Review Reports - Sequence-Controlled Synthesis of Heteroaryl-Substituted Decahydroacridine-1,8-diones: Comparative Evaluation of Preformed Enaminone and Multicomponent Routes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear editor of Molecule,

I hope you are well.

I have evaluated the manuscript titled "Sequence-Controlled Synthesis of Heteroaryl-Substituted Decahydroacridine-1,8-diones: Comparative Evaluation of Preformed Enaminone and Multicomponent Routes."

The paper presents a clear, well-structured investigation into the optimization of acridine-1,8-dione synthesis. The comparative study between the preformed-enaminone route and the one-pot multicomponent route is logical, and the identification of by-products adds depth to the mechanistic discussion.

To improve the clarity and rigor of the manuscript before publication, I suggest addressing the following questions and points:

You acknowledge that the compared procedures were historical and differed in scale. Could you provide a table or clear statement comparing the absolute concentrations (M) and catalyst loadings (mol%) for the sequential vs. multicomponent routes? If these are not identical, how can the reader be certain that the yield improvements are due to the "sequence-control" rather than differences in concentration-dependent kinetics?
Beyond final isolated yields, was any kinetic monitoring (e.g., TLC at 1h, 2h, 4h intervals) performed to determine if the one-pot multicomponent route reaches completion or if the yields are simply limited by premature degradation or by-product diversion?
Both procedures used the same solvent (glacial acetic acid) and. Were any control experiments performed in a different solvent (e.g., ethanol or acetonitrile) to see if the superiority of the sequential enaminone route is maintained, or is it specific to the acidic conditions?
You suggest the formation of a xanthene-type species (B). Aside from trace MS, did you observe any diagnostic peaks in the crude 1H NMR spectra of the one-pot reaction (e.g., specific shifts for xanthene-type CH protons) that distinguish it from the final product?
You state that by-products 6a-c contain no heteroaryl fragment. If the aldehyde is excluded from this pathway, what is the fate of the heteroaryl aldehyde in the one-pot mixture? Did you identify any other products (e.g., heteroaryl-dimedone adducts or simple aldehyde self-condensation products) in the mother liquors?
Regarding the description of 6c (page 9), Please clarify if the methyl signal at 2.57 ppm belongs to the 4-methylphenyl group and verify that no overlap occurs with a potential thienyl-methyl if using aldehyde 4b.
You emphasize the "strongly deshielded aromatic proton at 9.17-9.21 ppm" in 6a-c as a diagnostic marker. Can you definitively assign this proton based on the structure (e.g., is it the H-9 position in the aromatized tetrahydroacridinone)? A more detailed correlation (e.g., COSY or HMBC data) would strengthen this assignment.
Figure 3 implies that the by-product pathway (E) originates from the same enaminone precursor (3). Given that the one-pot route leads to significantly higher amounts of 6a-c, do you have evidence that the enaminone 3 is indeed the common intermediate, or could the one-pot route be partitioning into 6a-c before the enaminone is fully formed?
Given that some original printouts are lost and numerical data was transcribed from two-decade-old records, what specific validation process did you use to ensure the accuracy of the transcribed shifts and coupling constants? Did you compare these against known shifts for similar compounds in the literature?
For the compounds where original spectral printouts are missing (5b, 5d, 5e, 6b, 6c), are there any gaps in the provided characterization (e.g., missing IR or MS fragments) that could be addressed by citing similar, well-characterized literature analogues to support the structures?

Best regards

Author Response

General response: We thank the reviewer for the careful and constructive assessment. The revised manuscript and Supporting Information now explicitly distinguish directly supported experimental observations from mechanistic interpretation; provide the absolute concentrations and effective catalyst loadings of the two archived protocols; clarify the nonexclusive status of the proposed reaction network; and document the provenance, availability, and limitations of the historical spectroscopic records. The synthetic work was performed during the author’s post doctoral research and its subsequent continuation approximately two decades ago. A systematic search of the surviving post doctoral and laboratory archives was undertaken, all recoverable original instrument printouts are reproduced in the Supporting Information, and no unavailable spectrum or integral trace has been reconstructed or simulated. All revisions made in response to the reviewers are highlighted in green in the revised manuscript and Supporting Information.

