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Controlling the regiochemistry of radical cyclizations
1Division of Pharmaceutical Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan. isibasi@p.kanazawa-u.ac.jp
Chemical Record (New York, N.Y.)
|February 14, 2006
Summary
Researchers explored controlling regiochemistry in radical cyclizations, yielding diverse lactams and enabling natural product synthesis. Strategic modifications directed cyclization pathways for creating complex molecular architectures.
Area of Science:
- Organic Chemistry
- Synthetic Chemistry
- Methodology Development
Background:
- Radical cyclizations are pivotal in organic synthesis for forming cyclic compounds.
- Controlling the regiochemical outcome of these reactions is crucial for targeted synthesis.
- Previous methods often lacked precise control over cyclization pathways.
Purpose of the Study:
- To investigate and control the regiochemistry of radical cyclization reactions.
- To apply these controlled cyclizations to the synthesis of complex natural products.
- To develop new synthetic strategies based on regioselective radical cyclizations.
Main Methods:
- Systematic study of N-vinylic alpha-chloroacetamides and enamide substrates.
- Exploration of radical cyclizations involving aryl and alkyl radicals.
- Strategic manipulation of substrate structure, particularly the carbonyl group position, to influence cyclization modes.
- Application of developed methods in radical cascade sequences.
Main Results:
- N-vinylic alpha-chloroacetamides typically undergo 5-endo-trig cyclization to form five-membered lactams.
- Highly stabilized radical intermediates (phenyl or phenylthio) favor 4-exo-trig cyclization yielding beta-lactams.
- Positional changes of the carbonyl group in enamides successfully shifted aryl radical cyclizations from exo to endo modes (5-exo/6-exo to 6-endo/7-endo).
- Similar regiochemical control was achieved for alkyl radical cyclizations onto enamides, shifting from 5-exo-trig to 6-endo.
- Demonstrated successful application of regioselective cyclizations in radical cascades for synthesizing alkaloids (phenanthroindolizidine, cephalotaxine, lennoxamine) and cylindricine skeletons.
Conclusions:
- The regiochemistry of radical cyclizations can be effectively controlled by substrate design and substituent effects.
- Strategic placement of the carbonyl group in enamides is a powerful tool for directing cyclization pathways.
- These controlled radical cyclizations provide efficient routes for the synthesis of diverse and complex natural products.
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