Synthesis and Reactivity of Uhle’s Ketone and Its Derivatives

AbstractUhle’s ketone and its derivatives are highly versatile intermediates for the synthesis of a variety of 3,4-fused tricyclic indole frameworks, i.e. indole alkaloids of the ergot family, that are found in various bioactive natural products and pharmaceuticals. Therefore, the development of a convenient preparative method for this structural motif as well as its opportune/useful derivatization have been the subject of longstanding interest in the fields of synthetic organic chemistry and medicinal chemistry. Herein, we summarize recent and less recent methods for the preparation of Uhle’s ketone and its derivatives as well as its main reactivity towards the synthesis of bioactive substances. Regarding the preparation, it can be roughly classified into two categories: (a) using 4-unfunctionalized and 4-functionalized indole derivatives as starting materials to construct a fused six-member ring, and (b) constructing the indole ring through intramolecular cycloaddition. Principally, the reactivity of the cyclic Uhle’s ketone shown here is derived from the classical electrophilicity of the carbonyl carbon or the acidity of the α-hydrogen and, though less intensively investigated, chemical reactions that induce ring expansion to form novel ring skeletons.1 Introduction2 Synthesis2.1 Disconnection A: Cyclization Reaction of the Opportune 3,4-Disubstituted Indole2.2 Disconnection B: Intramolecular Friedel–Crafts Cyclization2.3 Disconnection B: Intramolecular Cyclization via Metal–Halogen Exchange2.4 Disconnection C: Intramolecular Diels–Alder Furan Cycloaddition2.5 Disconnection D: Intramolecular Dearomatizing [3 + 2] Annulation3 Reactivity3.1 Use of Uhle’s Ketone for Lysergic Acid3.2 Use of Uhle’s Ketone for Rearranged Clavines3.3 Use of Uhle’s Ketone for Medicinal Chemistry4 Conclusion and Outlook