5 research outputs found
Kinetic Resolution of Chiral Cyclohex-2-enones by Rhodium(I)/binap-Catalyzed 1,2- and 1,4-Additions
The feasibility of kinetic resolutions of racemic monosubstituted cyclohex-2-enones by Rh/binap-catalyzed reactions was investigated. 1,2-Addition of AlMe<sub>3</sub> to the 5-substituted derivatives furnished allylic alcohols in the matched case, while the less reactive enantiomers were either left over or transformed into <i>trans</i>-3,5-disubstituted cyclohexanones in parallel or sequential 1,4-additions. Altogether, these represent regiodivergent reactions on racemic mixtures. In contrast, 1,4-addition of aryl groups led to inferior results since either catalyst or substrate control dominated
Total Synthesis of (<i>R</i>)‑Sarkomycin via Asymmetric Rhodium-Catalyzed Conjugate Addition
(<i>R</i>)-Sarkomycin was prepared using a five-step
total synthesis. Key steps in the enantioselective construction of
the targeted scaffold were a rhodium-catalyzed asymmetric conjugate
alkenyl addition with subsequent silyl trapping and a Mukaiyama aldol
reaction with aqueous formaldehyde. Protection of the hydroxy group
as a THP acetal and oxidative cleavage of the C,C-double bond provided
a stable direct precursor to the natural product. The final liberation
was carried out under slightly acidic conditions in a microwave-assisted
reaction, resulting in a high yield of the “deceptive”
sarkomycin. This represents the shortest enantioselective synthesis
of this rather unstable compound to date and the first to employ asymmetric
catalysis to introduce the stereogenic center
Enantioselective Synthesis of 3,4-Disubstituted <i>cis</i>- and <i>trans</i>-1,2,5-Thiadiazolidine-1,1-dioxides as Precursors for Chiral 1,2-Diamines
Both, <i>cis</i>- and <i>trans</i>-3,4-disubstituted thiadiazolidines <b>5</b> and <b>6</b> can enantioselectively be obtained from thiadiazoles <b>2</b> which, in turn, are efficiently prepared from the respective 1,2-diketone by an improved protocol. An asymmetric ruthenium-catalyzed transfer hydrogenation followed by a diastereoselective hydride addition furnishes exclusively the <i>cis</i>-isomers <b>5</b> which, under acidic conditions, undergo a novel isomerization into the <i>trans</i>-isomers <b>6</b>. These cyclic sulfamides can be transformed into 1,2-diamines as well as 2,3-diamino acids
Enantioselective Synthesis of 3,4-Disubstituted <i>cis</i>- and <i>trans</i>-1,2,5-Thiadiazolidine-1,1-dioxides as Precursors for Chiral 1,2-Diamines
Both, <i>cis</i>- and <i>trans</i>-3,4-disubstituted thiadiazolidines <b>5</b> and <b>6</b> can enantioselectively be obtained from thiadiazoles <b>2</b> which, in turn, are efficiently prepared from the respective 1,2-diketone by an improved protocol. An asymmetric ruthenium-catalyzed transfer hydrogenation followed by a diastereoselective hydride addition furnishes exclusively the <i>cis</i>-isomers <b>5</b> which, under acidic conditions, undergo a novel isomerization into the <i>trans</i>-isomers <b>6</b>. These cyclic sulfamides can be transformed into 1,2-diamines as well as 2,3-diamino acids
Rhodium-Catalyzed Enantioselective Addition of Organoaluminum Reagents to <i>N</i>‑Tosyl Ketimines
Rhodium(I)/Binap complexes catalyze
highly enantioselective additions
of methyl- and arylaluminum reagents to cyclic α,β-unsaturated <i>N</i>-tosyl ketimines. Depending on the solvent and substituents
at the ring, the reaction occurs either in a 1,2-manner to deliver
α-tertiary allylic amines or in a 1,4-manner to yield, after
subsequent reduction, 3-substituted cycloalkyl amines. Well known
in the case of the respective cycloalkenones, these first transformations
of the aza-analogues enable the synthesis of amine structures of pharmaceutical
and biochemical interest