Journal
SYNTHESIS-STUTTGART
Volume 53, Issue 7, Pages 1247-1261Publisher
GEORG THIEME VERLAG KG
DOI: 10.1055/s-0040-1706003
Keywords
radical reaction; natural product modification; alkenes; boronic ester; 1,3-diol protecting group; triethylborane
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Funding
- Swiss National Science Foundation [IZ73Z0_152346/1, 200020_172621]
- State Secretariat for Education and Innovation (SERI) via a Swiss Government Excellence Scholarships for Foreign Scholars and Artists
- Swiss National Science Foundation (SNF) [IZ73Z0_152346] Funding Source: Swiss National Science Foundation (SNF)
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The modification of highly oxygenated forskolin and two less functionalized model substrates was investigated via intermolecular carbon-centered radical addition, leading to highly regio- and reasonably stereoselective iodine atom transfer radical addition reactions. Unexpected cyclic ether derivative was obtained from unprotected forskolin, while in situ protection of the 1,3-diol moiety as a cyclic boronic ester occurred during the iodine ATRA process by changing the mode of radical process initiation. This mild radical-mediated in situ protection of 1,3-diol may have broad applications in radical and non-radical transformations.
The modification of highly oxygenated forskolin as well as manoyl and epi-manoyl oxide, two less functionalized model substrates sharing the same polycyclic skeleton, via intermolecular carbon-centered radical addition to the vinyl moiety has been investigated. Highly regio- and reasonably stereoselective iodine atom transfer radical addition (ATRA) reactions were developed. Unprotected forskolin afforded an unexpected cyclic ether derivative. Protection of the 1,3-diol as an acetonide led the formation of the iodine ATRA product. Interestingly, by changing the mode of initiation of the radical process, in situ protection of the forskolin 1,3-diol moiety as a cyclic boronic ester took place during the iodine ATRA process without disruption of the radical chain process. This very mild radical-mediated in situ protection of 1,3-diol is expected to be of interest for a broad range of radical and non-radical transformations. Finally, by using our recently developed tert-butyl-catechol-mediated hydroalkylation procedure, highly efficient preparation of forskolin derivatives bearing an extra ester or sulfone group was achieved.
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