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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Organic synthesis complexity can be estimated, but it is not a timeless property captured by a molecule’s structure alone. A 2015 proposal by Jun Li and Martin D. Eastgate, called “current complexity,” combines chemists’ judgments with structural and route-related features. Its central insight is that a synthesis may seem less difficult when a better route or new technology becomes available.
What does “current complexity” measure?
The proposal aims to quantify the perceived challenge of making a molecule, taking account of both the molecule and the route used to synthesize it. In the 2015 report, 18 synthetic chemists ranked 40 molecules. Li and Eastgate considered multiple intrinsic and extrinsic factors, then used Bayesian regression to identify five major factors. Chemistry World’s 2015 report describes the resulting scale as running from 1, most complex, to 10, least complex.
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- Topological index: a feature of the molecule’s structure.
- Stereogenic centers established during synthesis: a route-related factor, since the route determines how these centers are formed.
- Heteroatoms on and in aromatic rings: structural features of the molecule.
- Number of synthesis steps: a feature of the chosen route.
- Route ideality: an assessment of how favorable or effective the route is.
The first and third factors are relatively intrinsic: they describe aspects of the molecule. The other factors can depend on synthetic choices and the methods available. That distinction is why the proposal is called “current” complexity rather than a fixed structural score. The underlying paper is Jun Li and Martin D. Eastgate, “Current complexity: a tool for assessing the complexity of organic molecules,” published in Organic & Biomolecular Chemistry in 2015: doi:10.1039/C5OB00709G.
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Why can the same molecule receive different complexity assessments?
A molecule’s structure may remain unchanged while the practical challenge of making it changes. A new reaction, a shorter route, or improved control over a difficult step can alter how demanding a synthesis appears. The score therefore reflects a combination of molecular features, route design, and the state of synthetic chemistry—not simply a permanent property of the molecule.
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The 2015 coverage gives strychnine as an illustration. It reports a current-complexity score of 2.14 for Robert Woodward’s original synthesis and 3.75 for Chris Vanderwal’s 2011 synthesis. Because 1 represents the most complex end of the scale, the higher score corresponds to a less complex assessment. These are figures reported in the 2015 article, not newly measured values or evidence that every chemist would rank the routes identically. See the Chemistry World account.
Eastgate also recounted how a new transformation reduced the route to BMS-911543 from 19 steps to eight. Reflecting on that change, he said: “When I reflected on what we had achieved, the molecule no longer looked as tough as it once had.” The example illustrates the proposal’s premise: route improvements can change perceived difficulty even when the target molecule does not change. The step counts and quotation are reported in Chemistry World’s 2015 coverage.
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Can a chemist’s intuition be quantified?
Li and Eastgate’s method turns expert judgments into a structured index, which could make comparisons more explicit and help chemists discuss route difficulty or plan syntheses. A common framework can be useful even when it does not capture every reason a chemist finds a route challenging.
The difficulty is that the input includes human judgment. The 2015 report describes wide distributions in chemists’ assessments of the same molecule, and organic chemists may differ in experience, priorities, or what they consider an ideal route. Scott Snyder, then an organic chemist at the Scripps Research Institute, likened judging complexity to “deciding which painting is superior or which piece of music is more pleasing to the ear.” Johann Gasteiger, a cheminformatics expert at the University of Erlangen-Nürnberg, observed that “even with the advent of computers, no system has found broad acceptance among the organic community.” Both quotations are from the 2015 report.
A single number also compresses distinct considerations. It can obscure whether a difficult score comes from a complex molecular structure, a long route, challenging stereochemistry, or the limits of available methods. The index is best understood as a proposed way to organize and compare assessments, not as a definitive verdict on a molecule or a substitute for route-specific expertise.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What was the method’s status?
The 2015 reporting described current complexity as a proof of method and said it was already in use at Bristol-Myers Squibb. At the time, the researchers’ ambitions included ranking a larger set of molecules and incorporating the method into a synthetic-route design engine. Those statements describe the position in 2015; the sources cited here do not establish present-day uptake, independent validation, or whether later methods superseded the proposal.
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The Royal Society of Chemistry also published an account titled “Organic chemistry’s complexity conundrum” on 26 May 2015. The article identifies the underlying scholarly paper and frames the broader question: whether a simple number can describe the complexity of an organic structure. The proposal addresses a narrower, more practical question—how difficult synthesis appears given routes and technology available at a particular time.
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