Research Article

Asymmetric phosphoric acid–catalyzed four-component Ugi reaction

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Science  14 Sep 2018:
Vol. 361, Issue 6407, eaas8707
DOI: 10.1126/science.aas8707

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Steering together all four Ugi pieces

The nearly 60-year-old Ugi reaction is a remarkably efficient means of linking together four molecular building blocks: an aldehyde, an amine, a carboxylic acid, and an isocyanide. Because each component is independently tunable, the reaction is especially well suited to the assembly of diverse compound libraries. However, stereoselectivity has been a challenge. Zhang et al. now show that chiral phosphoric acids can catalyze the four-component coupling with high enantioselectivity (see the Perspective by Riva). Theory suggests that a hydrogen-bonded complex involving the phosphoric acid and carboxylic acid sets the stereochemistry for isocyanide attack on an imine intermediate.

Science, this issue p. eaas8707; see also p. 1072

Structured Abstract

INTRODUCTION

The four-component Ugi reaction (Ugi-4CR) assembles peptide-like α-acylaminoamides through one-pot reaction of a carbonyl compound, an amine, an acid, and an isocyanide. Ugi-4CR is well suited for diversity-oriented synthesis applicable in drug discovery, as it facilitates rapid access to diverse libraries of biologically important molecules. The high step economy and atom efficiency of the reaction, as well as its convergent nature, foster its wide use in the synthesis of heterocyclic scaffolds, natural products, macrocycles, polymers, and other target molecules. Despite these practical advantages, the long-standing stereochemical challenges of the Ugi reaction have yet to be fully addressed. Consequently, access to chiral Ugi products for drug candidate exploration is hindered.

RATIONALE

The chiral phosphoric acid (CPA) framework was targeted as a catalyst for asymmetric Ugi-4CR. The heightened acidity of CPAs over carboxylic acids is perceived to accelerate the kinetics of the enantioselective Ugi reaction so as to outcompete the background reaction. Also, self-assembled heterodimerization between the CPA and carboxylic acid brings about a dual effect: enhanced acidity of the catalyst and nucleophilicity of the carboxylic acid. Both of these favor the catalytic enantioselective Ugi-4CR. A myriad of well-established or custom CPAs with well-defined chiral pockets could be readily applied, potentially leading to complete stereocontrol. A CPA that could suppress the Passerini and other side reactions would enable rapid imine formation and its preferential activation over the carbonyl group.

RESULTS

A catalytic asymmetric Ugi-4CR was accomplished with 1,1′-spirobiindane-7,7′-diol (SPINOL)–derived CPA4 and CPA6 as organocatalysts. The reaction exhibited broad substrate compatibility and good to excellent enantioselectivity [up to 99% enantiomeric excess (ee)]. Activation of the imine might be accomplished by CPA–carboxylic acid heterodimer catalysis via a bifunctional activation mode, which was supported by experiments (carboxylic acids with varying pKa values and steric properties yielded products with a range of ee values) and density functional theory (DFT) calculations (lowest energy among all the considered activation modes). The calculated free energy profile for the catalytic Ugi reaction gave three CPA-combined key transition states, which highlighted the bifunctional property of the CPA. In the favored enantio-determining transition states, the aryl groups fit into the pocket formed by the two substituents (cyclohexyl rings) of the catalyst, revealing the importance of noncovalent interactions in controlling the stereochemical outcome of this reaction.

CONCLUSION

This operationally simple one-pot enantioselective Ugi-4CR harnesses inherent benefits of multicomponent reaction and organocatalysis to access up to 86 enantioenriched α-acylaminoamides, which are otherwise challenging to obtain via conventional methods, from four achiral building blocks in excellent yields and enantioselectivities. DFT calculations gave a detailed catalytic mechanism, especially with respect to activation modes and enantio-determining transition states. Because amide functionality constitutes the defining primary linkage in proteins, we foresee multiple uses of this asymmetric four-component Ugi protocol for the synthesis of chiral peptides and components of natural products. We also anticipate that this work will initiate the further development of asymmetric multicomponent chemistry.

Design and exploration of catalytic asymmetric Ugi-4CR.

Abstract

The Ugi reaction constructs α-acylaminoamide compounds by combining an aldehyde or ketone, an amine, a carboxylic acid, and an isocyanide in a single flask. Its appealing features include inherent atom and step economy together with the potential to generate products of broad structural diversity. However, control of the stereochemistry in this reaction has proven to be a formidable challenge. We describe an efficient enantioselective four-component Ugi reaction catalyzed by a chiral phosphoric acid derivative that delivers more than 80 α-acylaminoamides in good to excellent enantiomeric excess. Experimental and computational studies establish the reaction mechanism and origins of stereoselectivity.

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