Alexey B. Zaitsev

Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Amino Acid Derived Rhodium Complexes: On the Origin of Enantioselectivity and Enantioswitchability

Amino acid based thioamides, hydroxamic acids, and hydrazides have been evaluated as ligands in the rhodium-catalyzed asymmetric transfer hydrogenation of ketones in 2-propanol. Catalysts containing thioamide ligands derived from L-valine were found to selectively generate the product with an R configuration (95 % ee), whereas the corresponding L-valine-based hydroxamic acids or hydrazides facilitated the formation of the (S)-alcohols (97 and 91 % ee, respectively). The catalytic reduction was examined by performing a structure-activity correlation investigation with differently functionalized or substituted ligands and the results obtained indicate that the major difference between the thioamide and hydroxamic acid based catalysts is the coordination mode of the ligands. Kinetic experiments were performed and the rate constants for the reduction reactions were determined by using rhodium-arene catalysts derived from amino acid thioamide and hydroxamic acid ligands. The data obtained show that the thioamide-based catalyst systems demonstrate a pseudo-first-order dependence on the substrate, whereas pseudo-zero-order dependence was observed for the hydroxamic acid containing catalysts. Furthermore, the kinetic experiments revealed that the rate-limiting steps of the two catalytic systems differ. From the data obtained in the structure-activity correlation investigation and along with the kinetic investigation it was concluded that the enantioswitchable nature of the catalysts studied originates from different ligand coordination, which affects the rate-limiting step of the catalytic reduction reaction.

1,2-Dioximes in the trofimov reaction

Abstract3,3′-Dimethyl-1,1′-divinyl-2,2′-dipyrrole was obtained during the reaction of 3,4-hexanedione dioximes with acetylene under pressure in the potassium hydroxide-DMSO system. In the case of 1,2-cyclohexanedione dioxime 2,2′-dipyrrole and 2-pyridyl-and 2-acylpyrroles were isolated. α-Benzil and α-furil dioximes give 3,4-diphenyl-and 3,4-di(2-furyl)-1,2,5-oxadiazoles respectively in addition to their mono-and divinyl derivatives.

Dioximes of 1,3-diketones in the trofimov reaction : New 3-substituted pyrroles

Dioximes of 1,3-diketones enter into the Trofimov reaction forming pyrroles containing an acyl or an O-vinyloxime substituent in position 3 of the pyrrole. In the case of sterically hindered dioximes the main reaction products are isoxazoles.

Trofimov Reaction with Oximes Derived from Ketosteroids: Steroid-Pyrrole Structures

Steroidal ketone oximes, namely pregnenolone oxime, Δ5-cholesten-3-one oxime, and progesterone dioxime react with acetylene in superbasic systems (Trofimov reaction) to afford steroid-pyrrole assemblies. The process is accompanied by prototropic migration of double bonds in the steroid fragment and vinylation of hydroxy groups in sterols with acetylene. The O-vinyl group can readily be removed by methanolysis.

Unusually Facile Pyrrolization of 2-Acetylcoumarone Oxime with Acetylene

The oxime of 2-acetylcoumarone reacts with acetylene under pressure in the system KOH-DMSO unusually readily forming 2-(2-pyrrolyl)coumarone and the corresponding O-vinyl oxime. Under more rigid conditions 2-(1-vinyl-2-pyrrolyl)coumarone is formed. The possibility of a two-stage transformation of 2-acylcoumarones into 2-pyrrolylcoumarones has therefore been demonstrated for the first time.

Annelation of the pyrrole to the steroid skeleton using the Trofimov reaction

The product of annelation of the N-vinyl pyrrole ring with steroid skeleton of 5-cholestene was obtained in a single regioselective step by the reaction of 5-cholesten-3-one oxime with acetylene in KOH-DMSO by the Trofimov reaction.

First Example of Construction of a Pyrrole Ring Bonded to a Steroid System by the Trofimov Reaction

The oxime of Δ5-pregnen-3β-ol-20-one reacts with acetylene in the KOH-DMSO system to form a mixture of products containing 3β-vinyloxy-17β-(1-vinyl-2-pyrrolyl)-Δ5-androstene as the major product (63% yield) and also the O-vinyloxime of 3β-vinyloxy-Δ5-pregnen-20-one (10%), 3β-vinyloxy-17α- and 3β-vinyloxy-17β-Δ5-pregnen-20-one (25%, 1:4 ratio).

Trifluoroacetylation of Ketone O-Vinyloximes

Ketone oxime O-vinyl ethers having alkyl or phenyl radicals react with trifluoroacetic anhydride in ether in the presence of pyridine, yielding 43-54% of the corresponding ketone oxime O-(trans-4,4,4-trifluoro-3-oxo-1-butenyl) ethers with high stereoselectivity.

Reactivity of benzophenone O-vinyloxime

Benzophenone O-vinyloxime readily takes up bromine and hydrogen chloride to give benzophenone O-(1,2-dibromoethyl)oxime and benzophenone O-(1-chloroethyl)oxime, respectively. Its reactions with methanol, trifluoroacetic acid, and acetic acid lead to formation of the corresponding O-(1-methoxyethyl), O-(1-trifluoroacetoxyethyl), and O-(1-acetoxyethyl) derivatives. Slow heating of the title compound induces its decomposition with formation of benzophenone as the major product; fast heating leads to a complex mixture of products containing benzophenone, benzophenone imine, and acetaldehyde.