Planning chemical syntheses with deep neural networks and symbolic AI


To plan the syntheses of small organic molecules, chemists use retrosynthesis, a problem-solving technique in which target molecules are recursively transformed into increasingly simpler precursors. Computer-aided retrosynthesis would be a valuable tool but at present it is slow and provides results of unsatisfactory quality. Here we use Monte Carlo tree search and symbolic artificial intelligence (AI) to discover retrosynthetic routes. We combined Monte Carlo tree search with an expansion policy network that guides the search, and a filter network to pre-select the most promising retrosynthetic steps. These deep neural networks were trained on essentially all reactions ever published in organic chemistry. Our system solves for almost twice as many molecules, thirty times faster than the traditional computer-aided search method, which is based on extracted rules and hand-designed heuristics. In a double-blind AB test, chemists on average considered our computer-generated routes to be equivalent to reported literature routes.

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Figure 1: Translation of the traditional chemists’ retrosynthetic route representation to the search tree representation.
Figure 2: Schematic of MCTS methodology.
Figure 3: An exemplary six-step synthesis route for an intermediate in a drug candidate synthesis.
Figure 4: Influence of the time per query on performance.
Figure 5: Double-blind AB testing of MCTS-derived routes against literature and BFS routes.


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M.H.S.S. and M.P.W. thank the Deutsche Forschungsgemeinschaft (SFB858) for funding. M.H.S.S. and M.P.W. also thank D. Evans (RELX Intellectual Properties) and J. Swienty-Busch (Elsevier Information Systems) for the reaction dataset. We thank all AB-test participants in Shanghai and Münster, and J. Guo for assistance in AB testing. M.H.S.S. thanks M. Wiesenfeldt, the Studer group, D. Barton, S. McAnanama-Brereton, R. Vidyadharan and T. Kogej for discussions. M.P. thanks M. Winands and J. Togelius for insights.

Author information




M.H.S.S. conceived the project, M.P.W. and M.P. contributed ideas. M.H.S.S., M.P. and M.P.W. designed the experiments. M.H.S.S. implemented the program. M.H.S.S. and M.P.W. conducted the experiments. M.P.W. supervised the project. All authors co-wrote the manuscript.

Corresponding authors

Correspondence to Marwin H. S. Segler or Mark P. Waller.

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The authors declare no competing financial interests.

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Reviewer Information thanks D. Duvenaud, W. H. Green and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Receiver operation characteristic curve for the in-scope filter.

The area under the curve is 0.99.

Extended Data Figure 2 An exemplary 10-step synthesis route for a complex intermediate in a drug synthesis.

It resembles the published route77 (with intermediates A and B) and was found by our algorithm autonomously within 30?s. The target was not contained in the training set.

Extended Data Figure 3 Example of reaction used in the AB testing, where the MCTS-derived route was less favoured.

In this task, the participants preferred the literature solution, as its key step was presumably perceived to be more convergent.

Extended Data Figure 4 Architectures of the employed neural networks.

(‘dim’, dimensions.)

Extended Data Figure 5 Rediscovering physicochemical properties with the in-scope filter.

The output logit score of the neural network correlates surprisingly well with calculated quantum-mechanical properties (LUMO energies, in Hartree) in Diels–Alder reactions (r2?=?0.74) (a) and with empirically measured Hammond parameters in electrophilic brominations (r2?=?0.78) (b), even though the input features (ECFP4 fingerprints) do not contain electronic information.

Extended Data Table 1 Metrics for the supervised neural network policies

Supplementary information

Supplementary Information

This file contains the DOE, route diversity analysis, and failed molecules (including Supplementary Figures 1-7, Supplementary Table 1 and Supplementary References). Available on figshare (DOI 10.6084/m9.figshare.5832054) are 2 files, mcts_examples.pdf which contains routes found by the 3N-MCTS algorithm and heuristicBFS_examples.pdf which contains routes found by heuristic best first search without policy network and in-scope filter. (PDF 692 kb)

Supplementary Information

This file contains the AB test. (PDF 5648 kb)

Supplementary Data

This file contains the experiment for correlating the in-scope filter to physicochemical properties. (XLSX 45 kb)

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Segler, M., Preuss, M. & Waller, M. Planning chemical syntheses with deep neural networks and symbolic AI. Nature 555, 604–610 (2018).

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