The catalytic aza-Michael addition is an important reaction within synthetic organic chemistry, given the significance of the biologically and synthetically interesting products, such as beta-amino acids and beta-lactams. In the last decade organocatalysis emerged as a powerful tool in asymmetric synthesis and had a large impact on the development of asymmetric and catalytic conjugate additions of nitrogen nucleophiles to Michael acceptors. In this review a first summary of the recent rapid progress of asymmetric organocatalyzed aza-Michael reactions is presented.
Benzothiazoline better than Hantzsch: A stereoselective Brønsted acid catalyzed reductive amination/aza‐Michael approach towards the important class of tetrahydroisoquinolines is presented (see scheme). A biphenyl‐substituted benzothiazoline is used as the reducing agent and leads to superior yields and enantiomeric excesses compared with the frequently used Hantzsch ester. The cyclization of the amine intermediate occurs smoothly with potassium tert‐butoxide as the base and affords the trans‐1,3‐disubstituted tetrahydroisoquinolines.
The direct pyrrolidine-catalyzed a-sulfenylation of aldehydes and ketones with the commercially available, very cheap chemical tetramethylthiuram disulfide (thiram) is described. The dithiocarbamoyl derivatives are obtained in good to excellent yields (47-98%). In the case of a-substituted aldehydes the protocol allows the generation of quaternary stereocenters. The sensibility of the a-sulfenylated carbonyl compounds for racemization has been investigated.
Combination of enantioselective allylation reactions with a tandem hydroformylation-Fischer indole synthesis sequence as a highly diversity-oriented strategy for the synthesis of tryptamines and homologues was explored. This modular approach allows the substituents at C3 of the indole core, the type of the amine moiety, and the distance of the amine moiety to the indole core in the final synthetic step to be defined. The starting materials required for the hydroformylation step were synthesized via iridium catalyzed enantioselective allylic substitution reactions in high yields and excellent enantioselectivities. The Rh catalyzed hydroformylation step in the presence of phenyl hydrazine, allows the in situ formed aldehyde to be trapped as the hydrazone. Subsequent acid catalyzed indolization furnishes the desired indole structures in moderate to good yields.
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