One-pot, catalytic, asymmetric synthesis of polypropionates

Chiral Lewis acid catalyzed asymmetric cycloadditions of disubstituted ketenes for the synthesis of β-lactones and δ-lactones

Highly diastereo- and enantioselective [2 + 2]- and [4 + 2]-cycloadditions of disubstituted ketenes were realized by chiral Lewis acid catalysis. A series of arylalkylketenes underwent the reaction smoothly with isatins and β,γ-unsaturated α-ketoesters, providing optically active β-lactones and δ-lactones with vicinal chiral centers in excellent yields (up to 99%) and enantioselectivities (up to 99% ee), as well as exclusively high diastereoselectivities under 0.2-2 mol % catalyst loading.

Asymmetric synthesis, structure, and reactivity of unexpectedly stable spiroepoxy-beta-lactones including facile conversion to tetronic acids: application to (+)-maculalactone A

A novel class of small spirocyclic heterocycles, spiroepoxy-beta-lactones (1,4-dioxaspiro[2.3]-hexan-5-ones), is described that exhibit a number of interesting reactivity patterns. These spiroheterocycles, including an optically active series, are readily synthesized by epoxidation of ketene dimers (4-alkylidene-2-oxetanones) available from homo- or heteroketene dimerization. An analysis of bond lengths in these systems by X-ray crystallography and comparison to data for known spirocycles and those determined computationally suggest that anomeric effects in these systems may be more pronounced due to their rigidity and may contribute to their surprising stability. The synthetic utility of spiroepoxy-beta-lactones was explored, and one facile rearrangement identified under several conditions provides a three-step route from acid chlorides to optically active tetronic acids, ubiquitous heterocycles in bioactive natural products. The addition of various nucleophiles to these spirocycles leads primarily to addition at C5 and C2. The utility of an optically active spiroepoxy-beta-lactone was demonstrated in the concise, enantioselective synthesis of the antifouling agent, (+)-maculalactone A, which proceeds in five steps from hydrocinnamoyl chloride by way of a tetronic acid intermediate.

Synthesis, Structure, and Reactivity of Unexpectedly Stable Spiroepoxy-β-Lactones Obtained by Epoxidation of 4-Alkylidene-2-Oxetanones

We describe the first synthesis of spiroepoxy-beta-lactones obtained via epoxidation of ketene dimers. These compounds display unexpected stability that may be due to a double anomeric effect garnered from analysis of bond lengths by X-ray crystallography of one spirocycle in comparison to calculated bond lengths of related structures. These new strained intermediates display interesting reactivity leading to a butenolide, an alpha-hydroxyketone, a triol, an alpha-chloroketone, and an alpha-azidoketone.

Catalytic, Asymmetric Synthesis and Diastereoselective Aldol Reactions of Dipropionate Equivalents

The dimer of methylketene can be conveniently prepared in one step and high enantiomeric excess from propionyl chloride, using a catalytic amount of a silylated cinchona alkaloid as a source of chirality. Opening of the dimer with a lithiated sulfonamide affords a beta-ketosulfonimide, which undergoes Sn(II)-mediated aldol reactions to diastereoselectively afford the anti,syn-aldol adduct. Alternatively, reduction of the dimer to the beta-hydroxy ketone, followed by acylation, affords a beta-acyloxyl ketone that undergoes diastereoselective, dialkylboron chloride-mediated aldol reactions to produce the anti,anti-aldol adduct.

First total synthesis of a natural product containing a chiral, beta-diketone: synthesis and stereochemical reassignment of siphonarienedione and siphonarienolone

The first total syntheses of siphonarienolone and siphonarienedione are described. The development of a stereoselective synthesis of beta-diketones facilitated the synthesis of the latter compound. The synthesis of the structures proposed for the natural products afforded compounds whose spectral data did not match those of the natural products. However, the synthesis of compounds isomeric to the proposed structures at C(4) and C(5) afforded compounds identical to the natural products, thereby reassigning the stereochemistry of the natural products.

Catalytic, asymmetric synthesis of siphonarienal

This paper describes the synthesis of the marine natural product, siphonarienal. The key step of this synthesis is an aldol reaction that constructs most of the skeleton and sets all three stereocenters of the target in one step from commercially available starting materials. Deoxygenation and chain homologation steps complete the synthesis.