The use of 13C NMR shifts in defining halogen regiochemistry or six-membered ring substituent stereochemistry in marine monoterpenes is considered in detail. New additivity parameters and model compound 13C NMR shift data are presented. Structures are proposed for three new red seaweed monoterpenes (3, 4, and 5), and revisions are suggested for four other published structures (6, 7, 8, and 9). Our approach can be extended to certain marine sesquiterpenes such as the chamigrane class. Comparisons between calculated and experimental chamigrane
A novel A-seco limonoid was isolated from methanolic extract of Croton jatrophoides and designated as zumsin. This compound showed potent antifeedant activity against two lepidopteran larvae, pink bollworm, Pectinophora gossypiella (PC(50) = 1 microg/cm(2), PC(95) = 8 microg/cm(2)), and fall armyworm, Spodoptera frugiperda (PC(50) = 2 microg/cm(2), PC(95) = 16 microg/cm(2)). The structure of zumsin was determined as 1 using a variety of spectroscopic methods including nuclear magnetic resonance, mass spectrometry, and circular dichroism. The structure consists of an A'-B trans-fused ring while dumsin (2), a constituent of the same source, maintains an A'-B cis-fused ring, and suggests two unique biosynthetic processes after A ring oxidative expansion.
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