Long Helicenes by Alkyne Cycloisomerization

doi:10.1055/s-0029-1218094

Synfacts 2009(11): 1221-1221
DOI: 10.1055/s-0029-1218094

Synthesis of Materials and Unnatural Products

Long Helicenes by Alkyne Cycloisomerization

Rezensent(en):Timothy M. Swager, David M. Chenoweth

P. Sehnal, I. G. Stará*, D. Šaman, M. Tichý, J. Míšek, J. Cvačka, L. Rulíšek, J. Chocholšouova, J. Vacek, G. Goryl, M. Szymonski*, I. Císařová, I. Star*
Academy of Sciences of the Czech Republic AND Charles University, Prague, Czech Republic and Jagiellonian University, Krakow, Poland
An Organometallic Route to Long Helicenes
Proc. Natl. Acad. Sci. U.S.A. 2009, 106: 13169-13174

Abstract

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Key words

helicenes - alkyne cycloisomerization - cobalt

Significance

Helicenes have many possible applications in the areas of materials science, asymmetric catalysis, and molecular recognition; however, long helicenes represent challenging synthetic targets. Reported here is a synthetic route to undecacyclic helicenes utilizing a cobalt-catalyzed intramolecular [2+2+2] alkyne cycloisomerization. This key step forms six new rings of the helicene backbone in a single transformation.

Comment

Cobalt catalysts performed better than nickel catalysts for the cycloisomerization reaction with CpCo(CO)₂/Ph₃P giving higher yields than CpCo(C₂H₄)₂ or Ni(cod)₂. A slightly modified version of hexyne 4 underwent diastereoselective cyclization to afford a 10:90 ratio of products similar to 5 in 26% yield. Racemic helicene 7 was purified by chiral HPLC and the racemization barrier at 230 ˚C was found to be 37.5 kcal/mol.

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