Indolizine family

The cycla[2.3.3] cyclazine heterocyclic products are new additions to the indolizine family, the skeleton of which is found in many naturally occurring alkaloids, such as crepidine and poranthericine (Chart 2). 

Indolizine, whose numbering system is shown in formula 1, is a ring system present in many families of natural alkaloids, mainly in the fully saturated form named indolizidine (Figure 1). 

Robinson and Sugasawa (8) selected dibenzo[b,g]pyrrocoline for the parent ring system, and this name was adopted for the alkaloid family. The numbering is shown on structure 3 although Robinson used a different pattern. The systematic name for the ring system is indolo[2,l-a]isoquinoline, but di-benzo[ib,g]indolizine also appears in the literature. 

Just as there is a family of heterocycles isoelectronic with the indene monoanion (13)27 (indole, isoindole, indolizine, pseudoazulenes, etc.), there are several hundred possible systems derived from the pentalene dianion (14).2 Scheme 3 shows some examples (15-20) derived (formally) from each canonical form of 14. 

Besides processes (1) and (2), the reader should be aware that nucleophilic attacks on alkynes are treated in other chapters of this book, dealing with rearrangements, cyclizations, polyacetylenes, cyclic acetylenes and perhaps others. A number of publications overlap with ours in different ways and at different levels -. They treat individual alkynes or families " , e.g. acetylene, diacetylenes , acetylene dicarboxylic esters haloacetylenes , alkynyl ethers and thioethers > ynamines , fluoro-alkynes ethynyl ketpnes , nitroalkynes , etc. synthetic targets, e.g. pyrazoles , if-l,2,3-triazoles , isothiazoles , indolizines S etc. reagents, e.g. nitrones , lithium aluminium hydride , heterocyclic A -oxides - , azomethine ylids - , tertiary phosphorus compounds , miscellaneous dipolar nucleophiles - , etc. The reader will appreciate that all of these constitute alternate entries into our subject. 

The structures of asperparalines A (123), B (124), and C (125) were determined to be spiro compounds made up of an A -methyl succinimide and a cyclopent[f]indolizine having an A methyl amide bridge. Another structural characteristic of asperparalines is a bicyclo[2,2,2]diazaoctane core, and various compounds, such as those of the paraherquamide family mentioned in the following section, contain the same core in their structures. However, all these compounds have an indole moiety in their structures so, it is of great interest that asperparalines have no indole part in their structures. 

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