘Tetrahedron Letters’ should be ‘Tetrahedron Lett.’.

Authors: This error has been corrected.

Round 2

Reviewer 1 Report

Because the authors have revised the manuscript according to the reviewers' comments, this manuscript should be accepted.