Abstract
Cucurbitacin A has been transformed into C(9)–C(10)–C(19) cyclopropane derivatives. By ring opening under anionic conditions the primary C(9)β-CH2OH group of cucurbitacin A has, in effect, been shifted and reduced to give a 10β-methyl group. A remarkable hydroxy-group participation in a carbanionic process is an essential element in the fission of the cyclopropane ring. By appropriate manipulation of the oxidation levels of the intermediates in this process, cucurbitacin A has been converted into 4,4,14α-trimethylpregn-8-ene-2,7,11,20-tetra-one, identical with authentic material prepared from the degradation of eburicoic acid. In this way the constitutions assigned to all the cucurbitacins have been confirmed. In addition, this work gives proof of the stereochemistry of the cucurbitacins at C-9, C-13, and C-14.
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