1. You acknowledge that the compared procedures were historical and differed in scale. Could you provide a table or clear statement comparing the absolute concentrations (M) and catalyst loadings (mol%) for the sequential vs. multicomponent routes? If these are not identical, how can the reader be certain that the yield improvements are due to the "sequence-control" rather than differences in concentration-dependent kinetics?

Author Response-1: We thank the reviewer for identifying this important limitation. A dedicated paragraph entitled “Nominal concentration and catalyst-loading comparison” has been added to Section 2.4 immediately after General Procedure III. In Procedure II, enaminone 3, aldehyde 4, and dimedone 2 were each present at 0.200 M (1.0 mmol in 5 mL), while p-toluenesulfonic acid was 0.016 M, corresponding to 8 mol% relative to each 1.0 mmol substrate; the total nominal substrate concentration was 0.600 M. In Procedure III, the amine and aldehyde were each 0.100 M (0.5 mmol in 5 mL), dimedone was 0.200 M, and p-toluenesulfonic acid was 0.016 M, corresponding to 16 mol% relative to the limiting amine/aldehyde and 8 mol% relative to dimedone; the total nominal substrate concentration was 0.400 M.

We fully agree that these differences prevent an unambiguous attribution of the yield advantage solely to component sequence or sequence-dependent kinetics. The Abstract, Section 1.2, the introductory paragraph of Section 3, Section 3.1, Section 3.5, and the Conclusions have therefore been revised to describe the study as an operational preparative comparison rather than an isomolar kinetic experiment. The consistent 28–33 percentage-point advantage across the six matched substrate combinations supports the practical robustness of the complete preformed-enaminone protocol under the investigated conditions, but concentration and effective catalyst loading remain possible contributing variables.

The product-forming second stage showed a 28–33 percentage-point advantage, while the calculated overall two-step yields remained 14.5–28.6 percentage points higher than the corresponding one-pot yields.

2. Beyond final isolated yields, was any kinetic monitoring (e.g., TLC at 1h, 2h, 4h intervals) performed to determine if the one-pot multicomponent route reaches completion or if the yields are simply limited by premature degradation or by-product diversion?

Author Response-2: The reactions were monitored qualitatively by TLC, as stated in Section 2.1, but no time-resolved TLC records at defined 1, 2, and 4 h intervals, densitometric data, conversion measurements, or aliquot-based kinetic profiles could be recovered from the surviving archives. Section 3.5 has been expanded to list these unavailable measurements explicitly and to state that the historical dataset cannot quantitatively distinguish incomplete conversion, degradation, and diversion into competing pathways.

The isolation of 6a–c in 13–15% yields and the tentative trace nominal-mass EI-MS features assigned to proposed species B and D show that at least part of the material entered alternative product manifolds. However, these observations do not constitute a complete mass balance and do not establish the relative contribution of the different processes. The manuscript therefore presents the network only as an evidence-based, nonexclusive interpretation, not as a demonstrated kinetic sequence.

3. Both procedures used the same solvent (glacial acetic acid) and. Were any control experiments performed in a different solvent (e.g., ethanol or acetonitrile) to see if the superiority of the sequential enaminone route is maintained, or is it specific to the acidic conditions?

Author Response-3: No comparative solvent-control experiments were performed in ethanol, acetonitrile, or another medium. Because glacial acetic acid is a strongly protic and acidic reaction medium that may influence aldehyde activation, enaminone formation, keto–enol equilibria, dehydration, and carbon- versus oxygen-centered cyclization, extrapolation to other solvents would not be justified.

The scope has therefore been restricted explicitly to the investigated p-toluenesulfonic acid/glacial acetic acid protocols. This limitation is now stated in Section 1.2, Section 3.1, Section 3.5, and the Conclusions. The revised manuscript does not claim solvent-independent superiority of the sequential route and identifies comparative solvent experiments as necessary future work.

4. You suggest the formation of a xanthene-type species (B). Aside from trace MS, did you observe any diagnostic peaks in the crude 1H NMR spectra of the one-pot reaction (e.g., specific shifts for xanthene-type CH protons) that distinguish it from the final product?

