Our synthesis of thapsigargin is out today in ACS Central Sci. Since we’ve prepared a detailed supporting information, we will forego discussion of the failed approaches & optimizations in this blog post (feel free to email us or comment if you’ve got any questions though). We will instead use this post as a means to openly discuss a defining aspect of not only this synthesis, but also of all of our 2016 syntheses: step-count. Thus, this blog is intended to stimulate an alternative type of peer reviewing–we present our reasoning behind a simple definition of a step and seek opinions from all readers.
Similar to other recent total syntheses (pallambins, maoecrystal V, and araiosamines) emerging this year from our lab, our manuscript was originally titled “11-Step Synthesis of (–)-Thapsigargin” (kinda gave away the punchline to the tweet here). In doing so, we abstained from using subjective and assertive descriptors such as “concise” and “short”. Here is the direct feedback we got from peer review:
"Their claim of 11 steps in a title is unhelpful, I would accept a “Short synthesis of…”, but not a step count. This sets a bad precedence in that we no longer claim “a first synthesis” as being valid. The same is true of step count since as the authors are well aware, step counting is just one of many criteria to judge a synthesis. If we allow a step count number to become normal practice, all total natural product synthesis will have to begin with this count of the synthesis steps. As we know this says nothing about efficiencies or the elegance or costs of an approach."
"The work includes the step count as primary consideration, something that has become a trademark of the author, as noted in the title. There are serious problems with this metric as practiced by the author: (1) the count is conducted in a manner not entirely transparent or logical, as such it does a disservice to the community and pioneering efforts of others that have previously worked on the targets; (2) it leads to erroneous conclusions. The principal investigator has repeatedly been engaged in such miscalculations; and prior ways of counting should not justify continued obfuscation."
This is really valuable feedback as it is likely that if these referees believe this then certain members of the community must also feel the same way. Although we are perplexed by the reviewer’s assertion that somehow the exact step-count included a title makes the first synthesis invalid, the other points raised by the referees deserve comment. To us, inclusion of the step-count in the title solely provides an immediate, objective, and unopinionated depiction of the synthesis – it informs the readers that the synthetic sequence disclosed comprises 11 steps. And we're certainly not alone as there have been dozens of high-profile total syntheses by other groups published with step counts in the title. By no means is this a Baran lab "trademark".
The more serious accusation, however, is that our definition of step-count is somehow not logical or transparent. Or even worse that we are engaging in outright deception. Open-Flask was started years ago specifically to bring more transparency to our research. We feel that the most transparent and fair way to discuss this is out in the open rather than in the comfortable anonymous basement of peer-review.
IUPAC defines a step as a process that "proceeds through a single transition state" which largely pertains to physical chemistry phenomena. When organic chemists refer to a step of a synthesis they are usually referring to what goes on in a flask (reagent additions, etc.) followed by some sort of work-up or purification (which signifies the end of a step). On several occasions we’ve explicitly defined that a single reaction step is one in which a substrate is converted to a product in a single reaction flask (irrespective of the number of transformations) without intermediate workup or purification. This definition seems to be the most pragmatic and encompasses what most organic chemists think of when speaking of a step (things go into a flask followed by a work-up which signifies the end of a step).
Although we can't rule out hacking from an outside group, or other sorts of rigging, the Twitter community seems to mostly agree as well based on this super-scientific poll we did the other day:
|Yes, some people will argue that amide-bond formation should be counted as 2 steps...|
Some of the discussion on Twitter is really revealing. Many people have different opinions as to what constitutes a step or not with some arguing that the whole concept of a step is outdated and should instead be replaced with other metrics, such as number of operations or even person-hours.
In our most recent syntheses as well as this one, we have strictly conducted our step-count according to the definition outlined above. While not perfect, it is a simple definition that accounts for many types of reactions. For example, a number of classic transformations (e.g. Swern, Ugi, Passerini, Strecker) involving multiple distinct intermediates (the last 3 isolable) and all fit into this definition. So do cascade or tandem-reactions. How many steps is Corey's legendary aspidophytine total synthesis if we break up the key step into individual components (multiple reagent additions and at least eight elementary intermediates, some of which are isolable)? How about Noyori's classic prostaglandin synthesis that introduced the world to vicinal difunctionalization (multiple reagents added and 2 new C–C bonds generated)? Let's take the Swern oxidation as a glaring example of this discontinuity in step count – this venerable reaction comprises three distinct transformations as shown here.
