Chemistry of Zerumbone. 1. Simplified Isolation, Conjugate Addition Reactions, and a Unique Ring Contracting Transannular Reaction of Its Dibromide.
Zerumbone (1) was isolated from fresh rhizomes of Zingiber zerumbet Smith in yields of 0.3-0.4% by simple steam distillation and recrystallization. 1 accepted 2 equiv of hydrogen cyanide at the C6 and C9 double bonds of the cross-conjugated dienone system to give a mixture of diastereomers 3a-d. In the presence of potassium cyanide, the dominant isomer 3a was isomerized to a mixture of 3a-d. Under controlled conditions, 1 added one mole of methanol regio- and stereoselectively at the C6 double bond to give adduct 4a. With potassium cyanide, 4a was transformed to the mixture of 3a-d. 1 took up one mole of bromine at C6 double bond to give a diastereomeric mixture of adducts 5a and 5b. Treatment of 5a with potassium cyanide gave a mixture of cyclopropanecarboxylic acid 6a and 6b. This unique ring-contracting cyclopropane formation is pictured as a sequential Favorskii type reaction. alpha-Cyclodextrin improved the selectivity and yields of the reactions conducted in an aqueous medium.
Kitayama
Takashi
T
Kyoto University of Education, Fushimi, Kyoto 612-0863, Japan.
Okamoto
Tadashi
T
Hill
Richard K.
RK
Kawai
Yasushi
Y
Takahashi
Sho
S
Yonemori
Shigetomo
S
Yamamoto
Yukio
Y
Ohe
Kouichi
K
Uemura
Sakae
S
Sawada
Seiji
S