Epimerization of C-1 alkynyl group on pyranose ring through dicobalt hexacarbonyl complexes

Alkynyldicobalt Derivatives of Dibenzosuberenol and Dibenzocyclooctatrien‐5‐ol: Ring Conformations, Ease of Carbonyl Elimination and Relevance to Pauson–Khand Cyclization

“…The changing conformation of cycloalkynol groups upon incorporation of a Co 2 (CO) 6 moiety has been noted earlier, in both five‐ and six‐membered ring systems . Thus, treatment of axial alkynylcyclohexanols with octacarbonyldicobalt brings about a ring flip to place the cluster fragment in the equatorial position.…”

Epoxide‐Opening Cascades Triggered by a Nicholas Reaction: Total Synthesis of Teurilene

“…However, 8 presented an inverted stereocenter at the propargylic position, which needed to be epimerized for the synthesis of 1. Precedents in the literature [22] and from our own laboratory [12f,h,i, 20] have shown that acidic treatment of propargyl hexacarbonyl dicobalt complexes is a suitable methodology to promote isomerization at the propargylic position. Unfortunately, we were unable to isomerize the hexacarbonyl dicobalt complex of 8 into the complex 9.…”

Acetylenosaccharides

“…The S N 1 reaction is initiated by loss of AcOsC(3), resulting in an intermediate allyloxycarbenium ion that is intercepted by a pseudoaxially attacking silylated acetylene. The a-d-hex-2-enopyranosylacetylene 61 was epimerized to the b-d-anomer by an acid-catalyzed isomerization of its dicobalt hexacarbonyl complex [104]. Under the same conditions as used for 60, the 2-acetoxyglucal 62 was transformed into moderately stable 3,7-anhydro-d-threo-oct-5-en-1-yn-5-uloses and, thence, by reduction to the 3,7-anhydro-d-arabino-oct-5-en-1-ynitols 63 [102].…”