Phosphine-Catalyzed β,γ-Umpolung Domino Reaction of Allenic Esters: Facile Synthesis of Tetrahydrobenzofuranones Bearing a Chiral Tetrasubstituted Stereogenic Carbon Center

Absolute Configuration Determination from Low ee Compounds by the Crystalline Sponge Method. Unusual Conglomerate Formation in a Pre-Determined Crystalline Lattice

When chiral compounds with low enantiomeric excess (ee, R:S=m:n) were absorbed into the void of the crystalline sponge (CS), enantiomerically pure [(R)_m (S)_n ] chiral composites were formed, changing the centrosymmetric space group into non-centrosymmetric one. The absolute configuration of the analyte compounds was elucidated with a reasonable Flack (Parsons) parameter value. This phenomenon is characteristic to the "post-crystallization" in the pre-determined CS crystalline lattice, seldom found in common crystallization where the crystalline lattice is defined by an analyte itself. The results highlight the potential of the CS method for absolute configuration determination of low ee samples, an often encountered situation in asymmetric synthesis studies.

Phosphine-Catalyzed (4+1) Annulation: Rearrangement of Allenylic Carbamates to 3-Pyrrolines through Phosphonium Diene Intermediates

We have developed a phosphine-catalyzed (4+1) annulative rearrangement for the preparation of 3-pyrrolines from allenylic carbamates via phosphonium diene intermediates. We employed this methodology to synthesize an array of 1,3-disubstituted- and 1,2,3-trisubstituted-3-pyrrolines, including the often difficult to prepare 2-alkyl variants. A mechanistic investigation employing allenylic acetates and mononucleophiles unexpectedly unveiled that a phosphine-catalyzed (4+1) reaction for the construction of cyclopentene products, previously reported by Tong, might not occur through a phosphonium diene, as had been proposed, but rather through multiple mechanisms working in concert. Consequently, our phosphine-catalyzed rearrangement is most likely the first transformation to involve the unequivocal formation of a phosphonium diene intermediate along the reaction pathway. To demonstrate the synthetic utility of this newly developed reaction, we have completed concise formal syntheses of the pyrrolizidine alkaloids (±)-trachelanthamidine and (±)-supinidine.

In Situ Observation of Thiol Michael Addition to a Reversible Covalent Drug in a Crystalline Sponge

A reversible Michael addition reaction between thiol nucleophiles and cyanoenones has been previously postulated to be the mechanism-of-action of a new family of reversible covalent drugs. However, the hypothetical Michael adducts in this mechanism have only been detected by spectroscopic methods in solution. Herein, the crystallographic observation of reversible Michael addition with a potent cyanoenone drug candidate by means of the crystalline-sponge method is reported. After inclusion of the cyanoenone substrate, the sponge crystal was treated with a thiol solution. Subsequent crystallographic analysis confirmed the single-crystal-to-single-crystal transformation of the substrate into the impermanent Michael adduct.