Absolute Configuration of Kuwanon L, a Natural Diels-Alder Type Adduct from the Morus Root Bark

Abstract

The absolute configuration of kuwanon L (1) isolated from the Morus root bark (Morus alba L., Moraceael was determined as 2s.3"R.4"R, 5"s by circular dichroism (CD) Spectroscopic study.Previously, we reported the structure determination HO of various Diels-Alder type adducts isolated from the Morus root bark.'Some of them, such as 2 kuwanon G , are the adducts of chalcones and dehydroprenylflavonoids.This type of adducts have three chiral centers on the methylcyclohexene ring, while the relative configuration of each substituents on the ring has been clarified by the nuclear magnetic resonance (nmr) spectroscopic studies.kuwanon G The plane structure of kuwanon L, one of the Diels-Alder type adducts, has been reported as formula 1 including the relative configuration on the methylcyclohexene ring.3In the course of our study, we attempt to determine the absolute structure of 1 by using the circular dicbroism (CD) spectroscopic study.Kuwanon L (11 possesses four chiral centers at C-2, C-3", C-4", and C-5" positions in the structure.In order to determine the absolute configurations of the three chiral centers on the methylcyclohexene ring, alkali degradation of 1 was carried out as follows: kuwanon L 11, 1a]i2 -227'.1.5 gl was dissolved in 50% potassium hydroxide aqeous solution (20 ml), and the solution was refluxed for 1 h to afford compound A I ; .870 mgl.The compound A (21 was obtained as colorless amorphous powder, 1 d i 5 -208.(c-0.097,MeOH).The molecular ion peak was observed at m/z 448 in mass spectrum.In the ' H nmr spectrum of 2 ( 6 in acetone-d6, 60DC), the proton signals on methylcyclohexene ring were observed at S 1.70 l3H, s , C-1-CH3).2.19 (1H.dd, J=4 and 18 Hz, C-6-H), 2.50-2.80(lH, m, C-6-H), 3.67 (1H.m, C-5-HI,