<P> Organocatalysis offers several advantages . There is no need for metal - based catalysis thus making a contribution to green chemistry . In this context, simple organic acids have been used as catalyst for the modification of cellulose in water on multi-ton scale . When the organocatalyst is chiral an avenue is opened to asymmetric catalysis, for example the use of proline in aldol reactions is an example of chirality and green chemistry . </P> <P> Regular achiral organocatalysts are based on nitrogen such as piperidine used in the Knoevenagel condensation . DMAP used in esterfications and DABCO used in the Baylis - Hillman reaction . Thiazolium salts are employed in the Stetter reaction . These catalysts and reactions have a long history but current interest in organocatalysis is focused on asymmetric catalysis with chiral catalysts, called asymmetric organocatalysis or enantioselective organocatalysis . A pioneering reaction developed in the 1970s is called the Hajos--Parrish--Eder--Sauer--Wiechert reaction . Between 1968 and 1997, there were only a few reports of the use of small organic molecules as catalysts for asymmetric reactions (the Hajos--Parrish reaction probably being the most famous), but these chemical studies were viewed more as unique chemical reactions than as integral parts of a larger, interconnected field . </P> <P> In this reaction, naturally occurring chiral proline is the chiral catalyst in an Aldol reaction . The starting material is an achiral triketone and it requires just 3% of proline to obtain the reaction product, a ketol in 93% enantiomeric excess . This is the first example of an amino acid - catalyzed asymmetric aldol reaction . </P> <P> The asymmetric synthesis of the Wieland - Miescher ketone (1985) is also based on proline and another early application was one of the transformations in the total synthesis of Erythromycin by Robert B. Woodward (1981). A mini-review digest article focuses on selected recent examples of total synthesis of natural and pharmaceutical products using organocatalytic reactions . </P>

What is an example of an organic catalyst