Synthesis and Pharmacological Evaluation of (+)-Usnic Acid Derivatives as Hypoglycemic Agents

Round 1

Reviewer 1 Report

 

The manuscript by Luzina and co-authors, describes synthesis of 2 novel usnic acid-derived enamines and their evaluation as hypoglycemic agents. For the latter bio-evaluation, also previously reported additional usnic acid-derived enamines were included.

The synthetic work is appropriate for Molbank, which tends to publish either one or few novel compounds that can be as supplement to a previously know compound series. As such the manuscript can be suggested for publishing, albeit after the revision process is done:

1) The abstract is too lengthy and contains too many general phrases that are more suitable for general introduction. On the other hand it lacks a more precise description of the obtained compounds, e.g. usnic acid-derived N-aryl enamines at C14 or similar description. Also some numerical values of biotests should be represented in the abstract.

2) The paper does not comment the E/Z-geometry of the newly formed enamine. Either original spectral data should be analyzed, or, alternatively, a reference on structure determination of similar compounds should be provided. The provided references on synthesis of previously known compounds 2-5 [13,15,16] do not contain such a spectral analysis.

3) Line 69-72: please indicate which compounds are new

4)    Scheme 1: please provide the yields in the scheme; please provide literature references in the scheme for the known compounds

5)    Please correct missing subscript/superscript format in lines 116, 118, 121, 122, 168, 169, 171, 173, 175, 179, 180, 182, 187.

6)    Please correct NHz to MHz in line 180

7)    Please check/revise the use of the English language.

8)    Please revise the sentence “Atomic numbering in compounds is given for assignment of resonances in NMR spectra and does not always agree with the nomenclature numbering.” and its consequences in the compound characterization. It is useless to designate C-6 or H-4, if it a) does not follow any rules; b) there is no alternative numbering provided. Please provide the atom numbering in Scheme 1. Moreover, it would be strongly recommended to use the numbering either by IUPAC rules, or by the rules, which are set for this class of natural products

9)    Please provide the ppm scale for the NMR spectrum in Figure S1 for ESI.

10) Please revise the NMR pictures in ESI: full spectral area should be shown for all compounds to evaluate the purity:

-0.5 …. 17 ppm for 1H-NMR

-5 … 220 ppm for 13C-NMR

11) The compounds contain sufficient number of functional groups that are observable in the IR spectra. Please include the IR spectra in the compound characterization.

12) Compounds 6 and 7 are characterized by HRMS, but the pictures in the ESI is provided only for DFS spectra with one decimal digit. One can maintain those pictures in the ESI, but those of HRMS with 4 decimal digits then also should be added. (there is also some text in Russian, which should be removed).

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

 

The manuscript entitled “ Synthesis and pharmacological evaluation of usnic acid derivatives as hypoglycemic agents “ by Sergey et al describes the synthesis of two novel usnic acid enamine derivatives (6 and 7) from (+)- usnic acid by simple boling of reagents in ethanol. Manuscript also describe their characterisation and biological properites as hypoglycimic agents in experimental model.

 

The manuscript need some changes and should be addressed.

1. In the title kindly change usnic acid to (+)- usnic acid. Also correct the title in the supporitng file.

2. Please comment on the significanc of (+)- usnic acid enamine derivatives over (-) - usnic acid enamines derivatives and importance of enantiomers of usnic acid derivatives in the biological stuides/hypoglycimic studies. This should be incorporated in introduction.

3. In the scheme 1. Please include the reaction time,  temperature, yields. 

4. Scheme 1. Caption is missing

5. Superscript and subscript for 1H NMR, 13C NMR and  molecular formulas should be corrected.

6. The reaction methodology  gives 41% yield for the derivatives. Which  means reaction is incomplete or some other side products formed ? Please comment 

7. Please include the catalog number for (+)- usnic acid in the methods.

8. Authors did not perform the purity assessments for the synthesised compounds. Kindly include the HPLC data for the compounds.  Also, the NMR data for synthesised compounds have some solvent or other traces of impurities (Figure S1). Which give serious concern to believe the biological activity of some of the  usnic acid enamine derivatives.

9. Line 155 change title to synthesis of  (+)- usnic acid enamine derivatives

10. Supporting files : In the figure legneds, kindly mention the solvent in which NMR is recorded 

11. Perform F19 NMR for compound 3 and 6 and include in the manuscript.

12.  Yields provided in the manuscript and supporting files should be corrected

13.  Kindly provide the ppm scale for figure S1. Also make the represenation of spectral data same for the all the figures.

14.  Mass data for the compounds does not show the actual masses. Authors reported the measured values which are not matching with spectral values.  Ex:-  compound 6 calculated m/z 487.1237, observed mass is 487.2. Mass data should be reperformed with acceptable < 6 ppm error.

15. Why 6 (having CF3) showed  better hypoglycemic effect compared to 7 (having CH3). please discuss in the conclusion.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript numbered molbank-1932567 by S.A. Borisov et al. describes the synthesis and biological studies of two new derivatives of usnic acid.
The manuscript is not well prepared from an editorial point of view. The title of the paper in the main part of the manuscript is different from that in the supporting materials. The use of superscripts and subscripts as well as the use of boldface type were repeatedly ignored. The Authors' initials do not match in the manuscript and supporting materials. The last page of the supporting materials has text written in cyrillic. In addition, there are repetitions of text regarding experiments in both parts of the manuscript: on the first page of the supporting materials regarding NMR and MS, and on the fourth page a repeated synthesis recipe. The designation of atoms was also missing. The Authors write that they used atom designations, and They also use them in describing the NMR spectra, but there is no showing of these numbers on the structural formulae.
The abstract should not contain an introduction to the subject matter, but should instead be a summary of the results obtained. In the conclusion, the Authors write about one new compound, while the article deals with two new compounds. The title of the paper in the supporting materials and the content of the conclusions indicate that the Authors planned to describe one compound, meanwhile this is not the case throughout the manuscript. Looking at the NMR spectra of compound 6, it is clear that it is not pure. If the Authors want to publish the synthesis of this compound first, they need to purify it. High-resolution mass spectrometry (HRMS) is an additional technique besides elemental analysis to confirm the structure. HRMS is used for small amounts of a substance; this technique does not confirm the purity of the compound. The Authors should also perform elemental analysis, but of course for compound 6 it will indicate the presence of impurities.
In the synthesis recipe, I would use "amine" instead of the word "aniline". In Scheme 1 there is a "Ph" designation, but this indicates a phenyl group, while here we are dealing with a phenylene group. The description of the syntheses does not indicate which compound is 6 and which is 7. Of the 16 literature references, 10 are self-citations. This is a lot. Although they are reasonably justified. Nevertheless, such a disproportion should be avoided.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Tha manuscript has been improved, but there are still some important passages that have to be corrected:

1) it is correct, that it is difficult to prove the E/Z geometry of the obtained enamines. Nevertheless, it does not liberate from the duty to characterize the products properly. The reference numbering is shifted, and this reviewer cannot find the ref. [13]

2) in some places authors have overdone the English corrections: e.g. line 123 - the original term "refluxing" was perfectly OK: Refluxing Refrigerated boiling... Please recheck the text. Usually also the clean version of the revised manuscript is provided, as in the track change mode it is difficult to follow the text. 

3) IS data should be included also in the characterization part of the manuscript. The same is also true about 19F-NMR.

4) In the ESI, Fig.S5: the 1H-NMR scale still does not start form -0.5 ppm !

5) In the ESI, Fig.S7: please correct to "standard"

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript has been adequately revised.

Author Response

Thank you.