Author Response-4: No crude-reaction ¹H NMR spectrum could be recovered from the surviving post doctoral and laboratory archives, and no diagnostic xanthene-type methine resonance was documented in the contemporaneous records. The only direct observation related to proposed species B is a trace nominal-mass EI-MS feature in archived reaction-mixture data. Nominal-mass EI-MS alone cannot exclude constitutional isomers or co-eluting species and is therefore insufficient for definitive structural identification.

Section 3.5 and the Supporting Information entry accompanying the trace EI-MS data for B have been revised to state explicitly that B was not isolated as a pure compound, that no crude-reaction ¹H NMR confirmation is available, and that the assignment remains tentative. Figure 3 depicts B only as an independent, mechanistically plausible xanthene-type competing sink; no NMR confirmation is claimed.

5. You state that by-products 6a-c contain no heteroaryl fragment. If the aldehyde is excluded from this pathway, what is the fate of the heteroaryl aldehyde in the one-pot mixture? Did you identify any other products (e.g., heteroaryl-dimedone adducts or simple aldehyde self-condensation products) in the mother liquors?

Author Response-5: The mother liquors were not subjected to exhaustive chromatographic fractionation, quantitative aldehyde recovery, or a complete carbon-balance analysis; consequently, the fate of the heteroaryl aldehyde cannot be assigned uniquely. Section 3.3 has been revised to state this limitation and to list the chemically plausible destinations supported by the reaction context: unreacted aldehyde, aldehyde–dimedone Knoevenagel adducts, bis-dimedone/xanthene-type material such as proposed B, non-isolated heteroaryl-containing nitrogen heterocycles such as proposed D, and unresolved degradation or oligomeric material.

The revised text also clarifies that formation of 6a–c is compatible either with exclusion of the aldehyde from that branch or with loss of a heteroaryl-containing unit after initial incorporation. The available data do not discriminate between these possibilities, and no complete aldehyde balance is claimed.

6. Regarding the description of 6c (page 9), Please clarify if the methyl signal at 2.57 ppm belongs to the 4-methylphenyl group and verify that no overlap occurs with a potential thienyl-methyl if using aldehyde 4b.

Author Response-6: The δ 2.57 ppm signal in 6c has been clarified as the C-7 methyl resonance (7-CH₃), inherited from the para-methyl substituent of the p-toluidine-derived ring. It is therefore no longer described as a “p-tolyl CH₃,” because an intact p-tolyl substituent is not present after annulation. The corresponding ¹³C resonance at δ 21.53 ppm is likewise assigned to 7-CH₃.

The characterization of 6c in Section 2.5 and the corresponding historical numerical-data entry in the Supporting Information have both been revised. An explicit sentence now states that the composition C₁₆H₁₇NO, the molecular ion at m/z 239, and the absence of thiophene resonances exclude assignment of this signal to a thienyl-methyl-containing species. Thus, no overlap with a 3-methylthienyl substituent is involved in the isolated compound 6c.

7. You emphasize the "strongly deshielded aromatic proton at 9.17-9.21 ppm" in 6a-c as a diagnostic marker. Can you definitively assign this proton based on the structure (e.g., is it the H-9 position in the aromatized tetrahydroacridinone)? A more detailed correlation (e.g., COSY or HMBC data) would strengthen this assignment.

Author Response-7: We agree that the assignment should not be presented as definitive in the absence of multidimensional NMR. Section 3.3 has been revised to state that the δ 9.17–9.21 ppm singlet is assigned to H-9 from the one-dimensional aromatic coupling pattern. In 6a–c, H-5 appears as an ortho-coupled doublet (J approximately 9.4 Hz), H-6 as a doublet of doublets (J approximately 9.4 and 2.7 Hz), and H-8 as a meta-coupled doublet (J approximately 2.7 Hz), leaving H-9 as the isolated aromatic proton expected to appear as a singlet.

The revised manuscript describes this assignment as strongly structure-consistent rather than independently proven and explicitly notes that HSQC and HMBC data are unavailable. The ¹³C signal at approximately δ 142.3–142.4 ppm is compatible with the assigned aromatic C-9 environment but does not provide a direct proton–carbon correlation.

