Other Systems containing Three Heteroatoms

Miscellaneous other Systems containing Three Heteroatoms. - Catechol is converted into the 1,3,2-benzodioxathiolan (503) by the action of EtNSFj. The cyclic iminodiacyl peroxide (504) is obtained from the imino-compound MeN(COCl)2 and hydrogen peroxide. The hydroxamic acid HOCPh2-CONHOH reacts with carbonyldi-imidazole to yield the dioxazolone (505). The i.r. and Raman spectra of l,3-dimethyl-l,3-diaza-2-boracyclopentanes (506 R = Cl, Br, or NMc2) have been reported. The zwitterionic salt... 

Other Systems containing Three Heteroatoms 266 Oxadiazoles 266... 

The majority of the compounds under discussion here have pyrrolidine as one of the 5-membered rings. The other ring, containing three heteroatoms, is sometimes a 6n aromatic system as in the tetrahydropyrrolotriazole (45), for example, but most of the bicyclic systems covered in this chapter, for example, (27), (31)-(33) have no aromatic character. 

In those systems where the five-membered ring contains three heteroatoms and is stable, the adjacent carbon atoms on the six-membered ring are subject to nucleophilic attack because of the reduction of electron density at these points. These reactions for l,2,3-triazolo[4,5-d]pyrimidines (7), for [l,2,5]oxadiazolo[3,4-d]pyrimidines (10), and for [ 1,2,5]thiadiazolo[3,4-d]pyrimidines (14) have been reviewed (see Table 5 for references). New reactions and reactions of other systems are discussed in the following sections. 

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. 

A cyclic conjugated system containing An + 2)ji electrons has an extra stability over that of a comparable number of isolated double bonds. This extra stabilization, known as aromaticity, leads to a characteristic pattern of reactivity which distinguishes the reactions of benzene (1.8) from, for example, the linear hexatriene (1.9) or cyclooctatetraene (1.10) An electrons, n = 2). The aromatic sextet may arise not just from the overlap of three double bonds as in benzene (1.8) or pyridine (1.11) but also from the participation of the lone pair of electrons on a heteroatom. Thus pyrrole (1.12), with effectively six n-electrons, shows some aromatic character. In allene (1.13) the double bonds are at 90° to each other and conjugation does not occur. 

The force field covers all alkenes, with the exception of those containing directing groups where additional double bonds or three-membered rings are at least three bonds (two saturated carbon atoms) and every other type of unsaturation or heteroatoms at least four bonds (three saturated carbon atoms) apart from the double bond to be epoxidized, and those where the double bond is not part of a cyclopropane or cyclobutane system. 

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