Yasutomo Yamamoto

Chiral N-Heterocyclic Carbene―Copper(I)-Catalyzed Asymmetric Allylic Arylation of Aliphatic Allylic Bromides: Steric and Electronic Effects on γ-Selectivity

Chiral N-heterocyclic carbene ligands were electronically and sterically tuned to improve γ-selectivity in copper(I)-catalyzed asymmetric allylic arylation of aliphatic allylic bromides with several aryl Grignard reagents. High γ-selectivity was realized when either the aryl group of the Grignard reagent or the aryl group on the N-substituent of the carbene ligand was electron-deficient or when either the carbene ligand or allylic bromide was bulky. The results indicated that electron deficiency and steric hindrance of the initially formed σ-allyl copper intermediate enhance the rate of the reductive elimination to give γ-products as major isomers.

Enantioselective conjugate addition of a lithium ester enolate catalyzed by chiral lithium amides: a possible intermediate characterized.

Two 1:1 noncovalent mixed aggregates between a lithium enolate and two diastereomeric lithium amides have been identified spectroscopically in THF. The NMR data, as well as DFT theoretical calculations, shine some light on a puzzling reversal of induction, observed when switching from one diastereomer of the amide to the other in the enantioselective Michael addition of the lithium enolate to an unsaturated ester.

Steric Tuning of the Amidomonophosphane-Rhodium(I) Catalyst in Asymmetric Addition of Arylboroxines to N-Phosphinoyl Aldimines

Highly enantioselective rhodium-catalyzed addition of arylboroxines to N-phosphinoylaldimines was realized by the steric tuning of a diphenylphosphorus moiety to a di(o-tolyl)phosphorus moiety of a chiral amidomonophosphane. The presence of MS 4 A in a 5:1 solvent mixture of dioxane-propanol was essential to afford the corresponding diarylmethylamines in high yield.

Chemoselective conjugate addition of dimethylzinc-mediated ether and acetal radicals to alkylidenemalonates and asymmetric reactions.

Cyclic and acyclic ether or acetal radicals were generated directly from ethers or acetals by the action of dimethylzinc-air, and their subsequent conjugate addition reaction with alkylidenemalonates afforded the corresponding conjugate adducts in reasonably high yields. The reaction with benzylidenemalonates bearing formyl and imino groups gave chemoselectively the conjugate addition products. The asymmetric reaction of bis(8-phenylmenthyl) benzylidenemalonate proceeded diastereoselectively to provide the adduct with 93:7 dr.

Consecutive cyclization of allylaminoalkene by intramolecular aminolithiation-carbolithiation.

Consecutive cyclization of allylaminoalkenes by tandem aminolithiation-carbolithiation proceeded smoothly by using a lithium amide as a lithiating agent as well as protonating agent to give bicyclic amines, octahydroindolizine and hexahydro-1 H-pyrrolizine, in reasonably high yield and diastereoselectivity.

High performance of N-alkoxycarbonyl-imines in triethylborane-mediated tin-free radical addition.

Triethylborane-mediated tin-free radical alkylation of N-alkoxycarbonyl-imines, such as N-Boc-, N-Cbz-, and N-Teoc-imines, proceeded smoothly at a low temperature (-78 to -20 °C) to give the corresponding adducts in high yield. Although the formation of isocyanate was the major unfavorable reaction at room temperature, a one-pot conversion of N-Boc-imine to N-ethoxycarbonyl-adduct was possible through the corresponding isocyanate generated in situ. The higher performance of N-alkoxycarbonyl-imine than those of N-Ts- and N-PMP-imines is rationalized by a moderate electron-withdrawing character of an alkoxycarbonyl group that makes both addition of alkyl radical and trapping of the resulting aminyl radical by triethylborane efficiently fast.

General entry to asymmetric one-pot [N + 2 + n] cyclization for the synthesis of three- to seven-membered azacycloalkanes.

Enantio- and diastereoselective one-pot synthesis of three- to seven-membered cis-azaheterocycles was achieved using a triggered asymmetric conjugate addition reaction of lithium amide with an enoate, followed by alkylation of the resulting lithium enolate with α,ω-dihaloalkane and N-alkylation. Isomerization of cis-azaheterocycles with a base yielded the trans-product, constituting a one-pot synthesis of cis-azacycles and a two-step synthesis of trans-azacycles. The four-step asymmetric synthesis of nemonapride highlights the general utility of the method.

Radical one-pot α,β-dual and β-mono-oxymethylation of alkylidenemalonate.

Dimethylzinc-mediated radical conjugate addition reaction of dimethyl alkylidenemalonates with iodomethyl pivalate gave a high yield of the α,β-dual oxymethylation product in one pot under air and the β-pivaloyloxymethylation product under argon.

Aminolithiation of carbon-carbon double bonds as a powerful tool in organic synthesis*

A conjugate amination of α,β-unsaturated carbonyl compounds with lithium amides has become a powerful method of N-C bond-forming reactions. Chiral ligand-controlled asymmetric version of the conjugate amination of enoates was developed for practical bench chemistry, giving the enantioenriched amination product with over 99 % ee. In situ diastereoselective alkylation of resulting lithium enolates allowed us to form vicinal N-C and C-C bonds in a one-pot operation. This protocol enabled us to realize a short-step asymmetric synthesis of otamixaban key intermediate. Treatment of product 3-benzylamino- and 3-allylaminoesters with tert-butyllithium gave five- or seven-membered lactams through [1,2]- or [2,3]-rearrangement of intermediate β-lactams. Isolated C-C double bonds were also found to accept intramolecular aminolithiation affording the corresponding hydroamination products. Chiral lithiophilic ligand-catalyzed reaction gave enantioenriched hydroamination products with high ee. Stereoselective intramolecular aminolithiation of allylaminoalkenes was coupled with subsequent carbolithiation to give doubly cyclized product amines.

4.7 C–C Bond Formation (Transition Metal-Catalyzed Michael)

Transition metal-catalyzed asymmetric Michael reactions were described. The reactions were classified into those of prochiral donors and prochiral acceptors. A chiral coordinating reagent creates asymmetric environment around a transition metal, and this chiral Lewis acid could activate a Michael acceptor by coordination, and/or act as a counter ion of a donor enolate. The combination of a chiral ligand and transition metal is highly useful in an asymmetric Michael reaction.