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Yes, chemists can try to quantify how difficult an organic molecule is to synthesize—but the 2015 “current complexity” proposal treats that difficulty as partly dependent on available routes and technology, not as a fixed property of a molecular structure. Its score is a way to organize judgments about synthesis, not a universally accepted measurement.
What does “current complexity” measure?
Jun Li and Martin D. Eastgate proposed the method in a 2015 paper, “Current complexity: a tool for assessing the complexity of organic molecules”. As described in contemporaneous coverage by Chemistry World, the researchers combined expert assessments with structural and synthesis-route features.
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The method reflects a useful distinction: some contributors are properties of the molecule, while others depend on how chemists make it. A new reaction or a better route can change the perceived challenge even though the target molecule itself has not changed.
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Chemistry World reported that 18 synthetic chemists ranked 40 molecules. Li and Eastgate considered a range of intrinsic and extrinsic factors, then used Bayesian regression to identify five major factors:
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- Topological index: a structural measure of the molecule’s connectivity.
- Stereogenic centers established during synthesis: the route-dependent challenge of creating the molecule’s three-dimensional arrangement.
- Heteroatoms on and in aromatic rings: structural features involving atoms other than carbon in or attached to aromatic rings.
- Number of synthesis steps: the length of the route being assessed.
- Route ideality: how favorably the synthesis route is judged.
The reported scale runs from 1, the most complex, to 10, the least complex. A higher score therefore indicates lower assessed complexity. The reported factors mix relatively fixed structural characteristics with choices and constraints that can change from one route or period to another.
Why can a molecule’s complexity score change?
The proposal’s central idea is that “complexity” here means perceived synthesis difficulty under current conditions. A molecule’s topology and aromatic-ring heteroatoms do not change when a new synthesis is developed; step count, stereocenter-forming strategy, and route ideality can. A better method can make the same target seem less challenging to prepare.
Rank #2
The strychnine example illustrates this point. Chemistry World reported a score of 2.14 for Robert Woodward’s original synthesis and 3.75 for Chris Vanderwal’s 2011 synthesis. On the proposal’s scale, the higher score corresponds to a less complex assessment. These are figures reported in 2015 coverage, not newly measured values or proof that every chemist would rank the routes identically.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesThe motivation was practical as well as conceptual. Martin Eastgate recounted that a new transformation cut the reported synthesis of BMS-911543 from 19 steps to eight. Reflecting on the improved route, he said, “When I reflected on what we had achieved, the molecule no longer looked as tough as it once had.” This example shows how advances can alter the perceived work involved in reaching a target.
Why is measuring complexity still a conundrum?
A numerical index can make comparisons easier, but it compresses judgments that may vary with a chemist’s experience, the chosen route, and what counts as an ideal synthesis. The 2015 report noted wide distributions in chemists’ assessments of the same molecule. Scott Snyder, then an organic chemist at the Scripps Research Institute, compared such judgments to “deciding which painting is superior or which piece of music is more pleasing to the ear.”
The index’s reliance on expert rankings is a strength insofar as it recognizes that synthesis difficulty is not captured by structure alone. It is also a limitation: a score derived from judgments does not make those judgments universal. Johann Gasteiger, a cheminformatics expert at the University of Erlangen-Nürnberg, observed in the same report that “even with the advent of computers, no system has found broad acceptance among the organic community”.
Rank #4
According to that 2015 coverage, the researchers described the method as a proof-of-method and said it was already in use at Bristol-Myers Squibb. The article presented a larger ranking set and integration into a synthetic-route design engine as future ambitions. Those historical statements do not establish present-day adoption, independent validation, or whether later methods have superseded the proposal.
What can the index help a chemist do?
A proposed complexity score can serve as a compact way to discuss how challenging a synthesis appears, or to compare routes when the assumptions behind the score are made clear. For practical use, the important questions are what the assessment includes, which route is being scored, and how much the result depends on expert judgment. It should not be mistaken for a timeless rating of a molecule or a definitive prediction of how difficult a synthesis will be for every team.
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