|How many steps is this step?|
However, since all of them take place in a single flask, it has always been considered as one step. So, three different transformations actually happen, but the net result is that an alcohol is oxidized to a ketone. Finally, what about the most used reaction in all of organic chemistry: Amide bond formation. There, one adds an activating agent like DCC to form an activated ester (sometimes isolable) followed by addition of an amine. Is that two-steps or one (a few people apparently think 2, see above poll)? By now you might be rolling your eyes and thats the point. This is common sense. Most people will agree that the sequential addition of reagents or solvents to the same flask does not constitute a new step. Filtering over silica or Celite, workup of any kind, adding scavenger resins – all of those things signal the end of a step with further operations on crude or semi-crude material representing a "telescope". For example, in Wender's synthesis of Phorbol (summarized graphically here), the following procedure is characterized as a single step in the overall step count (reported as 36 total) as presented the manuscript but I think most people would agree that the SI clearly describes a two step process (ketone to enol ether to alpha-bromoketone):
|Flash Chromatography, rapid or slow, signifies the end of a step.|
Now lets take a look at one of the steps in our synthesis that triggered the referee comments above. It accomplishes two transformations in a single flask (TBS installation and allylic oxidation). By the logic outlined above, this should also be counted as one step. There is no deception here. We are uncertain how alternative definitions of a step can be rewritten – after all, each single transformation within the “step” can be further divided into combinations of elementary steps.
|How many steps here?|
Indeed, one-pot multi-transformation steps are embedded into the strategy of a synthesis, meaning that when steps in which one pot reactions can be engineered (e.g. without sacrificing yields significantly & saving solvents, purifications, and manual labor), we went ahead and performed them. As a result, there’s a very clear logic behind conducting these one pot sequences. Although orchestrating such one-pot sequences can conceivably improve the overall efficiency of a concise synthesis, the improvements will be proportionally less substantial for longer syntheses. In other words, attempting to shorten a 40-step synthesis with just this tactic alone will be fruitless just like speeding up a conveyor belt does not always lead to a higher production rate.
Thus, we believe the discussion on step count is entirely perched on the overall strategy of the synthesis.
Although step-count is an important metric for measuring the efficiency of a synthesis, we are not claiming that as long as it’s short, one can go ahead and ignore all the other criteria for a good synthesis. The best example we can think of where a longer synthesis can be better is in the commercial synthesis of Halavan where the heroic Eisai team, led by process legend Frank Fang, favored a longer route due to the identification of crystalline intermediates that facilitated purification. In the case of thapsigargin, we actually report an alternative longer sequence (14-steps) that offers a distinct advantage for the production of certain analogs (in this case a higher yielding photo-rearrangement and the ability to incorporate different esters via simple acylations) over the shorter sequence. Details of this other route are included in the manuscript & the SI.
Some might argue that a better way to present step-count in synthesis would be to report actual isolated intermediates instead of actual steps. At the end of the day, someone (referees, students in a group meeting somewhere, readers, your parents?) will be counting the actual steps (which begin and end with isolated intermediates) so we're not sure it makes any difference. Also, we are still on the fence regarding the removal of a solvent as representing a new step (there was some fruitful debate on Twitter about this - the classic Arndt-Eistert reaction for example usually involves a solvent swap but no workup) and lean towards a definition where PURIFICATION of any type signifies the end of a step as the evaporation of a solvent is no different than refluxing a solution (without the cap).
In any event, the short or concise or 11-step or 14-step (your preference) scalable synthesis reported here facilitated LEO Pharma's interest in a medicinal chemistry campaign based on this chemotype. They filed a provisional patent on the route, have successfully outsourced it on 100-gram scale, and we are currently working on interesting analogs with them using advanced intermediates they have provided.
|However one counts it, the synthesis of Thapsigargin is now scalable.|
For some other recent reviews on the topic of efficiency that incorporate step count into the equation, see these from process grandmasters Martin Eastgate and Chris Senanayake. In addition, the Krische lab's impressive perspective published earlier this year in JACS defines a step in a similar fashion (see SI - "a step is defined as an operation that does not involve any intervening purification/separation, including removal of solvent, commencing with compounds that are over $50/gram.").
We welcome any critique and feedback and look forward to an open dialogue of step-count here at Open Flask or on Twitter. We realize and respect the fact that not everyone will agree with this simple definition of a step and thus, as the referee above stated, it is important to evaluate syntheses based on more than a single variable.
(thanks to Phil and other Baran Lab members for help with this post)