Ynals, reductive cyclization

The Ni(COD)2/PCy3-catalysed and Et3SiH-mediated reductive cyclization of ynals in THF is first order in catalyst and ynal and zero order in silane, which was used only in the presence of alkyne and aldehyde. The kinetic isotope effect, = 1 ... 

More successful results were obtained in the nickel-catalysed reductive cyclization of ynals 137 to macrocyclic allylic alcohols with 14- to 22-mem-bered rings (Scheme 11.17). This reaction can provide both endocyclic (138) and exocyclic (139) alkenes depending upon the ligand employed, the nature of the alkyne (terminal or internal) and the reducing agent. The optimized process has been subsequently applied to the preparation of modified macrolide structures. " ... 

Even so, the cyclic product was obtained in only 25% yield. A significant improvement was realized by employing the Co2(CO)g complex of the ynal for the cyclization. This improvement can be attributed to a reduction in transition-state energy by virtue of the more favorable geometry of the complex and the increased electrophilic character of the carbonyl group. 

Furthermore, this methodology was developed to afford the synthesis of e t-lycoricidine (248) using photolysis instead of a chemically mediated radical reaction (160). Photolysis of 4-(4/ )-hydroxy-6-[(2-methoxycar-bonyl-4,5-methylenedioxy)phenyl]-25,35-0-isopropylidenehex-5-ynal O-benzyloxime 245 (derived from D-glucose) in the presence of thiophenol under went intramolecular cyclization to produce l-(A-benzyloxy-amino)-5-phenylthiocyclohex-5-ene 246. Cleavage of the N-O bond and reductive elimination of the phenylthio group in 246 with Sml2 in THF... 

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