Known Small Heterocycles

STRUCTURE OF SMALL HETEROCYCLIC RINGS 5.01.2.1 Known Small Heterocycles... 

Anions of small heterocyclics are little known. They seem to be involved in some elimination reactions of oxetan-2-ones (80JA3620). Anions of large heterocycles often resemble their acyclic counterparts. However, anion formation can adjust the number of electrons in suitable systems so as to make a system conform to the Hiickel rule, and render it aromatic if flat geometry can be attained. Examples are found in Chapter 5.20. Anion formation in selected large heterocycles can also initiate transannular reactions (see also Section 5.02.7 below). 

A wide variety of natural toxins, from small heterocyclic molecules to large proteins, occurs in marine organisms. The phyletic diversity of plants in the ocean is far less than on land, while the number of marine animal phyla significantly exceeds that on land. Thus, it is not surprising that many of the known marine toxins are of animal origin. In this article, we will not only focus upon the toxins of unicellular organisms and marine animals, but also consider a few seaweed toxins. 

Structure of Small and Large Rings Table 2 Structures of Known Four-membered Heterocyclic Compounds... 

Radicals of most small and many large heterocycles are known, but their chemistry has not always been explored in depth. The ESR spectra of small ring radicals have been measured and generally found to be in good agreement with theoretical predictions. Table 1 gives some data for three-membered heterocyclics. Noteworthy is the close similarity of the ESR spectra of 1-aziridinyl, 1-azetidinyl and dimethylaminyl (71TL2247). The radicals in the table are all tt-radicals. 

Surprisingly little is known about the attack of radicals on small and large heterocycles. Hydrogen abstraction from the heteroatom of small rings leads to ring opening, and in the... 

The first member of the three-membered ring sulfones was synthesized about 70 years ago5, and its unsaturated analogue has been known for only 20 years6. Since the midsixties, an explosive expansion in the chemistry of some of these small- to middle-sized sulfone and sulfoxide heterocycles has taken place. 

As already described for the all-carbon-Diels-Alder reaction, a hetero-Diels-Alder reaction can also be followed by a retro-hetero-Diels-Alder reaction. This type of process, which has long been known, is especially useful for the synthesis of heterocyclic compounds. Sanchez and coworkers described the synthesis of 2-aminopyridines [48] and 2-glycosylaminopyridines 4-144 [49] by a hetero-Diels-Alder reaction of pyrimidines as 4-143 with dimethyl acetylenedicarboxylate followed by extrusion of methyl isocyanate to give the desired compounds (Scheme 4.30). This approach represents a new method for the synthesis of 2-aminopyridine nucleoside analogues. In addition to the pyridines 4-144, small amounts of pyrimidine derivatives are formed by a Michael-type addition. 

---