Enantioselective Synthesis of α-Amino-γ-sulfonyl Phosphonates with a Tetrasubstituted Chiral α-CarbonviaQuinine-Squaramide-Catalyzed Michael Addition of Nitrophosphonates to Vinyl Sulfones

Asymmetric Synthesis of Tetrasubstituted α-Aminophosphonic Acid Derivatives

Due to their structural similarity with natural α-amino acids, α-aminophosphonic acid derivatives are known biologically active molecules. In view of the relevance of tetrasubstituted carbons in nature and medicine and the strong dependence of the biological activity of chiral molecules into their absolute configuration, the synthesis of α-aminophosphonates bearing tetrasubstituted carbons in an asymmetric fashion has grown in interest in the past few decades. In the following lines, the existing literatures for the synthesis of optically active tetrasubstituted α-aminophosphonates are summarized, comprising diastereoselective and enantioselective approaches.

Recent Progress on the Synthesis of Chiral Sulfones

Chiral sulfones extensively exist in drugs, agricultural chemicals, chiral organic intermediates, and functional materials. Their importance causes the rapid development of their synthetic methods in recent years. Many transition metal complex catalysts with chiral ligands and chiral organocatalysts are adopted in synthesis of chiral sulfones. Most of the methods to construct chiral sulfones are based on the reduction of unsaturated sulfones and the introduction of sulfone groups into unsaturated hydrocarbons. This review describes all classes of asymmetric reactions for synthesis of chiral sulfones.

Deconjugated butenolide: a versatile building block for asymmetric catalysis

Deconjugated butenolides have emerged as a popular synthon for the enantioselective synthesis of γ-lactones. This review provides a comprehensive overview on the catalytic asymmetric reactions of deconjugated butenolides reported till date.

Catalytic asymmetric formal γ-allylation of deconjugated butenolides

A formal γ-allylation of deconjugated butenolides is reported based on a two-step sequence consisting of a catalytic diastereo- and enantioselective vinylogous nucleophilic addition to vinyl sulfones and Julia-Kocienski olefination. This highly modular approach delivers densely functionalized butenolides containing a quaternary stereogenic centre in excellent yield with high enantioselectivity.

Expanding the Potential of Heteroaryl Vinyl Sulfones

The easily available vinyl sulfone 3 showed great potential for new applications in several fields such as organic synthesis and bioconjugate formation. This was demonstrated by performing a systematic assessment of its reactivity in Michael, radical, and cycloaddition reactions. Heteroaryl vinyl sulfone 3 presented excellent output in terms of reactivity and selectivity, proving superior to phenyl vinyl sulfone 1 and with clear advantages over bis-sulfone 2. This behavior might be due to the conformational and orbital control exerted by the tetrazole unit according to DFT calculations. Moreover, some alternative transformations to the Julia-Kocienski olefination on the obtained products are also described.

Enantioselective Synthesis of Quaternary α-Amino Acids via l-tert-Leucine-Derived Squaramide-Catalyzed Conjugate Addition of α-Nitrocarboxylates to Enones

Enantioselective Michael addition of tertiary α-nitroesters to β-unsubstituted vinyl ketones has been carried out in the presence of an l-tert-leucine-derived squaramide as organocatalyst. The products, quaternary α-nitroesters, were formed in excellent yield and moderate to good ee's in most cases. Scale-up of the reaction and synthetic applications of the products, including transformation to representative quaternary α-amino acids, have also been demonstrated.

Quinine-derived thiourea and squaramide catalyzed conjugate addition of α-nitrophosphonates to enones: asymmetric synthesis of quaternary α-aminophosphonates

Conjugate addition of α-nitrophosphonates to enones was carried out in the presence of two sets of organocatalysts, viz. a quinine-thiourea and a quinine-squaramide. The quinine-thiourea provided the products possessing an α-quaternary chiral center in high enantioselectivities only in the case of electron rich enones. On the other hand, the quinine-squaramide was more efficient in that a wide variety of electron rich and electron poor enones underwent Michael addition of nitrophosphonates to afford the quaternary α-nitrophosphonates in excellent yields and enantioselectivities. The hydrogen bonding donor ability of the bifunctional catalyst, as shown in the proposed transition states, appears primarily responsible for the observed selectivity. However, a favorable π-stacking between the aryl groups of thiourea/squaramide and aryl vinyl ketone also appeared favorable. The reaction was amenable to scale up, and the enantioenriched quaternary α-nitrophosphonates could be easily transformed to synthetically and biologically useful quaternary α-aminophosphonates and other multifunctional molecules.

Catalytic asymmetric 1,2-addition of α-isothiocyanato phosphonates: synthesis of chiral β-hydroxy- or β-amino-substituted α-amino phosphonic acid derivatives

α-Amino phosphonic acid derivatives are considered to be the most important structural analogues of α-amino acids and have a very wide range of applications. However, approaches for the catalytic asymmetric synthesis of such useful compounds are very limited. In this work, simple, efficient, and versatile organocatalytic asymmetric 1,2-addition reactions of α-isothiocyanato phosphonate were developed. Through these processes, derivatives of β-hydroxy-α-amino phosphonic acid and α,β-diamino phosphonic acid, as well as highly functionalized phosphonate-substituted spirooxindole, can be efficiently constructed (up to 99 % yield, d.r. >20:1, and >99 % ee). This novel method provides a new route for the enantioselective functionalization of α-phosphonic acid derivatives.

Enantioselective formal α-allylation of nitroalkanes through a chiral iminophosphorane-catalyzed Michael reaction–Julia–Kocienski olefination sequence

A chiral iminophosphorane-catalyzed Michael addition–Julia–Kocienski olefination sequence for the asymmetric formal α-allylation of nitroalkanes is developed.

Hydrogen bond donor–acceptor–donor organocatalysis for conjugate addition of benzylidene barbiturates via complementary DAD–ADA hydrogen bonding

Organocatalysts featuring a hydrogen bond donor–acceptor–donor structural element catalyze conjugate addition via complementary DAD–ADA hydrogen bonding.