Chemistry of indoles carrying a basic function. Part 8: A new approach to the ergoline skeleton

Synthesis and Reactivity of Uhle’s Ketone and Its Derivatives

Uhle’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 Introduction

2 Synthesis

2.1 Disconnection A: Cyclization Reaction of the Opportune 3,4-Disubstituted Indole

2.2 Disconnection B: Intramolecular Friedel–Crafts Cyclization

2.3 Disconnection B: Intramolecular Cyclization via Metal–Halogen Exchange

2.4 Disconnection C: Intramolecular Diels–Alder Furan Cycloaddition

2.5 Disconnection D: Intramolecular Dearomatizing [3 + 2] Annulation

3 Reactivity

3.1 Use of Uhle’s Ketone for Lysergic Acid

3.2 Use of Uhle’s Ketone for Rearranged Clavines

3.3 Use of Uhle’s Ketone for Medicinal Chemistry

4 Conclusion and Outlook

Construction of the tetracyclic core of (±)-cycloclavine and 4-amino Uhle's ketone

Construction of the tetracyclic core of (±)-cycloclavine and 4-amino Uhle's ketone.

Enantioefficient synthesis of alpha-ergocryptine: first direct synthesis of (+)-lysergic acid

The first direct synthesis of (+)-lysergic acid (2a) suitable for scale-up has been achieved by the following reaction sequence. Bromoketones 4d or 4g were allowed to react with amine 5 followed by deprotection, and the resulting diketone 6c was transformed into the unsaturated ketone (+/-)-7 by the LiBr/Et(3)N system. Resolution afforded (+)-7, which was further transformed by Schöllkopf's method into the mixture of esters 2e and 2f. Upon hydrolysis the latter mixture afforded (+)-2a. The peptide part of alpha-ergocryptine (1) was prepared according to the Sandoz method; the stereoefficiency, however, has been significantly improved by applying a new resolution method and recycling the undesired enantiomer. Coupling the peptide part with lysergic acid afforded 1. Having synthetic (+)-7 in hand, we can claim the total synthesis of all the alkaloids which were prepared earlier from (+)-7 that had been obtained through degradation of natural lysergic acid.