Transformations of carboxonium compounds in carbohydrate and polyol chemistry
By attaching 1,3-dioxolenium or 1,3-dioxenium rings to polyfunctional compounds, cationic species are obtained which are prone to undergo several different transformations, such as 1,2and 1,3-neighbouring group reactions as well as ring contraction reactions. The synthesis and characterization of such carboxonium ions is discussed. In a series of monosaccharides and also polyols the enhanced reactivity of carboxonium ions is the reason for manifold rearrangements. During the last 10 years it has been possible to find conditions under which more or less free carbenium ions with an enhanced lifetime can be synthesized. In this connection one first of all thinks of reactions in super acids studied by Olah1 and his collaborators. These studies have demonstrated that free carbenium ions are rather reactive and often undergo extremely fast carbon skeletal rearrangements to yield more stable carbenium ions. Naturally these investigations were only possible in the range of the rather stable hydrocarbons and not in the field of polyfunctional compounds. However, by resonance stabilization of the cationic centre with one or several hetero atoms the reactivity of the resulting cation can gradually be lowered. This kind of stabilized cation can be obtained with polyfunctional compounds. There, the reactivity is lowered so far that uncontrolled degradations are excluded but is still high enough to give rise to interesting and directed rearrangements. The purpose of this lecture will be to show you how carefully this line-—that is between too 'hot' and too 'cold' cations— must be manipulated in order to control the manifold applications of transformations of polyfunctional compounds. Particularly suitable cations are represented by the carboxonium ions which are resonance-stabilized by two oxygen atoms. These are ions of the dioxolenium type, as shown in Figure 1. More simply, such an ion related to a carbonic acid can be named acyloxonium ion—in this case acetoxonium ion—related to acetic acid. It was Meerwein23 who first succeeded in isolating crystalline acyloxonium salts. The general methods by which acyloxonium ions can be generated have been summarized in Figure 1. As shown here, the acetoxonium ion, for 69