Enantiomeric synthesis of pyran subunits of marine trans-fused polyether toxins :Epimerization of Alkynyl sugars through cobalt complexes

Enantiodivergent Cyclization by Inversion of the Reactivity in Ambiphilic Molecules

Inverting the reactivity of the functional groups in ambiphilic molecules provides a new synthetic strategy to perform late-stage enantiodivergence. Both enantiomers of the final compound can be obtained from a common chiral precursor. As a proof of concept, the synthesis of substituted five- and six-membered oxacycles is described. The key step is the cyclization of an ambiphilic linear precursor bearing a propargylic alcohol and an epoxide linked through an alkyl chain. Through a slight modification of these linear precursors and employing different reaction conditions, these functional groups can inverse their chemical reactivity, producing one enantiomer or another of the final product. This enantiodivergent cyclization involves three stereogenic centers that can undergo fully controlled retention or inversion of their configuration depending on the cyclization pathway that is activated. The cyclization provides late-stage enantiodivergence, enabling the synthesis of either enantiomers of the oxacycles from a common chiral substrate with total transfer of the enantiomeric purity.

Extension of the 5-alkynyluridine side chain via C-C-bond formation in modified organometallic nucleosides using the Nicholas reaction

Dicobalt hexacarbonyl nucleoside complexes of propargyl ether or esters of 5-substituted uridines react with diverse C-nucleophiles. Synthetic outcomes confirmed that the Nicholas reaction can be carried out in a nucleoside presence, leading to a divergent synthesis of novel metallo-nucleosides enriched with alkene, arene, arylketo, and heterocyclic functions, in the deoxy and ribo series.

Advances in C-alkynylation of sugars and its application in organic synthesis

C-Glycosidation plays a significant role in the synthesis of optically active scaffolds.

Zinc mediated activation of terminal alkynes: stereoselective synthesis of alkynyl glycosides

Zinc mediated alkynylation reaction was studied for the preparation of C-glycosides from unactivated aromatic and aliphatic acetylenes. Different glycosyl donors such as glycals and anomeric acetates were tested towards in situ generated alkynyl zinc reagent using zinc dust and ethyl bromoacetate. The method provides a simple, mild and stereoselective access of alkynyl glycosides.

Stereocontrolled total synthesis of polygalolide A

The total synthesis of polygalolide A, a secondary metabolite that was isolated from a Chinese medicinal plant, is reported. A key issue in this synthesis was construction of an oxabicyclo[3.2.1] skeleton, which was solved by the development of an intramolecular Ferrier-type C-glycosylation of a glucal with siloxyfuran as an internal nucleophile. The substrate was prepared from D-glucal by the introduction of trimethylsilylacetylene and siloxyfuran groups. Although C-glycosylation did not occur under the conditions found from model experiments, further examination revealed that the combination of trimethylsilyl trifluoromethanesulfonate (TMSOTf) and 2,4,6-collidine successfully afforded the desired product as a single diastereomer. The siloxy group at the C3 position played a crucial role in the stereocontrol of this reaction. The product was further transformed into a tetracyclic compound as follows: The vinyl ether and acetylenic moieties were reduced and the siloxy group was removed with a Barton-McCombie reaction. The construction of the six-membered ether and the γ-lactone provided the tetracyclic compound. Finally, a phenolic moiety was introduced by using a Mukaiyama aldol reaction to furnish polygalolide A.

Ferrier-Type Alkynylation Reaction Mediated by Indium

An efficient Ferrier-type alkynylation reaction between glycals and iodoalkynes using Barbier conditions is described. These conditions require In0, InI, or InII and lead to alpha-2,3-unsaturated-C-glycosides with good stereoselectivity. When glycosyliodoalkynes are used, trehalose-derived compounds and alpha-(1-->6)-C-disaccharides are obtained.

Alkynylcyclohexanol chairs and twist-boats: Co(2)(CO)(6) as a conformational switch

Treatment of 1-[axial]-(trimethylsilylethynyl)cyclohexan-1-ol with dicobalt octacarbonyl results in a conformational ring flip such that the bulky dicobalt-alkyne cluster moiety now occupies the favored equatorial site. However, when a 4-tert-butyl substituent is present, the coordinated alkynyl group retains its original axial or equatorial position. Complexation of trans-[diaxial]-1,4-bis(triphenylsilylethynyl)cyclohexane-1,4-diol brings about a chair-to-chair conformational inversion such that both cluster fragments now occupy equatorial sites. In contrast, cis-1,4-bis(triphenylsilylethynyl)cyclohexane-1,4-diol reacts with Co(2)(CO)(8) to yield the twist-boat conformer in which the two axial hydroxy substituents exhibit intra-molecular hydrogen bonding. Likewise, the corresponding reaction of cis-1,4-bis(trimethylsilylethynyl)cyclohexane-1,4-diol with Co(2)(CO)(8) leads to a twist-boat, but in this case, the molecules are linked through inter-molecular hydrogen bonds. Eight of these cobalt clusters have been characterized by X-ray crystallography, and the potential use of twist-boats in synthesis is discussed.

Mirror Image Synthesis of Left Ends of Ciguatoxin and Gambiertoxin 4b

Three compounds related to the AB fragments of ciguatoxin and gambiertoxin 4b and two diastereomers (at the C-2 position) of the ABC fragment of ciguatoxin have been synthesized in enantiomeric form. The stereochemistry of the C-2 position was introduced selectively from the corresponding pentose derivative. Construction of the A ring with its side chain was completed by Nicholas type cyclization of an acetylene bis(cobalthexacarbonyl) complex followed by reductive decomplexation.