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Synfacts 2014; 10(3): 0263
DOI: 10.1055/s-0033-1340780
Synthesis of Materials and Unnatural Products
© Georg Thieme Verlag Stuttgart · New York

Can You exo-dig It? Radical-Cascade Synthesis of Polycyclic Frameworks

Contributor(s):
Timothy M. Swager
,
Elizabeth S. Sterner
Pati K, Hughes AM, Phan H, Alabugin IV * Florida State University, Tallahassee, USA
Exo-Dig Radical Cascades of Skipped Enediynes: Building a Naphthalene Moiety within a Polycyclic Framework.

Chem. Eur. J. 2014;
20: 390-393
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Publication History

Publication Date:
17 February 2014 (online)

 
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Key words

radical cyclization - polycyclic aromatic hydrocarbons - cascade reaction

Significance

Alabugin and co-workers report a substrate structure which allows for 6-exo-dig ring closing of skipped diynes that produces fused six-membered rings rather than previously reported 5-exo-dig cascades. The use of a radical cascade expands the reaction’s functional-group tolerance beyond established ionic cascade techniques. The authors describe conjugated systems of four and five rings but provide a rational basis for extending the method to longer ribbons.


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Comment

The choice of the radical-generation system greatly affects the product yield, and it has been optimized in the report. The reaction tolerates a variety of functionality, and a facile synthesis of the relevant substrates is provided. A mechanism is ­proposed to account for the high regioisomeric ­selectivity of the reaction when performed on asymmetric alkynes. In the authors’ mechanism the last step involves a radical abstraction by Bu3Sn•, which generates Bu3SnH. This is questionable as this mechanism is a radical-chain process, as would be expected with 0.3 equivalents of initiator. An alternative scheme would be for Bu3SnI to react with the radical and effect related processes that would be equivalent to a formal elimination of HI.


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