Enantioselective Mukaiyama-Michael Reaction of β,γ-Unsaturated α-Keto Esters with Silyl Ketene Acetals Catalyzed by a Chiral Magnesium Phosphate

We would like to describe an efficient and highly enantioselective Mukaiyama-Michael reaction of silyl ketene acetals with β,γ-unsaturated α-keto esters catalyzed by a chiral magnesium BINOL-derived phosphate. The resulting functionalized 1,5-dicarbonyl adducts are obtained in high yields (up to 96%) and with excellent enantioselectivities (up to 98%) under mild conditions. Two plausible mechanistic pathways were proposed, including a 1,4-addition and a hetero Diels-Alder [4 + 2] cycloaddition.

Bijvoet in solution reveals unexpected stereoselectivity in a Michael addition

The absolute configuration of small crystallizable molecules can be determined with anomalous X-ray diffraction as shown by Bijvoet in 1951. For the majority of compounds that can neither be crystallized nor easily be converted into crystallizable derivatives, stereocontrolled organic synthesis is still required to establish their absolute configuration. In this contribution, a new fundamental methodology for resolving the absolute configuration will be presented that does not require crystallization. With residual dipolar coupling enhanced NMR spectroscopy, ensembles of a limited number of structures are created reflecting the correct conformations and relative configuration. Subsequently, from these ensembles, optical rotation dispersion (ORD) spectra are predicted by DFT calculations and compared to experimental results. The combination of these two steps reveals the absolute configuration of a flexible molecule in solution, which is a big challenge to chiroptical methods and DFT in the absence of NMR spectroscopy. Here the absolute stereochemistry of the product of a new Michael addition, synthesized via a niobium(V) chiral enolate, will be elucidated by using the new methodology.