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Unexpected Regiochemical Control in the Nugent-RajanBabu Reductive Epoxide Cyclization
Nantamon Supantanapong1,2, Scott W Niman1, Christopher D Vanderwal1,3
1Department of Chemistry, 1102 Natural Sciences II, University of California, Irvine, CA 92697, USA.
Synthesis
|September 25, 2025
Summary
The Nugent-RajanBabu reductive epoxide cyclization offers new control over reaction pathways. Protecting group size dictates whether 6-endo or 5-exo cyclizations occur, enabling novel syntheses.
Area of Science:
- Organic Chemistry
- Synthetic Methodology
- Natural Product Synthesis
Background:
- The Nugent-RajanBabu reductive epoxide cyclization is a key alternative to classical cationic polyene cyclizations.
- This method typically exhibits exquisite regiochemical control, favoring 6-endo cyclization even against potential 5-exo pathways.
- Applications include the synthesis of complex decalins and perhydrophenanthrenes.
Purpose of the Study:
- To investigate the behavior of α-alkoxy epoxides in Nugent-RajanBabu reductive epoxide cyclizations.
- To uncover a method for controlling the regioselectivity between 6-endo and 5-exo cyclization pathways.
- To explore new synthetic routes toward complex, polyoxygenated terpenoid natural products and related structures.
Main Methods:
- Evaluation of α-alkoxy epoxides in the Nugent-RajanBabu reductive epoxide cyclization.
- Systematic variation of protecting group size on the α-oxygen substituent.
- Analysis of reaction products to determine regiochemical outcomes (6-endo vs. 5-exo).
Main Results:
- A novel method for controlling regioselectivity was discovered, dependent on the size of the α-oxygen protecting group.
- Specific protecting group sizes were found to favor either the 6-endo or 5-exo cyclization pathway.
- This control allows for predictable formation of different cyclic scaffolds.
Conclusions:
- The size of the protecting group on α-alkoxy epoxides is a critical determinant of regioselectivity in Nugent-RajanBabu cyclizations.
- This finding enables the targeted synthesis of highly oxygenated cyclopentane systems, expanding the utility of this reaction.
- The methodology provides a new avenue for the rapid construction of complex molecular architectures relevant to natural product synthesis.
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