Selectivity guidelines and a reductive elimination-based model for predicting the stereochemical course of conjugate addition reactions of organocuprates to gamma-alkoxy-alpha,beta-enoates

HBTU mediated 1-hydroxybenzotriazole (HOBt) conjugate addition: synthesis and stereochemical analysis of β-benzotriazole N-oxide substituted γ-amino acids and hybrid peptides

Synthesis and conformational analysis of β-benzotriazole N-oxide (β-BtO) substituted γ-amino acids by direct conjugate addition of HOBt to E-vinylogous γ-amino acids are reported.

Diastereoselective synthesis of nitroso acetals from (S,E)-γ-aminated nitroalkenes via multicomponent [4 + 2]/[3 + 2] cycloadditions promoted by LiCl or LiClO4

Chiral nonracemic aminated nitroso acetals were synthesized via diastereoselective multicomponent [4 + 2]/[3 + 2] cycloadditions employing new (S,E)-γ-nitrogenated nitroalkenes 5a–c as heterodienes, ethyl vinyl ether (EVE) as a dienophile, and selected electron-deficient alkenes as 1,3-dipolarophiles. The employment of different organic solutions of LiClO₄ or LiCl as promoter systems provided the respective nitroso acetals with yields from 34–72% and good levels of diastereoselectivity. In addition, the nitroso acetal 9c was transformed to the pyrrolizidin-3-one derivative 14c, proving the usefulness of the route in the synthesis of an interesting chiral compound. The elucidation of the stereostructures was based on 2D COSY, NOESY and HSQC NMR experiments as well as an X-ray diffraction experiment.

Parallel kinetic resolution of acyclic γ-amino-α,β-unsaturated esters: application to the asymmetric synthesis of 4-aminopyrrolidin-2-ones

Conjugate addition of a 50:50 pseudoenantiomeric mixture of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide and lithium (S)-N-3,4-dimethoxybenzyl-N-(α-methylbenzyl)amide to a range of racemic acyclic γ-amino-α,β-unsaturated esters (derived from the corresponding α-amino acids) effects their efficient parallel kinetic resolution, allowing the preparation of enantiopure β,γ-diamino esters. The β,γ-diamino ester products of these reactions are readily converted into the corresponding substituted 4-aminopyrrolidin-2-ones via N-debenzylation and cyclization.

Synthesis of structurally simplified analogues of pancratistatin: truncation of the cyclitol ring

Pancratistatin is a phenanthridone-type natural product isolated from several plants of the Amaryllidaceae family. Its potent antiproliferative, antivascular, antiviral, and antiparasitic properties have attracted the attention of synthetic, biological, and medicinal chemists. Pancratistatin's low natural availability and complex structure have steered many of these research projects toward the preparation of its simplified synthetic analogues with useful levels of activity. In this work we have developed synthetic chemistry aimed at the preparation of pancratistatin analogues with a truncated cyclitol portion of the molecule. The described synthetic pathways are based on a highly anti-diastereoselective arylcuprate conjugate addition to gamma-alkoxy-alpha,beta-enoates and syn-selective azidation at the alpha-position of ester enolates. Analogues with the formally cleaved C3-C4 bond, and thus containing an open ring C, as well as a compound containing a truncated lactol moiety in lieu of the cyclitol, were prepared. Several of the analogues exhibited weak antiproliferative activity, with the highest potency observed in the case of the lactol analogue. From these results implications for the design of future pancratistatin analogues are discussed. Furthermore, the synthetic pathways can be used to construct pancratistatin-mimetic libraries, in which the cyclitol moiety is replaced by other cyclic motifs.

Stereoselective Conjugate Addition of Aryl- and Alkenylboronic Acids to Acyclic γ,δ-Oxygen-Substituted α,β-Enoates

The substrate-controlled RhI-catalyzed conjugate addition of aryl- and alkenylboronic acids to alpha,beta-unsaturated esters which bear gamma- and delta-oxygen substituents takes place in a highly anti diastereoselective fashion either when using gamma-hydroxyl unprotected starting materials or when the gamma-oxygen substituent is protected with a nonbulky group. The delta-oxygen substituent plays a role in the stereoselectivity of the reaction, and better results are obtained when this OH-group is protected.

A solution to the stereochemical problems posed by amaryllidaceae constituents using a highly syn-selective arylcuprate conjugate addition to γ-amino and γ-carbamato-α,β-enoates

Various substituted arylcuprates undergo stereocontrolled additions to L-serine-derived γamino- and γ-carbamato-α,β-enoates with high syn-selectivities. The stereochemical outcome of these reactions is fully consistent with the reductive elimination-based model proposed previously. This method is well suited for the preparation of a broad range of biologically active amaryllidaceae constituents and their aromatic analogues.