Triflic Acid Promoted Decarboxylation of Adamantane-oxazolidine-2-one: Access to Chiral Amines and Heterocycles

Synthesis of 1,2-Disubstituted Adamantane Derivatives by Construction of the Adamantane Framework

This review summarizes achievements in the synthesis of 1,2-disubstituted adamantane derivatives by the construction of the tricyclic framework either by total synthesis or by ring expansion/contraction reactions of corresponding adamantane homologues. It is intended to complement reviews focusing on the preparation of 1,2-disubstituted derivatives by C-H functionalization methods.

Towards a qualitative understanding of the carbonyl reactivity of α-substituted ethyl 5-oxohomoadamantyl-4-carboxylates

Substituted ethyl 5-oxohomoadamantane-4-carboxylates were subjected to reactions with several nucleophiles to establish some aspects of the carbonyl reactivity. However, only one example of the desired Claisen retro-reaction was observed as 3,7-disubstituted bicyclo[3.3.1]nonane formation. Most of the reactions yielded α-substituted homoadamantan-5-ones or some products of their further transformations. Reductive amination of substituted homoadamantane-5-ones gave several homoadamantane-fused nitrogen heterocycles, which could be considered as GABA- and/or ∂-aminovaleric acid analogues. Reduction products of several substituted ketones were observed as single products when treated with organomagnesium reagents. These deviations from the general chemical reactivity patterns are associated with steric factors and the geometry of the cage unit and could be a particular case, which is reflecting the overall specificity of the chemistry of cage carbonyl compounds.

Annulated carbamates are precursors for the ring contraction of the adamantane framework

We report a protocol for the one-pot two-step synthesis of noradamantane methylene amines. The first step is the triflic acid-promoted decarboxylation of adamantane carbamates, which causes rearrangement of the adamantane framework to form noradamantane iminium salts, which are reduced to amines in the second separate step.

Synthesis of noradamantane derivatives by ring-contraction of the adamantane framework

We describe a triflic acid promoted cascade reaction of adamantane derivatives consisting of a decarboxylation of N-methyl protected cyclic carbamates and a subsequent intramolecular nucleophilic 1,2-alkyl shift to generate ring contracted iminium triflates. This reaction expands the family of similar transformations, such as Wagner-Meerwein-, Demjanov-Tiffeneau-, Meinwald- or (semi-)pinacol-rearrangement. It allows the preparation of noradamantane derivatives in a few steps, starting from simple hydroxy-substituted adamantane precursors.

[1,2]-Rearrangement of iminium salts provides access to heterocycles with adamantane scaffold

We describe a Brønsted acid-catalysed cascade reaction consisting of a Wagner-Meerwein rearrangement and a subsequent intra- or intermolecular Friedel-Crafts reaction leading to adamantane-based heterocycles. In contrast to the reported W.-M. rearrangements, in this case an iminium moiety serves as the acceptor of a migrating nucleophilic alkyl group in a [1,2]-alkyl shift.

Synthesis of Sterically Congested 2,2'-Bi(Adamantyl)-Based Alcohol and Amines

Sterically congested chiral alcohol and amines have gained tremendous attention in the design of asymmetric catalysts. Herein, the synthesis of a sterically congested bis-adamantane framework-based chiral alcohol, (1R,2S,3S,4R)-4-(2-adamantyl)adamantan-2-ol, and amine, (1R,2S,3S,4R)-4-(2-adamantyl)adamantan-2-amine, is described. Access to these sterically encumbered compounds is found via the preparation of an enantioenriched 4-adamantyladamantan-2-one intermediate, which was synthesized in 6 steps from adamantan-2-one. The key step involved enzyme-catalyzed ester hydrolysis in giving unsaturated alcohol with an enantiomeric excess of >95%. This adamantylidene adamantanol was subjected to an acid-catalyzed intramolecular [1,4] shift to give the key chiral intermediate without racemization. This ketone intermediate was transformed into the target compounds via reduction and reductive amination protocols.