Ring formation three-heteroatomic rings

Beyond the scope of this chapter, but similar in mode of formation and reactivity to the title compounds, are some three-membered rings with three heteroatoms. Oxadiaziridine... 

Since syntheses of three-membered rings with two heteroatoms almost always proceed under mild conditions with good yields and from simple starting materials, they are useful intermediates in synthesis. The easy formation of N—O and N—N bonds offers synthetic potentialities. Furthermore, the three-membered ring may serve as a protecting group during additional operations of substitution. 

As with the other procedures for the preparation of six-membered heterocyclic systems which proceed via formation of only one ring bond there are relatively few methods which involve formation of a ring bond y to the heteroatom and which can best be classified as [6 + 0] processes rather than [4 + 2], [3 + 3], etc, processes. Of those which can be so represented, however, a number are important processes which are widely used for the synthesis of saturated, partially saturated and aromatic six-membered heterocyclic systems and their benzo derivatives. Mechanistically, the nucleophile —> electrophile approach is by far the most common, but in contrast to the reactions discussed in the previous three sections, radical cyclizations are of considerable utility here. 

A carbene or nitrene transfer reaction to a carbon-carbon or carbon-heteroatom double bond system leads to the formation of three-membered rings, such as a cyclopropane, an aziridine or an epoxide. These processes can be catalyzed by applying iron catalysts and the different cyclic systems are discussed here. 

One of the most fascinating reaction modes of disilenes is the ready formation of disiliranes, three-membered ring systems containing two silicon atoms and one heteroatom in the ring, which are hardly accessible by other routes. They are mostly formed by [2 + 1] cycloadditions of atoms or molecular fragments to the Si=Si double bond. One of the few examples of other accesses to this ring system is the reaction of AMithio-2,4,6-trimethylanilide 53 with the disilane 52, which affords the azadisilacyclopropane 54 in modest yield (equation 8)75. 

Many syntheses of pyrrolo-fused bicyclic [5-5] systems of this type start from pyrrole derivatives and involve formation of the ring containing three heteroatoms, as illustrated by preparation of the 1,2,4-triazole derivative 72 (Scheme 44) . 

Solid-acid catalysts can also be used to prepare a three-heteroatom five-membered ring system, but relatively mild reaction conditions are required. A recent example is the liquid-phase formation of 4-aminotriazole (29) by reaction of hydrazine and formic acid in the presence of a strong-acid resin [54]. 

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