6-Unsubstituierte 2H-Pyran-2-one Synthese und Reaktivität

Abstract

Although the 2H-pyran-2-ones have been known for more than 100 years, their importance as versatile intermediates was not widely recognized before the sixties of this century. Until then, the C(6)-unsubstituted derivatives represented only a small minority due to their difficult accessibility. This review is dealing in the first part with the syntheses as well as, in the second part, with the reactivities of most of the C(6)-unsubstituted 2H-pyran-2-ones known so far. Of all reactions of the 2H-pyran-2-ones, the Diels-Alder cycloaddition is the most studied. Depending on conditions, the 2H-pyran-2-one ring can react as dienophile or as enophile. Furthermore, the subsequent elimination of CO₂ from the formed bicyclolactone can frequently be influenced. This opens the opportunity of stereoselective syntheses. Recently, there were also reports on [4+3] and [3+2] cycloadditions. The photochemistry of the 2H-pyran-2-ones was studied intensely in the seventies of this century. Again, the enormous versatility of this ring system was demonstrated. Depending on conditions and substituent patterns, some ring contraction, ring opening, or dimerization by [4+4], [2+2], or [4+2] cycloaddition occurs. However, the highest variability in the reactivity of the C(6)-unsubstituted 2H-pyran-2-ones is demonstrated in their reactions with nucleophiles. These reactions, mostly accompanied by ring opening, give different, often otherwise difficultly accessible products depending on the conditions. The reactions with N-nucleophiles were the most productive so far, but also H-, O-, and C-nucleophiles gave interesting products. In recent years, much more became known about the synthesis and reactivity of C(6)-unsubstituted 2H-pyran-2-ones. This opened further possibilities of their applications in all fields of modern organic synthesis.