Puzzle Maker: Building a Chemical from the Ground Up

For chemists like Sarah Reisman, professor of chemistry at Caltech, synthesizing molecules is like designing your own jigsaw puzzle. You know what the solved puzzle looks like—the molecule—and your job is to figure out the best pieces to use to put it together.

"We look at the molecule we want to build and think about how to cut it up into pieces. When we are in the lab, the question is: do your puzzle pieces go back together?" says Reisman.

Synthesizing molecules is a vital part of many chemical manufacturing industries, from producing fuels to dyes used in flatscreen TVs with organic light-emitting diode (OLED) displays. Scientists also create molecules from scratch to better understand how they work, as well as to design new drugs.

Reisman's team has been busy trying to crack the puzzle of the insecticide ryanodine, a complex molecule first isolated from a tropical plant in the 1940s. Ryanodine paralyzes insects by binding to a class of calcium-channel receptors called ryanodine receptors. In humans, these receptors play critical roles in muscle and neuronal function. Mutations in the genes that encode ryanodine receptors can lead to disease, including certain types of heart arrhythmias and possibly Alzheimer's disease.

As a stepping stone on the path to synthesizing ryanodine, Reisman, along with graduate student Kangway Chuang and postdoctoral researcher Chen Xu, first targeted a similar molecule, ryanodol. Ryanodol previously has been made by two other research groups: In the late 1970s, a research team made ryanodol in 41 steps, and in 2014, another team synthesized the chemical in 35 steps.

Now, reporting in the journal Science, Reisman's team has devised a route to synthesize ryanodol in just 15 steps. This significantly cuts the time required to make ryanodol, and presumably also ryanodine, which Reisman's team will try to synthesize next.

"Once you have the platform for making both of these molecules, it opens up a lot of possibilities," says Reisman. "In general, it is important that we know how to put molecules together. Without this, it's tough to think about how to study the biological function of molecules and develop new drugs."

Ryanodol and ryanodine belong to a class of molecules called terpenes. These are naturally occurring molecules that commonly contain between 10 and 30 carbon atoms. For example, 10-carbon terpenes include R-carvone, the molecule behind the flavor in spearmint leaves; and pinene, which is derived from pine trees and is the primary chemical in the paint solvent turpentine. The antimalarial drug artemisinin, derived from the wormwood shrub, is a 15-carbon terpene.

Ryanodol and ryanodine are some of the more chemically complex 20-carbon terpenes, with five different carbon rings and many carbon–oxygen bonds.

"The simplest forms of terpenes give you fragrances and flavors, but as you build upon the structure, you get more interesting biological compounds like ryanodol and ryanodine," says Reisman.

There are two big challenges in the synthesis of ryanodol. First, chemists have to build the five rings that make up the carbon backbone of the molecule, and second, they have to precisely decorate seven of the carbons with "OH" (or hydroxyl) groups, the chemical structure found in alcohols. Previous syntheses of ryanodol required multiple chemical reactions to introduce the OH groups, adding extra steps. Reisman's synthesis develops new reactions that brings in two or three alcohols at a time—a key discovery of the new synthesis that makes it more efficient. 

The Reisman team began with a simple commercially available terpene, then attached two of the OH groups. They then built up four of the five carbons rings in a series of reactions. Next, the team brought in two more OH groups, and a precursor to an OH group, again in a single step. The fifth and final ring was formed in two steps using conditions developed in a previous synthesis, which also introduced the remaining two OH groups.

"Five of the oxygen atoms are brought in with just two reactions. That is the key to streamlining the synthesis," says Reisman. "It's like building from Legos using the larger pieces instead of the small ones. You get there faster."

Reisman's team is now working on the final piece of the puzzle: creating ryanodine from ryanodol. They think the solution not only will help them to make ryanodine but also aid in the synthesis of new, designer analogues. This will lead to more precise studies of the ryanodine receptors and the possible development of drugs that can target them.

The Science study, entitled "A 15-Step Synthesis of (+)-Ryanodol," is also authored by Kangway V. Chuang and Chen Xu of Caltech. Reisman is a Heritage Principal Investigator, and the research is funded by the National Science Foundation, Shenzhen UV-ChemTech Inc., the National Institutes of Health, Eli Lilly, and Novartis.