Pentamethylphenyl (Ph*) ketones: Unique building blocks for organic synthesis

Borrowing hydrogen C-alkylation with secondary saturated heterocyclic alcohols

The borrowing hydrogen methodology (BH) has emerged as a powerful tool for the rapid construction of C-C bonds, offering a greener alternative to traditional multi-step syntheses. This methodology involves the activation of inactivated alcohols followed by condensation or aldolization, ultimately leading to the regeneration of the saturated product. Herein, we report the C-alkylation of a hindered ketone with challenging secondary saturated heterocyclic alcohols. Our study encompasses the optimization of reaction conditions using either an iridium or a ruthenium catalyst and exploration of substrate scope. We demonstrate the efficient synthesis of substituted pyrrolidines and piperidines directly from a triol precursor, showcasing the versatility of this methodology. Moreover, we illustrate the post-functionalization of BH products, significantly broadening their chemical utility.

Stereochemistry in All Its Shapes and Forms: The 56th Bürgenstock Conference

Acknowledging the crucial role of stereochemistry in fields as diverse as total synthesis, synthetic methodology, spectroscopy, and the study of the origin of life, the 56th SCS Conference on Stereochemistry, better known as the BÃ1/4rgenstock Conference, brought together a diverse range of chemistry expertise in Brunnen, Switzerland.

Redox Reorganization: Aluminium Promoted 1,5-Hydride Shifts Allow the Controlled Synthesis of Multisubstituted Cyclohexenes

An efficient synthesis of cyclohexenes has been achieved from easily accessible tetrahydropyrans via a tandem 1,5-hydride shift-aldol condensation. We discovered that readily available aluminium reagents, e.g. Al2 O3 or Al(Ot Bu)3 are essential for this process, promoting the 1,5-hydride shift with complete regio- and enantiospecificity (in stark contrast to results obtained under basic conditions). The mild conditions, coupled with multiple methods available to access the tetrahydropyran starting materials makes this a versatile method with exceptional functional group tolerance. A wide range of cyclohexenes (>40 examples) have been prepared, many in enantiopure form, showing our ability to selectively install a substituent at each position around the newly forged cyclohexene ring. Experimental and computational studies revealed that aluminium serves a dual role in facilitating the hydride shift, activating both the alkoxide nucleophile and the electrophilic carbonyl group.

Evolution of the Dearomative Functionalization of Activated Quinolines and Isoquinolines: Expansion of the Electrophile Scope

Herein we disclose a mild protocol for the reductive functionalisation of quinolinium and isoquinolinium salts. The reaction proceeds under transition-metal-free conditions as well as under rhodium catalysis with very low catalyst loadings (0.01 mol %) and uses inexpensive formic acid as the terminal reductant. A wide range of electrophiles, including enones, imides, unsaturated esters and sulfones, β-nitro styrenes and aldehydes are intercepted by the in situ formed enamine species forming a large variety of substituted tetrahydro(iso)quinolines. Electrophiles are incorporated at the C-3 and C-4 position for quinolines and isoquinolines respectively, providing access to substitution patterns which are not favoured in electrophilic or nucleophilic aromatic substitution. Finally, this reactivity was exploited to facilitate three types of annulation reactions, giving rise to complex polycyclic products of a formal [3+3] or [4+2] cycloaddition.

Evolution of the Dearomative Functionalization of Activated Quinolines and Isoquinolines: Expansion of the Electrophile Scope

Herein we disclose a mild protocol for the reductive functionalisation of quinolinium and isoquinolinium salts. The reaction proceeds under transition-metal-free conditions as well as under rhodium catalysis with very low catalyst loadings (0.01 mol %) and uses inexpensive formic acid as the terminal reductant. A wide range of electrophiles, including enones, imides, unsaturated esters and sulfones, β-nitro styrenes and aldehydes are intercepted by the in situ formed enamine species forming a large variety of substituted tetrahydro(iso)quinolines. Electrophiles are incorporated at the C-3 and C-4 position for quinolines and isoquinolines respectively, providing access to substitution patterns which are not favoured in electrophilic or nucleophilic aromatic substitution. Finally, this reactivity was exploited to facilitate three types of annulation reactions, giving rise to complex polycyclic products of a formal [3+3] or [4+2] cycloaddition.

Extension of hydrogen borrowing alkylation reactions for the total synthesis of (-)-γ-lycorane

The total synthesis of (-)-γ-lycorane (10 steps) and synthesis of (±)-γ-lycorane (8 steps) was completed from cyclohexenone. A new two step hydrogen borrowing alkylation of an aziridinyl alcohol, coupled with a Ph* (Me₅C₆) deprotection/cyclisation procedure was developed for de novo formation of the fused 6,5 heterocyclic ring. This work is one of the first examples of hydrogen borrowing C-C bond formation being used as a key step in a total synthesis project.

Ligand-redox assisted nickel catalysis toward stereoselective synthesis of (n+1)-membered cycloalkanes from 1,n-diols with methyl ketones

A well-defined, bench-stable nickel catalyst is presented here, that can facilitate double alkylation of a methyl ketone to realize a wide variety of cycloalkanes. The performance of the catalyst depends on the ligand redox process comprising an azo-hydrazo couple. The source of the bis electrophile in this double alkylation is a 1,n-diol, so that (n+1)-membered cycloalkanes can be furnished in a stereoselective manner. The reaction follows a cascade of dehydrogenation/hydrogenation reactions and adopts a borrowing hydrogen (BH) method. A thorough mechanistic analysis including the interception of key radical intermediates and DFT calculations supports the ligand radical-mediated dehydrogenation and hydrogenation reactions, which is quite rare in BH chemistry. In particular, this radical-promoted hydrogenation is distinctly different from conventional hydrogenations involving a metal hydride and complementary to the ubiquitous two-electron driven dehydrogenation/hydrogenation reactions.