Bjarke S. Donslund

The Diarylprolinol Silyl Ethers: Ten Years After

Asymmetric organocatalysis has experienced an incredible development since the beginning of this century. The expansion of the field has led to a large number of efficient types of catalysts. One group, the diarylprolinol silyl ethers, was introduced in 2005 and has been established as one of the most frequently used in aminocatalysis. In this Minireview, we will take a look in the rear-view mirror, ten years after the introduction of the diarylprolinol silyl ethers. We will focus on the perspectives of the different activation modes made available by this catalytic system. Starting with a short introduction to aminocatalysis, we will outline the properties that have made the diarylprolinol silyl ethers a common choice of catalyst. Furthermore, we will describe the major tendencies in the activation and reaction concepts developed with regard to reactivity patterns and combinations with other activation concepts.

Organocatalytic enamine-activation of cyclopropanes for highly stereoselective formation of cyclobutanes.

A novel organocatalytic activation mode of cyclopropanes is presented. The reaction concept is based on a design in which a reactive donor-acceptor cyclopropane intermediate is generated by in situ condensation of cyclopropylacetaldehydes with an aminocatalyst. The mechanism of this enamine-based activation of cyclopropylacetaldehydes is investigated by the application of a combined computational and experimental approach. The activation can be traced to a favorable orbital interaction between the π-orbital of the enamine and the σ*C-C orbital of the cyclopropyl ring. Furthermore, the synthetic potential of the developed system has been evaluated. By the application of a chiral secondary amine catalyst, the organocatalytically activated cyclopropanes show an unexpected and highly stereoselective formation of cyclobutanes, functionalizing at the usually inert sites of the donor-acceptor cyclopropane. By the application of 3-olefinic oxindoles and benzofuranone, biologically relevant spirocyclobutaneoxindoles and spirocyclobutanebenzofuranone can be obtained in good yields, high diastereomeric ratios, and excellent enantiomeric excesses. The mechanism of the reaction is discussed and two mechanistic proposals are presented.

Organocatalytic Asymmetric Formation of Steroids

A novel and simple one-step approach for the construction of optically active steroids in a highly stereoselective manner by using organocatalysis is presented. The reaction of (di)enals with cyclic dienophiles in the presence of a TMS-protected prolinol catalyst leads to the construction of important 14 β-steroids. This new reaction allows an easy access to optically active steroids with a variety of substituents in the A ring in high yields and up to greater than 99 % ee. The reaction has been extended to include the construction of B- and D-homosteroids as well as steroids containing heteroatoms in the B ring. The angular substituent at C13 can be varied and alkyl, ester, and sulfone functionalities are introduced with excellent stereoselectivities. Simple synthetic procedures provide access to a range of naturally occurring steroids such as estrone and related analogues.

Asymmetric synthesis of γ-nitroesters by an organocatalytic one-pot strategy.

An enantioselective synthesis of γ-nitroesters by a one-pot asymmetric Michael addition/oxidative esterification of α,β-unsaturated aldehydes is presented. The procedure is based on merging the enantioselective organocatalytic nitroalkane addition with an N-bromosuccinimide-based oxidation. The γ-nitroesters are obtained in good yields and enantioselectivities, and the method provides an attractive entry to optically active γ-aminoesters, 2-piperidones, and 2-pyrrolidones.

Benzofulvenes in Trienamine Catalysis: Stereoselective Spiroindene Synthesis

The asymmetric formation of spiroindenes containing up to four contiguous stereocenters from the reaction of benzofulvenes with 2,4-dienals through trienamine catalysis is described. The benzofulvene core was found to be an excellent starting point for the synthesis of interesting spiroindenes through a formal cycloaddition pathway. The reaction was mediated by a diphenylprolinol silyl ether catalyst, and a diverse array of spiroindenes were obtained in high yields with excellent stereoselectivity. An attractive feature of the developed system is the possibility to diversify the product scaffold significantly by further manipulation of the chiral spiroindenes. Thus, three intramolecular ring-closing reactions following the organocatalytic step resulted in highly complex polycyclic systems.

Enantioselective formation of cyclopropane spiroindenes from benzofulvenes by phase transfer catalysis

The enantio- and diastereoselective formation of indenes spirofused to highly substituted cyclopropanes is described. The application of a new cinchona alkaloid derived ammonium salt in a phase transfer catalysis setup facilitates high selectivity and excellent yields at low catalyst loadings (0.1-1.0 mol%).

Asymmetric Organocatalytic Synthesis of Complex Cyclopenta[b]quinoline Derivatives

An efficient one-pot procedure that provides a direct access to polycyclic hexahydrocyclopenta[b]quinoline derivatives having five stereogenic centers has been developed. The system displays great tolerance toward different aldehydes, anilines, and nitroalkenes. The products are obtained in high yields and excellent enantio- and diastereoselectivities.

Organocatalytic Enantioselective Higher-Order Cycloadditions of In Situ Generated Amino Isobenzofulvenes

The [8+2] cycloaddition of indene-2-carbaldehydes and nitro olefins is described to provide benzonorbornene scaffolds in a highly peri-, diastereo-, and enantioselective fashion in the presence of a C2 -symmetric aminocatalyst. This reaction, which proceeds through a transient semi-aromatic amino isobenzofulvene, represents the first example of catalytic formation and transformation of these species. Quantum chemical calculations suggest a kinetically controlled stepwise mechanism where the stereochemistry is determined in the first bond-forming event. Beyond the useful [8+2] cycloadducts, [10+4] cycloadducts have been identified in silico as potential off-pathway intermediates.

Catalytic Enantioselective [10+4] Cycloadditions

The first peri- and stereoselective [10+4] cycloaddition between catalytically generated amino isobenzofulvenes and electron-deficient dienes is described. The highly stereoselective catalytic [10+4] cycloaddition exhibits a broad scope with high yields, reflecting a robust higher-order cycloaddition. Experimental and computational investigations support a kinetic distribution of intermediate rotamers dictating the enantioselectivity, which relies heavily on additive effects.