Stereospecific Ring Contraction of Bromocycloheptenes through Dyotropic Rearrangements via Nonclassical Carbocation-Anion Pairs.

Experimental and theoretical evidence is reported for a rare type I dyotropic rearrangement involving a [1,2]-alkene shift, leading to the regio- and stereospecific ring contraction of bromocycloheptenes. This reaction occurs under mild conditions, with or without a Lewis acid catalyst. DFT calculations show that the reaction proceeds through a nonclassical carbocation-anion pair, which is crucial for the low activation barrier and enantiospecificity. The chiral cyclopropylcarbinyl cation may be a transition state or an intermediate, depending on the reaction conditions.

Desymmetrization of meso-Dibromocycloalkenes through Copper(I)-Catalyzed Asymmetric Allylic Substitution with Organolithium Reagents.

The highly regio- and enantioselective (up to >99:1 dr, up to 99:1 er) desymmetrization of meso-1,4-dibromocycloalk-2-enes using asymmetric allylic substitution with organolithium reagents to afford enantioenriched bromocycloalkenes (ring size of 5 to 7) has been achieved. The cycloheptene products undergo an unusual ring contraction. The synthetic versatility of this Cu(I)-catalyzed reaction is demonstrated by the concise stereocontrolled preparation of cyclic amino alcohols, which are privileged chiral structures in natural products and pharmaceuticals and widely used in synthesis and catalysis.

Synthesis of optically active ß- or γ-alkyl-substituted alcohols through copper-catalyzed asymmetric allylic alkylation with organolithium reagents.

An efficient one-pot synthesis of optically active ß-alkyl-substituted alcohols through a tandem copper-catalyzed asymmetric allylic alkylation (AAA) with organolithium reagents and reductive ozonolysis is presented. Furthermore, hydroboration-oxidation following the Cu-catalyzed AAA leads to the corresponding homochiral γ-alkyl-substituted alcohols.

Cu-catalyzed enantioselective allylic alkylation with organolithium reagents.

This protocol describes a method for the catalytic enantioselective synthesis of tertiary and quaternary carbon stereogenic centers, which are widely present in pharmaceutical and natural products. The method is based on the direct reaction between organolithium compounds, which are cheap, readily available and broadly used in chemical synthesis, and allylic electrophiles, using chiral copper catalysts. The methodology involves the asymmetric allylic alkylation (AAA) of allyl bromides, chlorides and ethers with organolithium compounds using catalyst systems based on Cu-Taniaphos and Cu-phosphoramidites. The protocol contains a complete description of the reaction setup, a method based on 1H-NMR, gas chromatography-mass spectrometry (GC-MS) and chiral HPLC for assaying the regioselectivity and enantioselectivity of the product, and isolation, purification and characterization procedures. Six Cu-catalyzed AAA reactions between different organolithium reagents and allylic systems are detailed in the text as representative examples of these procedures. These reactions proceed within 1-10 h, depending on the nature of the allylic substrate (bromide, chloride, or ether and disubstituted or trisubstituted) or the chiral ligand used (Taniaphos or phosphoramidite). However, the entire protocol, including workup and purification, generally requires an additional 4-7 h to complete.

Chiral Diarylmethanes via Copper-Catalyzed Asymmetric Allylic Arylation with Organolithium Compounds.

A highly enantioselective copper/N-heterocyclic carbene catalyzed allylic arylation with organolithium compounds is presented. The use of commercial or readily prepared aryllithium reagents in the reaction with allyl bromides affords a variety of chiral diarylvinylmethanes, comprising a privileged structural motif in pharmaceuticals, in high yields with good to excellent regio- and enantioselectivities. The versatility of this new transformation is illustrated in the formal synthesis of the marketed drug tolterodine (Detrol).