Zinc-mediated α-regioselective Barbier-type cinnamylation reactions of aldehydes, ketones and esters

Enantioselective Desymmetrization Triggered by Iminium-Enamine Activation: Access to Complex Cyclohepta[b]indoles

The expeditious construction of complex molecules having multiple stereocentres is highly desirable in organic chemistry. In the present communication, we report the development of an organocatalytic asymmetric desymmetrization of prochiral enal-tethered cyclohexadienones via the C3-selective Friedel-Crafts alkylation of indoles triggered by LUMO-lowering iminium activation/HOMO-raising enamine activation. The reaction provides access to bicyclic enones, which further undergo acid-mediated intramolecular annulation from C2-position to afford highly strained cyclohepta[b]indoles with five contiguous stereocentres and three new C-C bonds in excellent enantioselectivity and diastereoselectivity.

MnO2 as a terminal oxidant in Wacker oxidation of homoallyl alcohols and terminal olefins

Efficient and mild reaction conditions for Wacker-type oxidation of terminal olefins of less explored homoallyl alcohols to β-hydroxy-methyl ketones have been developed by using a Pd(ii) catalyst and MnO2 as a co-oxidant. The method involves mild reaction conditions and shows good functional group compatibility along with high regio- and chemoselectivity. While our earlier system of PdCl2/CrO3/HCl produced α,β-unsaturated ketones from homoallyl alcohols, the present method provided orthogonally the β-hydroxy-methyl ketones. No overoxidation or elimination of benzylic and/or β-hydroxy groups was observed. The method could be extended to the oxidation of simple terminal olefins as well, to methyl ketones, displaying its versatility. An application to the regioselective synthesis of gingerol is demonstrated.

One-Pot Synthesis of Aza-Morita-Baylis-Hillman Adducts via Zinc-Mediated Allylation of 4-Bromocrotonates and Imines

An approach to the synthetically valuable β-substituted aza-Morita-Baylis-Hillman (MBH) type products, through zinc-mediated allylation of imines with 4-bromocrotonates followed by subsequent isomerization of the double bond, was developed. Complementary to classical MBH reaction, this reaction utilizes easily available 4-bromocrotonates as the reaction partners, providing a rapid alternative to the traditional use of α,β-unsaturated carbonyl compounds. The short reaction time, one-pot operation, broad substrate scopes, gram-scale synthesis, and synthetic application exemplified the utility and practicability of this method.

Cobalt-catalyzed asymmetric reactions of heterobicyclic alkenes with in situ generated organozinc halides

A general strategy for the asymmetric allylation and benzylation of heterobicyclic alkenes is described by employing in situ generated organozinc halides.