Synthesis of optically active aldol derivatives through chirality transfer type 1,2-Wittig rearrangement of α-alkoxycarboxamides

Synthesis of Unnatural α-Amino Acids from (Pyridin-2-yl) Carbamate via CIPE-Induced Carbonyl Migration

Synthetic methods of unnatural α-amino acids have always been the focus of extensive research due to their significant bioactivities. However, convenient transition-metal-free catalyzed methods are still in demand. Herein, we report a novel strategy for the construction of an unnatural α-amino acid skeleton via intramolecular rearrangement of carbamates, which are readily available from amines and their common protecting groups. This rearrangement could afford a variety of amino ester products in up to 98% yield, even in gram-scale reaction. The reaction mechanism was studied in detail through experiments and theoretical calculations. The complex-induced proximity effect (CIPE) from the 2-pyridyl group is shown to be indispensable for this transformation.

Further Studies on the [1,2]-Wittig Rearrangement of 2-(2-Benzyloxy)aryloxazolines

The behaviour of 14 ortho-functionalised 2-aryloxazolines (11 of them prepared and characterised for the first time) with butyllithium has been examined. Significant limitations to the Wittig rearrangement of such systems are revealed. In terms of asymmetric Wittig rearrangement, good diastereoselectivity is obtained with a valine-derived 4-isopropyl oxazoline, but this is compromised by racemisation upon hydrolysis. More encouraging selectivity is achieved in the Wittig rearrangement of an acyclic phenylalanine-derived ortho-benzyloxy benzamide.

Access to Diarylmethanols by Wittig Rearrangement of ortho-, meta-, and para-Benzyloxy-N-Butylbenzamides

The N-butyl amide group, CONHBu, has been found to be an effective promoter of the [1,2]-Wittig rearrangement of aryl benzyl ethers and thus allow the two-step synthesis of isomerically pure substituted diarylmethanols starting from simple hydroxybenzoic acid derivatives. The method is compatible with a wide range of functional groups including methyl, methoxy, and fluoro, although not with nitro and, unexpectedly, is applicable to meta as well as ortho and para isomeric series.

Chemoselective Palladium-Catalyzed Deprotonative Arylation/[1,2]-Wittig Rearrangement of Pyridylmethyl Ethers

Control of chemoselectivity is one of the most challenging problems facing chemists and is particularly important in the synthesis of bioactive compounds and medications. Herein, the first highly chemoselective tandem C(sp3)-H arylation/[1,2]-Wittig rearrangement of pyridylmethyl ethers is presented. The efficient and operationally simple protocols enable generation of either arylation products or tandem arylation/[1,2]-Wittig rearrangement products with remarkable selectivity and good to excellent yields (60-99%). Choice of base, solvent, and reaction temperature play a pivotal role in tuning the reactivity of intermediates and controlling the relative rates of competing processes. The novel arylation step is catalyzed by a Pd(OAc)2/NIXANTPHOS-based system via a deprotonative cross-coupling process. The method provides rapid access to skeletally diverse aryl(pyridyl)methanol core structures, which are central components of several medications.