8. Figure 3 implies that the by-product pathway (E) originates from the same enaminone precursor (3). Given that the one-pot route leads to significantly higher amounts of 6a-c, do you have evidence that the enaminone 3 is indeed the common intermediate, or could the one-pot route be partitioning into 6a-c before the enaminone is fully formed?

Author Response-8: We agree that the one-pot reaction is not demonstrated to proceed through a single obligatory enaminone intermediate. Figure 3 and its caption have been revised as a nonexclusive connectivity network originating from a common reagent pool. In the revised scheme, A denotes the isolated target products 5a–f, E denotes the isolated by-products 6a–c, B is an independent tentative xanthene-type competing sink, and C–D–E form a separate mono-dimedone nitrogen-heterocycle branch. The scheme no longer requires every branch to pass through isolated enaminone 3.

Sections 3.4 and 3.5 now state that aldehyde–dimedone condensation, amine–dimedone enaminone formation, and 1:1:1 nitrogen-heterocycle formation may occur in parallel before complete enaminone formation. The relative rates were not measured; B–D were not independently isolated, C was not directly detected, and more than one pathway may converge on E.

9. Given that some original printouts are lost and numerical data was transcribed from two-decade-old records, what specific validation process did you use to ensure the accuracy of the transcribed shifts and coupling constants? Did you compare these against known shifts for similar compounds in the literature?

Author Response-9: Two dedicated paragraphs, “Historical data and integrity note” and “Validation of the historical numerical records,” have been added to Section 2.1. Parallel data-status and validation statements have also been added to the Supporting Information, and the Supplementary Materials and Data Availability statements have been expanded. These revisions state that the numerical data were transcribed contemporaneously into the original laboratory records and were not reconstructed during preparation of the present manuscript.

Validation was documentary and internally comparative rather than a modern remeasurement. The transcribed values were checked against: (i) surviving original printouts for related members of the same compound classes; (ii) molecular formulas, nominal molecular ions, and characteristic fragment losses; (iii) proton counts, multiplicities, coupling relationships, symmetry-equivalent carbon environments, and substituent-dependent 14 Da homologous shifts; (iv) the enaminone data in Ref. [13] and related acridine-1,8-dione literature [1–8,11,12,15]; and (v) consistency among the contemporaneous laboratory records, manuscript, and Supporting Information. The revised text explicitly states that these checks test chemical plausibility and identify transcriptional inconsistencies but are not equivalent to reacquiring spectra or reprocessing raw FID data.

10. For the compounds where original spectral printouts are missing (5b, 5d, 5e, 6b, 6c), are there any gaps in the provided characterization (e.g., missing IR or MS fragments) that could be addressed by citing similar, well-characterized literature analogues to support the structures?

Author Response-10: For compounds 5b, 5d, 5e, 6b, and 6c, the original printouts, raw electronic files, and residual samples could not be recovered despite a systematic archive search. Nevertheless, contemporaneously transcribed numerical IR, ¹H NMR, ¹³C NMR, and nominal-mass EI-MS values remain, so no complete characterization modality is absent. The unrecoverable elements are the primary printouts and raw files, reliable numerical integral traces, HRMS or elemental analysis, and multidimensional NMR; these cannot be recreated retrospectively.

The Supporting Information has been reorganized to distinguish clearly between surviving original printouts for 3a–c, 5a, 5c, 5f, and 6a and historical numerical records for 5b, 5d, 5e, 6b, and 6c. A data-status statement is provided for each missing-printout compound. Structural plausibility is supported internally by the archived spectra of closely related series members and externally by the enaminone and acridine-1,8-dione literature cited in Refs. [1–8,11–13,15]. The manuscript and Supporting Information explicitly acknowledge that literature analogues support scaffold plausibility but do not replace the missing primary spectra. No unavailable spectrum has been reconstructed or simulated.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript reports the synthesis of 9-thienyl-10-aryl-substituted 3,3,6,6-tetramethyldecahydroacridine-1,8-diones via two acid-mediated routes. Although the reaction mechanism and procedure are relatively straightforward, the authors have provided detailed experimental descriptions. In my opinion, these syntheses are still of guiding significance. Overall, I consider this work to be meaningful and recommend its publication after minor revision.

