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Treatment of cis-dihydroxykawainol = rel (5R, 6R)-(±)-5-hydroxy-4-methoxy-6- phenylethyl-5,6-dihydro-2H-pyran-2-on (cis-3) with potassium hydroxide in methanol (2.5%, 1 h, room temp.) leads to a 1:1 mixture of the corresponding erythro-butenolide 7 and threo-butenolide 8 by a ring contraction process that may have analogies with the biosynthesis of the piperolides 15. Trans-3 gives the same yields (about 70%) of the same mixture (7 and S), which can be separated by silica-gel columns. Starting from mixtures of cis-5-hydroxykawain (cis-4) and trans-5-hydroxykawain (trans-4), there were prepared and described the two epimeric dehydro-butenolides 9 and 10 and their O-methylderivatives 11 and 12. The threo- and erythro-series show consistant differences between J 5,6  and the  1 H NMR spectra. The constitution and configuration of (± )-erythro-5- (1-methoxy-3-phenylallyl)-4-methoxy-2(5H)-furanone (11) were established by X-ray analysis.

Über das Verhalten von 5-Hydroxy-Kawainen gegen Alkali / Behaviour of 5-Hydroxykawain Derivatives under Mild Alkaline Conditions