Involving other than five and six-membered rings

A. R., Tautomerism Involving Other Than Five- and Six-Membered Rings, 77, 1. 

Tautomerism involving other than five- and six-membered rings 00AHC(77)1. 

Benzene is aromatic because it has six electrons in a cyclic conjugated system. We know it is aromatic because it is exceptionally stable and it has a ring current and hence large chemical shifts in the proton NMR spectrum as well as a special chemistry involving substitution rather than addition with electrophiles. This chapter and the next are about the very large number of other aromatic systems in which one or more atoms in the benzene ring are replaced by heteroatoms such as N, O, and S. There are thousands of these systems with five- and six-membered rings, and we will examine just a few. 

Scheme 11.13. A representation of a portion of the pathway of gluconeogenesis. The 6-phosphate of glucose is considered capable of hydrolysis to glucose with the appropriate hydroiyase (EC 3.1.3.9) catalyst. The anomer shown is a (because the hydroxyl on the ano-meric carbon is cis or syn to the hydroxyl at C5, Scheme 11.14). A caution concerning nomen-ciature is needed. In the acyclic form, it is clear which carbon is number six. Cyclization to the six-membered ring pyranose (oxane) (or furanose [oxolane] when the five-membered ring forms) leaves ns with a nomenclature problem since the carbon that was C-1 (the ano-meric carbon) is often referred to as C-1 in both structures that involve formation of bonds to that carbon as well as in reactions that may occur at that carbon. However, it will be recalled (Chapter 5) that the nomenclature of cyclic ethers (and other heterocycles) generally requires that the heteroatom (or a heteroatom if more than one is present) receive the lowest number. So, the lUPAC and the lUBMB numbering system is used for archiving purposes, while the trivial nomenclature is commonly used in the literature. Both systems will be found here as appropriate.

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