It is recommended that all compounds in the scheme be clearly numbered to allow easy cross-referencing with the corresponding compounds in the text. Currently, readers need to scrutinize the scheme carefully to identify the correct entries.
In Figure 3, B, C, and D are described as mechanistically plausible competing species or intermediates. These intermediates can also be expected to form. However, the scheme indicates that B can be converted to C, which I find questionable. I would therefore suggest that this transformation be omitted from the scheme.
The NMR integration data are missing in the Supporting Information. I would recommend that the authors provide the integral values for the NMR spectra. In addition, the NMR spectra of several compounds, including compound A, are also absent.

Author Response

1. It is recommended that all compounds in the scheme be clearly numbered to allow easy cross-referencing with the corresponding compounds in the text. Currently, readers need to scrutinize the scheme carefully to identify the correct entries.

Author Response-1: We agree and have standardized the compound identifiers throughout the revised schemes and captions. The aromatic amines are designated 1a–c, dimedone as 2, the isolated enaminones as 3a–c, the heteroaryl aldehydes as 4a,b, the target decahydroacridine-1,8-diones as 5a–f, and the isolated tetrahydroacridinones as 6a–c. The revised captions for Schemes 1–4 explicitly identify the compound classes shown, enabling direct cross-referencing with Sections 2.2–2.5, Tables 1 and 2, Figure 3, and the spectral descriptions.

2. In Figure 3, B, C, and D are described as mechanistically plausible competing species or intermediates. These intermediates can also be expected to form. However, the scheme indicates that B can be converted to C, which I find questionable. I would therefore suggest that this transformation be omitted from the scheme.

Author Response-2: We agree with the reviewer. The unsupported direct B→C transformation has been removed from Figure 3. In the revised nonexclusive connectivity network, B is depicted as an independent, tentative oxygen-containing bis-dimedone/xanthene-type competing sink, whereas C–D–E constitute a separate mono-dimedone nitrogen-heterocycle branch originating from the common reagent pool.

The Figure 3 caption and Sections 3.4 and 3.5 have been revised accordingly. They now state explicitly that no direct B-to-C conversion is proposed; B and D were detected only as tentative trace nominal-mass EI-MS features, C was not directly detected, and the network is an interpretive connectivity model rather than a demonstrated linear mechanism.

3. The NMR integration data are missing in the Supporting Information. I would recommend that the authors provide the integral values for the NMR spectra. In addition, the NMR spectra of several compounds, including compound A, are also absent.

Author Response-3: We appreciate this request. Authentic numerical integrations cannot be supplied because the surviving original ¹H NMR printouts do not contain reliable numerical integral traces and the raw FID files could not be recovered from the approximately two-decade-old post doctoral and laboratory archives. Reconstructed or normalized integral curves have therefore not been added, because they would incorrectly imply retrospective reprocessing of unavailable raw data. Instead, the numerical ¹H NMR descriptions report the assigned proton count for every resonance (for example, s, 6H; d, 2H; m, 7H), allowing the expected proton balance to be evaluated transparently. This limitation is now stated explicitly in Section 2.4, the Supplementary Materials statement, and the Supporting Information data-status section.

The Supporting Information reproduces every original archived IR, ¹H NMR, ¹³C NMR, and EI-MS printout that could be recovered for compounds 3a–c, 5a, 5c, 5f, and 6a (Figures S1–S28). For 5b, 5d, 5e, 6b, and 6c, the original printouts, raw electronic data, and residual samples could not be recovered; their contemporaneously transcribed numerical IR, ¹H NMR, ¹³C NMR, and nominal-mass EI-MS records are presented in a clearly separated historical-data section with an individual data-status statement for each compound. No spectrum has been simulated or reconstructed.

We have also clarified in the Supporting Information that the label A in Figure 3 denotes the isolated product class 5a–f, not an additional discrete compound whose spectrum is missing. Representative original spectra for this class are supplied for 5a, 5c, and 5f.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Editor of Molecule,

Thank you for the opportunity to review this manuscript.

The authors have satisfactorily addressed all of the comments and concerns raised during the peer-review process. I have carefully evaluated the revised version and believe that the manuscript has been significantly improved.

Based on the revisions made, I recommend that this manuscript be accepted for publication in Molecules.

I appreciate your consideration.

Kind regards,