Saturated heterocycles defined

The n.m.r. spectrum of ethyl 2-methyl-5,6,7,8-tetrafluorochromone-3-carboxylate [122] has been discussed briefly. An attempt has been made using n.m.r. to define the conformation of a saturated heterocyclic ring fused at the 7,8-positions of chromone-2-carboxylic acid [37]. 

In this chapter we summarize the reactivity of the six-membered heterocyclic compounds. We describe first the aromatic compounds, where aromatic is defined as fully conjugated round the ring, and then the partially and fully saturated compounds. Within each of these sections we discuss first the reactivity at the ring atoms, then the reactivity of substituent groups. 

Very simple experimental conditions (palladium(II) chloride (catalyst, 0.1 equivalent) copper(II) chloride (oxidant, 3 equivalents) and sodium acetate (buffer, 3 equivalents) in acetic acid under carbon monoxide at normal pressure and temperature) are necessary for this asymmetric route to saturated fused heterocycles with defined stereochemistry. 

This chapter and the next two will revisit the ring theme, but the rings will all be heterocycles rings containing not just carbon atoms, but oxygen, nitrogen, or sulfur as well. It may seem strange that this rather narrowly defined class of compounds deserves three whole chapters, but you will soon see that this is justified both by the sheer number and variety of heterocycles that exist and by their special chemical features. Chapters 43 and 44 cover heterocycles that are aromatic, and in this chapter we look at heterocycles that are saturated and flexible. Some examples, a few of which may be familiar to you, are shown below and overleaf. 

The presence of an amine or amide group in cyclic substrates greatly fiicilitates the hydroxylation by Beauveria sulfurescens. Numerous mono-, di- and tri-cyclic amides and saturated nitrogen heterocycles have been stuped and a rational basis for the position at vriiich the hydroxy group is inttoduced into the substrate molecule has been put forward however, yet more work is required to define all the factors controlling the selectivi of hydroxylation. Neverdieless useful regio-, stereo- and in some cases... 

From the examples described in the preceding Sections and in Chapter 3, Section 3.3.2 (p. 84), it can be concluded that the hydrogenation of N-heterocyles is much more facile than C-N bond breaking reactions when soluble transition metal complexes are employed as models for HDN catalysis this is in parallel to what has been observed on metal sulfide surfaces. Indeed, while several examples of efficacious homogeneous catalysts for the hydrogenation of N-heterocycles to the corresponding cyclic amines (partially or fully saturated) are available (see Chapter 3), only one case of C-N bond hydrogenolysis promoted by a metal-complex in solution has been described so far. Moreover, the real nature of this process is not clearly defined. 

Within the plant secondary metabolites of flavonoids, flavanones define one of the minor subclasses. They may be called dihydroflavones. The basic chemical structure of flavanones involves two benzene rings (A and B), which are linked by a heterocyclic ring (C). The most characteristic point of flavanone structures is that the C-ring is saturated. Flavanones have an asymmetric carbon at C2-position. 

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