Pyrrolizines and Related Systems

The saturated or partially saturated pyrrolizidine alkaloids are the main naturally occurring pyrrolizines senecionine is an example. 

The relatively high of 29 for deprotonation of 3//-pyrrolizine cf. indene pA a 18.5) indicates that formation of the 10-electron pyrrolizine anion adds only minor stabilisation relative to the simple pyrrole originally present. Its reactions are those of a highly reactive carbanion, for example benzophenone condenses to generate a fulvene-like product.  

Isoelectronic replacement of a carbanionic carbon by a heteroatom gives much more stable compounds, and such 5,5-bicyclic aromatic systems have received considerable attention. In these compounds, sulfur and oxygen can also be incorporated into fully conjugated systems, unlike the 5,6-compounds where only nitrogen can be used. Because of the variety of such systems, it is difficult to generalise about reactivity but electrophilic substitution, which can take place in either ring, has been most widely reported with occasional examples of nucleophilic displacements and lithiations. Some representative reactions and self-explanatory syntheses are shown below.  

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This class of tricyclic compound is dominated by the 377-pyrrolizines (systematically named 3//-pyrrolo[l,2- ]-pyrroles) and related systems, which are fused to another six-membered heterocyclic ring (Figure 1). 

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