Pictet-Spengler-reactie

De Pictet-Spengler-reactie is een belangrijke organische reactie voor de synthese van alkaloïden en farmaceutische stoffen. Het is een condensatiereactie tussen een β-arylethylamine, zoals fenylethylamine of tryptamine, en een aldehyde of keton, waarbij ringvorming optreedt. Voor de reactie is in de regel een zuur als katalysator en verwarming nodig.

Algemeen schema Pictet-Spengler-reactie
Algemeen schema Pictet-Spengler-reactie

De reactie werd ontdekt door Amé Pictet en Theodor Spengler aan de universiteit van Genève in 1911. Ze verwarmden β-fenylethylamine en dimethoxymethaan (het dimethylacetaal van formaldehyde) in aanwezigheid van zoutzuur. De twee verbindingen ondergingen een cycloadditiereactie met afsplitsing van water en vormden het alkaloïde 1,2,3,4-tetrahydro-isochinoline (THI).[1]

De Pictet-Spengler-reactie kan beschouwd worden als een speciaal geval van de meer algemene Mannich-reactie. Ze kan uitgevoerd worden met een grote variëteit aan tryptamines, aldehyden en ketonen. Ze wordt beschouwd als de belangrijkste reactie voor de synthese van alkaloïden.[2]

Biologische Pictet-Spengler-reactie

In 1977 ontdekte men dat de Pictet-Spengler-reactie ook in de natuur optreedt bij de biosynthese van alkaloïden. Daar wordt ze gekatalyseerd door enzymen, die men Pictet-Spenglerases noemt. Een voorbeeld is de biosynthese van morfine, waarin een enzym-gekatalyseerde Pictet-Spengler-reactie een sleutelrol speelt.[2] De eerste Pictet-Spenglerase die in de natuur ontdekt werd was strictosidinesynthase, dat de synthese katalyseert van strictosidine, een precursor van indoolalkaloïden zoals strychnine.

Biologische Pictet-Spengler-reacties zijn stereoselectief: ze vormen een welbepaalde stereo-isomeer. De klassieke Pictet-Spengler-reactie is niet stereoselectief, tenzij er chirale katalysatoren gebruikt worden.[3]

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