Systems containing Two Different Heteroatoms

Systems containing Two Different Heteroatoms Oxazepines. - In an unusual reaction, theiV-hydroxyphthalimido-ether (170) rearranges (under basic conditions) to give the 1,2-oxazepine derivative (171) in moderate yield, together with (172)/ ... 

Systems Containing Two Different Heteroatoms 6.1 Oxazepines.- 2-Substituted triaryl-1,2-oxazepinium perchlorates, the first examples of seven-membered heterocyclic cations with eight n-electrons, have been prepared by the reaction of 2,4,6-triarylpyrylium salts with nitrones, or N-arylhydroxylamines,... 

Systems containing Two Different Heteroatoms 256 Oxathioles, Oxatelluroles, an Oxaphosphole, and Selenathioles 256 Isoxazoles 258 Formation 258 Reactions 258 258... 

This chapter is subdivided into sections mainly as in previous volumes but with the exception that there is no section on ring systems containing two different heteroatoms due to the dearth of publications in this area. Recent developments in the chemistry of saturated... 

This chapter covers bicyclic ring systems containing two five-membered rings where each ring contains one heteroatom and neither heteroatom is situated at the position of the ring junction. These 5 5 fused heterocyclic systems are represented by the general structures 1-4, wherein X and Y may be the same or different heteroatoms and represent O, NH, S, Se, and Te. The fully conjugated title compounds have a central C-C bond and are isoelectronic with the 10-7i-electron pentalene dianion 5 (Fig. 1). 

These substitutions are facilitated by electron release from the heteroatom pyrroles are more reactive than furans, which are in turn more reactive than thiophenes. Quantitative comparisons of the relative reactivities of the three heterocycles vary from electrophile to electrophile, but for trifluoroacetylation, for example, the pyrrole furan thiophene ratio is 5 x 10 1.5 x 10 I " in formylation, furan is 12 times more reactive than thiophene, and for acetylation, the value is 9.3. In hydrogen exchange (deuteriodeproton-ation), the partial rate factors for the a and p positions of A-methylpyrrole are 3.9 x 10 ° and 2.0 x 10 ° respectively for this same process, the values for furan are 1.6 x 10 and 3.2 x l(f and for thiophene, 3.9 X 10 and 1.0 x 10 respectively, and in a study of thiophene, a P ratios ranging from 100 1 to 1000 1 were found for different electrophiles. Relative substrate reactivity parallels positional selectivity i.e. the a P ratio decreases in the order furan > thiophene > pyrrole. ° Nice illustrations of these relative reactivities are found in acylations of compounds containing two different systems linked together. ... 

Systems containing Nitrogen as the only Heteroatom.—One Nitrogen Atom. The aza[14]annulene (28) has been synthesized by the photolysis of (29) in pentane at —70°C. Its aromatic character is demonstrated by the large difference in chemical shifts of the three inner and the ten outer protons of ca 9.5 p.p.m. Variable-temperature H n.m.r. moreover demonstrates that the ring is confor-mationally mobile, producing an equilibrium of two degenerate species. 

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