Highly Efficient and Practical Pyrrolidine-Camphor-Derived Organocatalysts for the Direct α-Amination of Aldehydes

Recent Advances in Asymmetric Synthesis of Pyrrolidine-Based Organocatalysts and Their Application: A 15-Year Update

In 1971, chemists from Hoffmann-La Roche and Schering AG independently discovered a new asymmetric intramolecular aldol reaction catalyzed by the natural amino acid proline, a transformation now known as the Hajos-Parrish-Eder-Sauer-Wiechert reaction. These remarkable results remained forgotten until List and Barbas reported in 2000 that L-proline was also able to catalyze intermolecular aldol reactions with non-negligible enantioselectivities. In the same year, MacMillan reported on asymmetric Diels-Alder cycloadditions which were efficiently catalyzed by imidazolidinones deriving from natural amino acids. These two seminal reports marked the birth of modern asymmetric organocatalysis. A further important breakthrough in this field happened in 2005, when Jørgensen and Hayashi independently proposed the use of diarylprolinol silyl ethers for the asymmetric functionalization of aldehydes. During the last 20 years, asymmetric organocatalysis has emerged as a very powerful tool for the facile construction of complex molecular architectures. Along the way, a deeper knowledge of organocatalytic reaction mechanisms has been acquired, allowing for the fine-tuning of the structures of privileged catalysts or proposing completely new molecular entities that are able to efficiently catalyze these transformations. This review highlights the most recent advances in the asymmetric synthesis of organocatalysts deriving from or related to proline, starting from 2008.

Enantioselective Synthesis of β-Hydrazino Alcohols Using Alcohols and N-Boc-Hydrazine as Substrates

An enantioselective approach for the synthesis of α-hydrazino aldehydes is described that utilizes alcohols and N-Boc hydrazine instead of the conventional combination of aldehydes with azodicarboxylates. This protocol is enabled by merging in situ aerobic dual oxidation with asymmetric organocatalysis. This reaction also exhibits a high tolerance for varieties of substituents on the alcohol component. This approach features excellent enantiocontrol, cheap starting materials, operational simplicity, and scalability. The corresponding chiral β-hydrazino alcohols were obtained by sequential reduction with excellent enantioselectivity (up to 98% ee).

Chemoselective room temperature E1cB N–N cleavage of oxazolidinone hydrazides from enantioselective aldehyde α-hydrazination: synthesis of (+)-1,4-dideoxyallonojirimycin

A room temperature method for hydrazide N–N cleavage is presented that completes enantioselective aldehyde α-hydrazination methodology.

Enzyme-catalyzed kinetic resolution of N-Boc-trans-3-hydroxy-4-phenylpyrrolidine

The first enzyme-catalyzed kinetic resolution of tert-butyl-3-hydroxy-4-phenylpyrrolidine-1-carboxylate is presented. Enzyme, solvent and temperature optimization resulted in a new resolution method with E = 40 enantioselectivity. The acetate derivative of the (+)-(3S,4R) enantiomer formed while the (−)-(3R,4S) isomer remained intact. Very good enantioselectivities (E > 200) were achieved in the enzyme-catalyzed alcoholysis of the racemic acetate in i-propanol and t-butanol where the (+)-(3S,4R) enantiomer was prepared in pure form (ee > 99.7%). Absolute configuration of the (−)-(3R,4S)-enantiomer was determined by single crystal X-ray diffraction method.

Synthesis and structural characterization of novel camphor-derived amines

Two novel 4-substituted camphidine derivatives 10a,b have been prepared from (+)-camphor (1) in five steps, the Beckmann rearrangement being the bottleneck of the synthesis. Isoborneol derivative 5b, formed as a side product during the hydrogenation of arylidene ketone 3b, under Beckmann rearrangement conditions yielded interesting novel rearrangement products 11 and 12. (1S)-(+)-Camphorquinone (13) was transformed into diamines 15 and 16 in two steps, the former being cyclized into an imidazoline salt 17, an N-heterocyclic carbene precursor. The structures of all novel compounds have been meticulously characterized using NMR techniques and/or single crystal X-ray analysis.

Synthesis and structural elucidation of novel camphor-derived thioureas

Nine novel (+)-camphor-derived thioureas have been prepared. 3-((Dimethylamino)methylene)camphor (2) served as the common precursor for the preparation of both, 2-thiourea 15-20 and 3-thiourea functionalized camphor derivatives 6, 7/7', respectively. Starting from 2, the latter were prepared in two or three steps whereas the former in five steps, respectively. Configuration of all novel compounds has been meticulously determined using NMR techniques and/or single crystal X-ray crystallography.