Chain-ring interconversions

In the second group of ring-chain tautomeric interconversions, an open-chain system is transformed into a cyclic system through the intramolecular reversible addition of a functional group to a polar multiple bond lA IB 2A 2B 3A 3B and 4A 4B. The book (I) and this article deal with... 

Ring-chain tautomeric interconversions proceeding by intramolecular reversible addition reactions to the C=0 group (Scheme 1) have been well studied, particularly with respect to the 3- and 4-oxocarboxylic acids containing five- or six-membered rings, respectively. Relatively few new investigations have appeared in the literature. 

Section 3.4.3.1.9 gives details of ring interconversions involving three-atom side chain displacements at both N and S ring atoms. 

An interesting example of the synthesis of a pyrazoline via ring interconversion involving a saturated side chain has been studied in detail (113- 114).65,66 This reaction is unlike the classical mhrs in this case, the starting ring and side chain do not form a continuous 7r-electron system, and the conversion is azole-to-azoline and not azole-to-azole, as in the original scheme 1 - 2. 

Even before Kekule published this speculation, another approach was tried not a method for absolute determinations of ring positions, but rather for relative determinations. Let us say that one of the three isomers of toluidine (methyl aniline) is shown to convert into a particular isomer of nitrobenzoic acid when it is vigorously oxidized, and that this nitrobenzoic acid can be shown to reduce to a particular one of the three isomers of aminobenzoic acid. Since all three of these isomers are mutually interconvertible, one can infer that all three molecules have the same relative positions of their two substituents, even though the absolute identity of that positional assignment remains unknown they are all, say, "ortho" compounds (where in this case "ortho" is intended arbitrarily to express only relative and not absolute positions). Such "genetic" chains of interconversions were rapidly developed after January 1865, so that one could make a table of three different series, the compounds in each series being theoretically or actually all mutually interconvertible. These acquired the then-arbitrary names "ortho," "meta," and "para" families of aromatic isomers. At first no one at-... 

The chain-ring equilibria are often observed in systems of polyamides. Independently of the route of preparation of polymers (polycondensation, polymerization of lactams, or other cyclic amides, including cyclic oligomers) the ring-chain interconversions are often important and cannot be neglected. However, the equilibrium conditions usually are not reached and often only kinetic distributions can be observed. 

Because six-membered rings aie nonnally less strained than five-membered ones, pyranose forms are usually present in greater anounts than furanose forms at equilibrium, and the concentration of the open-chain form is quite small. The distribution of carbohydrates among then- various herniacetal forms has been examined by using H and NMR spectroscopy. In aqueous solution, for exanple, D-ribose is found to contain the various a- and p-furanose and pyranose forms in the amounts shown in Figure 25.5. The concentration of the open-chain form at equilibr ium is too small to measure directly. Nevertheless, it occupies a central position, in that interconversions of a and p anorners and furanose and pyranose forms take place by way of the open-chain form as an intermediate. As will be seen later, certain chemical reactions also proceed by way of the open-chain form. 

The Cornforth rearrangement involves the thermal interconversion of 4-carbonyl substituted oxazoles, with exchange between the C-C-O side-chain and the C-C-O fragment of the oxazole ring. These reactions generally involve compounds where a heteroatom (-OR, -SR, -Cl) is attached to the 5-position (R2) of the starting oxazole. 

A ring-chain transformation with slow interconversion (compared with the NMR time-scale) has been reported in the solution of (144) and related derivatives. On the other hand, no tautomerism was observed when the benzene ring was replaced by a thiophene ring or an aliphatic double bond. ... 

---