This is Prilosec OTC (omeprazole, also marketed as losec, antra, gastroloc, and a number of other names in other countries): This is Nexium (esomeprazole, also marketed as sompraz, zoleri, lucen, etc): Did you note the difference?  I’ll give you a second to look again. Omeprazole was brought to market by (what is now) AstraZenica in 1989.  It is used in treatment of gastroesophageal reflux desease (GERD, commonly known as acid reflux).  It is a proton pump inhibitor which works by blocking the production of gastric acid by parietal cells.  It was a blockbuster drug.  In the year 2000 alone, omeprazole made $6.26 billion.  But in 2001, omeprazole’s patent was set to expire.  AstraZenica needed something to fill their pipeline, so they looked at their data for omeprazole. Omeprazole is an example of a little-known class of chiral molecules where the stereogenic atom is not carbon.  We  typically think of stereocenters with carbon as the central atom.  But this is an example of a stereogenic sulfur atom (in case you’re wondering where the fourth ‘thing’ bonded to sulfur is, it is a pair of electrons.  See the explanation of chiral sulfoxides for more information.  Ellman’s chiral auxiliary is a good example of chiral sulfoxides in use).  Omeprazole is a racemic mixture: an equal mixture of (R) and (S) enantiomers. In looking at their data on omeprazole, they noticed that the (S) enantiomer was more potent than the (R) enantiomer...