A convenient synthetic method of spiroketal enol ether by dehydration-spiroketalization reaction from furan intermediate and its application for the syntheses of three natural products, tonghaosu (1) and 2, 3, was reported.
Tonghaosu (1), a lead for a botanical antifeedant, and its 22 analogues were synthesized according to a previously reported concise and straightforward procedure. The structures of all new compounds were confirmed by NMR, IR, MS, and HREIMS or elemental analysis. Their insect antifeedant activities against the large white butterfly (Pieris brassicae L.) were examined, and six analogues (Z- and E-6h and Z-isomers of 6i-I), which contain 1,3-diyn or 3,4-methylenedioxyphenyl acetylene group, showed considerable antifeedant activity. Interestingly, Zisomers of 6i-k are much more active than their corresponding E-isomers.