Biosnythesis of some necines (pyrrolizidine alkaloids) from putrescin (1) with the key enzyme homospermidine synthetase (HSS). homospermidine (2); iminium ion (3a, 3b); trachelanthamidine (4); isoretronecanole (5); (-)-rosmarinecine (6); <b>retronecine</b> (7); otonecine (8); (+)-heliotridine (9).
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<span class="int-own-work" lang="en">Own work</span>; Current Organic Chemistry 1999 3(6) page 561 + <a class="external mw-magiclink-pmid" rel="nofollow" href="//www.ncbi.nlm.nih.gov/pubmed/7700976?dopt=Abstract">PMID 7700976</a>