Stereoselective Iodolactonization of 4-Allenoic Acids with Efficient Chirality Transfer: Development of a New Electrophilic Iodination Reagent

Catalytic enantioselective iodolactonization reactions

The halolactonization reaction is a useful chemical transformation for the construction of lactones from γ- or δ-substituted alkenoic carboxylic acids or carboxylic esters. Traditionally, the stereoselectivity of these reactions has been controlled by the substrates or the reagents. The substrate-controlled method has been extensively studied and applied in the synthesis of many natural products. However, catalytic, enantioselective iodolactonizations of γ- or δ-substituted alkenoic carboxylic acids have only recently been developed. This review article highlights the advances that have emerged over the last decade.

Synthesis of γ-Lactones via the Kowalski Homologation Reaction: Protecting-Group-Free Divergent Total Syntheses of Eupomatilones-2,5,6, and 3-epi-Eupomatilone-6

A highly efficient synthesis of functionalized chiral γ-butyrolactone scaffolds has been described. The basis of the approach is the Kowalski ester homologation that is modified for our proposed transformation. The newly developed methodology combines a divergent synthetic strategy to permit a straightforward protecting-group-free asymmetric total syntheses of eupomatilones-2,5,6, and 3-epi-eupomatilone-6 in five or six steps from commercial starting materials, making it one of the shortest syntheses reported to date.

Efficient Syntheses of Traumatic Lactone and Rhizobialide

Herein, we report the total synthesis of traumatic lactone and rhizobialide by utilizing allenoic acid to construct the lactone ring. The key starting materials, allenoic acids, could be prepared by the ATA (allenation of terminal alkynes) of a terminal alkyne with an aldehyde that contained a protected hydroxyl group followed by hydrolysis. Importantly, the asymmetric synthesis could be realized just by replacing racemic diphenylprinol with (R)- or (S)-diphenylprinol to deliver the optically active allenoate.

Gold-catalyzed stereoselective cycloisomerization of allenoic acids for two types of common natural γ-butyrolactones

γ-(E)-Vinylic and γ-alkylic γ-butyrolactones are two different types of lactones existing extensively in animals and plants and many of them show interesting biological activities. Nature makes alkylic γ-butyrolactones by many different enzymatic lactonization processes. Scientists have been mimicking the natural strategy by developing new catalysts. However, direct and efficient access to γ-(E)-vinylic γ-butyrolactones is still extremely limited. Here, we wish to present our modular allene approach, which provides an efficient asymmetric approach to (E)-vinylic γ-butyrolactones from allenoic acids by identifying a new gold complex as the catalyst. Based on this cycloisomerization strategy, the first syntheses of racemic xestospongiene and xestospongienes E, F, G, and H have been realized and the absolute configurations of the chiral centers in xestospongienes E and F have been revised. In addition, by applying a C–O bond cleavage-free hydrogenation, the syntheses of naturally occurring γ-alkylic γ-lactones, (R)-4-tetradecalactone, (S)-4-tetradecalactone, (R)-γ-palmitolactone, and (R)-4-decalactone, have also been achieved.

Gamma-butyrolactones are widespread in Nature, however direct catalytic methods to access them are limited. Here, the authors report a gold-catalyzed cycloisomerization of allenoic acids to acces γ-butyrolactones and apply it to the asymmetric synthesis of xestospongienes E, F, G, and H and other naturally occurring lactones.

Enantioselective Synthesis of Allenes by Catalytic Traceless Petasis Reactions

Allenes are useful functional groups in synthesis as a result of their inherent chemical properties and established reactivity patterns. One property of chemical bonding renders 1,3-substituted allenes chiral, making them attractive targets for asymmetric synthesis. While there are many enantioselective methods to synthesize chiral allenes from chiral starting materials, fewer methods exist to directly synthesize enantioenriched chiral allenes from achiral precursors. We report here an asymmetric boronate addition to sulfonyl hydrazones catalyzed by chiral biphenols to access enantioenriched allenes in a traceless Petasis reaction. The resulting Mannich product from nucleophilic addition eliminates sulfinic acid, yielding a propargylic diazene that performs an alkyne walk to afford the allene. Two enantioselective approaches have been developed; alkynyl boronates add to glycolaldehyde imine to afford allylic hydroxyl allenes, and allyl boronates add to alkynyl imines to form 1,3-alkenyl allenes. In both cases, the products are obtained in high yields and enantioselectivities.

Ligand- and Brønsted acid/base-switchable reaction pathways in gold(i)-catalyzed cycloisomerizations of allenoic acids

Competing gold-catalyzed cycloisomerizations of γ-allenoic acids are optimized through ligand and Brønsted acid/base effects, affording three distinct classes of lactones.

Enantioselective iodolactonization of allenoic acids

An enantioselective iodolactonization reaction of allenoic acids has been developed using a trisimidazoline catalyst and I₂.