Pyrrolizidine chemistry

Monocarboxylic [as in heliotrine (5) and trachelanthamine (6)] and dicarboxylic acids [in senecionine (7) and thesine (8)] are found among the carboxylic acid moieties. All the pyrrolizidine alkaloids afford pyrrolizidine alcohols under conventional conditions of ester hydrolysis. Formerly, these alcohols were used as starting materials for all the studies dealing with pyrrolizidine chemistry synthetic approaches have now been developed. Some pyrrolizidine alcohols, and other derivatives like 1-methylenepyrrolizidine, occur in plants in the free state, forming the special group of the so-called non-ester pyrrolizidine alkaloids. 

The most outstanding achievement of synthetic pyrrolizidine chemistry is the recently published69 total stereospecific synthesis of (+ )-retronecine, a constituent of many alkaloids (see Scheme 7). 

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