Abstract
The skeletal conformations of naturally occurring norditerpenoid alkaloids fix their substituent functional groups in space, thereby directing their bioactivities. Solution conformations of the A-rings of 4 selected norditerpenoid alkaloid free bases: mesaconitine, karacoline (karakoline), condelphine, and neoline (bullatine B), were analysed by NMR spectroscopy and single-crystal X-ray crystallography. They adopt twisted-chair, twisted-boat, twisted-boat, twisted-boat conformations, respectively. That the A-ring is stabilised in a boat conformer by an intramolecular H-bond from 1α-OH to the N-ethyl tertiary amine is also confirmed in the condelphine single crystal data. The conformations are a result of through-space repulsion between 12-He′ and atoms attached to C1 (in the equatorial positions). This causes the A-rings with 1α-OR always to be twisted whether in a chair or a boat conformation. The impact of these studies is in providing a detailed understanding of the shape of the A-ring of these important biologically active natural product alkaloids.
Highlights
The unambiguous structural identi cation of naturally occurring norditerpenoid alkaloids with 6 fused and bridged rings is complicated
Due to repulsion between 1a-OMe/12-He0 proposed by Pelletier and coworkers,13,21,22 the A-ring of norditerpenoid alkaloid free bases with 1a-OMe adopting chair conformations are twisted
It is highlighted that d (2-Ha) 1⁄4 2.31 ppm is unusually larger than d (2-Ha) 1⁄4 2.14 ppm by 0.17 ppm
Summary
The unambiguous structural identi cation of naturally occurring norditerpenoid alkaloids with 6 fused and bridged rings is complicated. The A-rings of the selected 1a-OH alkaloids, karacoline [6], condelphine [7], and neoline [8], adopt boat conformations stabilized by an H-bond between the tertiary amine and 1aOH,21,29,30 and they are proven to be in a twisted-boat conformation.
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