Ingo Schiffers

An alkaloid-mediated desymmetrization of meso-anhydrides via a nucleophilic ring opening with benzyl alcohol and its application in the synthesis of highly enantiomerically enriched β-amino acids

The cinchona alkaloid-mediated opening of prochiral cyclic anhydrides in the presence of benzyl alcohol leading to optically active hemiesters is described. Structurally diverse anhydrides are converted into their corresponding benzyl monoesters with either enantiomer being obtained with up to 99% e.e. by using quinine or quinidine as the directing additive. A simple aqueous work-up protocol permits the isolation of the products in analytically pure form and the recovery of the alkaloids almost quantitatively. These hemiesters can be converted to N-protected β-amino esters by means of Curtius degradation of the corresponding acyl azides. Subsequent cleavage of both protecting groups by a single reaction step leads to the free β-amino acids in excellent yields. The efficiency of this procedure is demonstrated by the short asymmetric synthesis of the fungicide cis-pentacin delivering the amino acid with >99.7% enantiomeric excess.

Catalytic, Asymmetric Synthesis of Phosphonic γ‐(Hydroxyalkyl)butenolides with Contiguous Quaternary and Tertiary Stereogenic Centers

A procedure that enables high yielding access to phosphonic γ-(hydroxyalkyl)butenolides with excellent regio-, diastereo- and enantiocontrol is reported. The simultaneous construction of up to two adjacent quaternary stereogenic centers by a catalytic asymmetric vinylogous Mukaiyama aldol reaction unites biologically and medicinally relevant entities, namely α-hydroxy phosphonates and γ-(hydroxyalkyl)butenolides. This is achieved by utilizing a readily available chiral copper-sulfoximine catalyst showing a broad functional group tolerance for both the electrophilic and nucleophilic reactants. A discussion about potential factors affecting the observed level of enantioselectivity, which stems from the enantiopure sulfoximine ligand, is also included.

The First Homoleptic Metallabenzene Sandwich Complex

Air-stable, yellow crystals are formed by the first bis(metallabenzene) sandwich complex 1. With regard to its properties, 1 shows many similarities to classical metallocenes, the syn-ecliptical arrangement of the metal atoms in the rings pointing to an additional interaction between the ring Ru atoms.

Asymmetric Michael additions of α-nitrocyclohexanone to aryl nitroalkenes catalyzed by natural amino acid-derived bifunctional thioureas.

A series of new thiourea catalysts prepared from natural amino acids have been applied in organocatalytic asymmetric Michael additions of α-nitrocyclohexanone to nitroalkenes. The resulting addition products are formed with excellent enantioselectivities (up to an er of 98:2) in good yields (up to 90%).

Mechanochemical Strecker Reaction: Access to α-Aminonitriles and Tetrahydroisoquinolines under Ball-Milling Conditions.

A mechanochemical version of the Strecker reaction for the synthesis of α-aminonitriles was developed. The milling of aldehydes, amines, and potassium cyanide in the presence of SiO2 gave the corresponding α-aminonitriles in good to high yields. The high efficiency of the mechanochemical Strecker-type multicomponent reaction allowed the one-pot synthesis of tetrahydroisoquinolines after a subsequent internal N-alkylation reaction.

A novel asymmetric synthesis of highly enantiomerically enriched norbornane-type diamine derivatives

The simple and highly enantioselective methanolysis of norbornene dicarboxylic acid anhydride mediated by quinidine leads to the corresponding cis-monomethyl ester with 98% ee. By means of selective ester epimerization, followed by Curtius degradation of the intermediate trans-diacyl azide, two optically active norbornane-type diamines are obtained as their hydrochloric salts. Liberating the amine with an excess of triethylamine in situ and subsequent derivatization affords potential C1-symmetric ligands for asymmetric catalysis in excellent yields.

Der erste homoleptische Metallabenzol‐Sandwichkomplex

Luftstabile, gelbe Kristalle bildet der erste Bis(metallabenzol)-Sandwichkomplex 1. In seinen Eigenschaften ahnelt er den klassischen Metallocenen, wobei die syn-ekliptische Anordnung der Metallatome in den Ringen fur eine zusatzliche Ru-Ru-Wechselwirkung spricht.

Asymmetric Copper-Catalyzed Vinylogous Mukaiyama Michael Addition of Cyclic Dienol Silanes to Unsaturated α-Keto Phosphonates

A highly stereoselective vinylogous Mukaiyama Michael reaction (VMMR) leading to α-keto phosphonate-containing γ-butenolides with two stereogenic centers is described. The presented transformation is catalyzed by a combination of a commercially available C2 -symmetric bisoxazoline (BOX) ligand and a copper salt and tolerates a variety of nucleophiles and electrophiles. The stereoselectivities of the reactions are good to excellent and the products are obtained in moderate to high yields.