Many molecules of life come in versions that look alike but are not exactly identical, like right and left hands.
One mirror-image of the chemical carvone smells like mint, for example, while the other has a whiff of caraway.
When designing drugs to treat diseases, scientists want to be choosy about which mirror image they are getting.
They want the version of a molecule that will treat a cold, not the kind that will make the sniffles worse.
Kagan and Soai found ways to make chemistry pick a side, designing and manipulating reactions to produce the desired mirror image, work that has been key for chemists who design reactions for the manufacture of pharmaceuticals.
"This is probably the coolest experiment in organic chemistry," said Peter Somfai, a member of the Nobel Committee for Chemistry.
"Taken together, the groundbreaking discoveries by Henri Kagan and Kenso Soai have reshaped our understanding of molecular chirality - how it is created, amplified and transmitted."
Chirality comes from the Greek word for hand.
In the 1980s, Kagan's work showed there might be a way to design reactions to produce one form of a molecule over the other.
Years later, Soai brought that dream to life.
He created the first-ever chemical reaction to produce only one of a molecule's mirror images.
"Other than life itself, no one had previously achieved this feat," the committee said in a news release.
"It might be difficult to say that this drug, or that drug, was developed using this," Somfai said.
"I would say all of them, because we use this as a tool, we use this as an understanding, how to develop catalysts, how they function."
Kagan, 95 years old and born in France, is affiliated with the Universite Paris-Sud in Orsay, France, and Soai, born in 1950 in Japan, with the Tokyo University of Science.
"This is the most exciting day in my life, I am very glad to share this prize with professor Henri Kagan," Soai said, speaking by phone to the Nobel Prize media conference, adding he had been out shopping when receiving the news.
The Royal Swedish Academy of Sciences announced the winner in Stockholm on Wednesday.
The 2026 prizes began with the Nobel for medicine, awarded on Monday to three scientists whose work led to a tool that uses light to help unravel how the brain works.
The physics award on Tuesday recognised a scientist's efforts to demystify a rare group of neutrinos.
The literature prize is set to be awarded on Thursday.
After Friday's peace prize is awarded, the Nobel Memorial Prize in Economic Sciences will be announced on Monday.
The prize money is 12 million Swedish kronor (about $A1.7 million), whether for an individual, a small group of laureates or an organisation.
with Reuters
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