Enantioselective Allenoate-Claisen Rearrangement Using Chiral Phosphate Catalysts.

Herein we report the first highly enantioselective allenoate-Claisen rearrangement using doubly axially chiral phosphate sodium salts as catalysts. This synthetic method provides access to ß-amino acid derivatives with vicinal stereocenters in up to 95% ee. We also investigated the mechanism of enantioinduction by transition state (TS) computations with DFT as well as statistical modeling of the relationship between selectivity and the molecular features of both the catalyst and substrate. The mutual interactions of charge-separated regions in both the zwitterionic intermediate generated by reaction of an amine to the allenoate and the Na+-salt of the chiral phosphate leads to an orientation of the TS in the catalytic pocket that maximizes favorable noncovalent interactions. Crucial arene-arene interactions at the periphery of the catalyst lead to a differentiation of the TS diastereomers. These interactions were interrogated using DFT calculations and validated through statistical modeling of parameters describing noncovalent interactions.

Preparation of Solid Polyfunctional Alkynylzinc Pivalates with Enhanced Air and Moisture Stability for Organic Synthesis.

We report the preparation of solid and air-stable polyfunctionalized alkynylzinc pivalates from the corresponding alkynes using TMPZnOPiv (TMP=2,2,6,6-tetramethylpiperidyl) as base. These organozinc pivalates are obtained as powders under mild conditions in excellent yields and can be manipulated in air for several hours without significant decomposition. These zinc reagents show an excellent reactivity in various carbon-carbon bond- forming reactions and 1,3-dipolar cycloadditions. An alkynylzinc pivalate has been used to prepare a carboxyamidotriazole with potential antineoplastic activity in eight steps and 38 % overall yield.

Preparation and Application of Solid, Salt-Stabilized Zinc Amide Enolates with Enhanced Air and Moisture Stability.

The treatment of various N-morpholino amides with TMPZnCl⋅LiCl (TMP=2,2,6,6-tetramethylpiperidyl) and Mg(OPiv)2 in THF at 25 °C provides solid zinc enolates with enhanced air and moisture stability (t1/2 in air: 1-3 h) after solvent evaporation. These enolates undergo Pd- and Cu-catalyzed cross-couplings with (hetero)aryl bromides as well as allylic and benzylic halides. The arylated N-morpholino amides were converted into various ketones by LaCl3 ⋅2 LiCl mediated acylation with Grignard reagents. The new, solid enolates were used to prepare a potent anti-breast-cancer drug candidate in six steps and 23 % overall yield.

Synthesis of Complex Druglike Molecules by the Use of Highly Functionalized Bench-Stable Organozinc Reagents.

The reactivity of a representative set of 17 organozinc pivalates with 18 polyfunctional druglike electrophiles (informers) in Negishi cross-coupling reactions was evaluated by high-throughput experimentation protocols. The high-fidelity scaleup of successful reactions in parallel enabled the isolation of sufficient material for biological testing, thus demonstrating the high value of these new solid zinc reagents in a drug-discovery setting and potentially for many other applications in chemistry. Principal component analysis (PCA) clearly defined the independent roles of the zincates and the informers toward druggable-space coverage.

Regioselective Transition-Metal-Free Allyl-Allyl Cross-Couplings.

Readily prepared allylic zinc halides undergo SN 2-type substitutions with allylic bromides in a 1:1 mixture of THF and DMPU providing 1,5-dienes regioselectively. The allylic zinc species reacts at the most branched end (γ-position) of the allylic system furnishing exclusively γ,α'-allyl-allyl cross-coupling products. Remarkably, the double bond stereochemistry of the allylic halide is maintained during the cross-coupling process. Also several functional groups (ester, nitrile) are tolerated. This cross-coupling of allylic zinc reagents can be extended to propargylic and benzylic halides. DFT calculations show the importance of lithium chloride in this substitution.

Preparation of Solid, Substituted Allylic Zinc Reagents and Their Reactions with Electrophiles.

The treatment of various allylic chlorides or bromides with zinc dust in the presence of lithium chloride and magnesium pivalate (Mg(OCOtBu)2) in THF affords allylic zinc reagents which, after evaporation of the solvent, produce solid zinc reagents that display excellent thermal stability. These allylic reagents undergo Pd-catalyzed cross-coupling reactions with PEPPSI-IPent, as well as highly regioselective and diastereoselective additions to aryl ketones and aldehydes. Acylation with various acid chlorides regioselectively produces the corresponding homoallylic ketones, with the new C-C bond always being formed on the most hindered carbon of the allylic system.

Preparation of tertiary amines by the reaction of iminium ions derived from unsymmetrical aminals with zinc and magnesium organometallics.

We report a convenient one-pot preparation of polyfunctional tertiary amines, including various biorelevant phenethylamines or ephedrine derivatives, via the reaction of new functionalized iminium ions with a variety of zinc and magnesium organometallic reagents. These iminium ions were generated from unsymmetrical aminals, obtained by the in situ addition of various amides to Tietze's iminium salt [Me2NCH2(+)CF3COO(-)]. A functionalized aniline, prepared by this method, was converted to a quinolidine via an intramolecular Heck reaction.

Air-stable solid aryl and heteroaryl organozinc pivalates: syntheses and applications in organic synthesis.

A wide range of air-stable, solid, polyfunctional aryl and heteroarylzinc pivalates were efficiently prepared by either magnesium insertion or Hal/Mg exchange followed by transmetalation with Zn(OPiv)2 (OPiv = pivalate). By reducing the amount of LiCl the air stability could be significantly enhanced compared with previously prepared reagents. An alternative route is directed magnesiation using TMPMgCl⋅LiCl (TMP = 2,2,6,6-tetramethylpiperidyl) followed by transmetalation with Zn(OPiv)2 or, for very sensitive substrates, direct zincation by using TMPZnOPiv. These zinc reagents not only show excellent stability towards air, but they also undergo a broad range of C-C bond-formation reactions, such as allylation and carbocupration reactions, as well as addition to aldehydes and 1,4-addition reactions. Acylation reactions can be performed by using an excess of TMSCl to overcome side reactions of the omnipresent pivalate anion.