Homoallylic rearrangements of 19-substituted steroids

Cyclopropylcarbinyl cation chemistry in synthetic method development and natural product synthesis: cyclopropane formation and skeletal rearrangement

“…Seminal studies of 19-substituted steroids by Tadanier have shown that C19-mesylated steroid 90 could undergo homoallylic cyclization to provide cyclopropylcarbinol 91 under solvolytic conditions. 51,52 This intermediate could then serve as an important branchpoint leading to a variety of products with distinct ring scaffolds (94-100) via the skeletal rearrangement of homoallyl cations and cyclopropylcarbinyl cations (90a-90f, Scheme 13). [51][52][53] It is noteworthy that the possible equilibrium of 90a-90f could lead to formation of sophisticated ring systems that are otherwise difficult to access at a late stage, i.e., cyclopropane rings from homoallylic cyclization as well as seven-membered ring (90f) and bridged ring system (90c) from ring expansion.…”

“…51,52 This intermediate could then serve as an important branchpoint leading to a variety of products with distinct ring scaffolds (94-100) via the skeletal rearrangement of homoallyl cations and cyclopropylcarbinyl cations (90a-90f, Scheme 13). [51][52][53] It is noteworthy that the possible equilibrium of 90a-90f could lead to formation of sophisticated ring systems that are otherwise difficult to access at a late stage, i.e., cyclopropane rings from homoallylic cyclization as well as seven-membered ring (90f) and bridged ring system (90c) from ring expansion. Shirahama et al utilized a cyclopropylcarbinyl cationmediated conformationally selective transannular cyclization to achieve the skeletal rearrangement of (±)-humulene-9,10epoxide (101, Scheme 14).…”