Niacin (vitamin B3)
Though niacin or nicotinic acid has been known to the organic chemists since 1867 and Funk isolated it from yeast and rice polishing in order to identify the anti-beriberi vitamin in 1913, it was finally isolated as nicotinamide from liver by Elvehjem and his co-workers in 1937. This was actually after the discovery by Rum and Smith in 1934 that the crude extract of liver was effective against pellagra (you might know that pellagra is the disease condition caused due to the deficiency of niacin in the body)