Koon-Sin Ngo

Stilbenes, monoterpenes, diarylheptanoids, labdanes and chalcones from Alpinia katsumadai

Abstract The aerial parts of Alpinia katsumadai yielded six novel compounds, including 1-(1-terpinen-4-olyl)-3-methoxystilbene (E) (1), a mixed metabolite comprising elements from the stilbene and menthane monoterpene classes; the stilbenes 3-methoxy-5-hydroxystilbene (Z) (7) and 3,5-dihydroxystilbene (Z) (8); the diaryl-heptanoids 5-hydroxy-1-(4′-hydroxyphenyl)-7-phenyl-hepta-6-en-3-one (10) and trans-3,5-dihydroxy-1,7-diphenyl-hept-1-ene (12b); and a mixed metabolite (13) comprising elements from the chalcone and labdane diterpene classes of natural products. Several known representatives of each of these classes of natural product were also isolated. The structures of the novel compounds were determined by 2D NMR spectroscopy and NMR assignments of several of the known compounds, deduced by the same methodology, are reported for the first time.

BIOMIMETIC SYNTHESIS OF ARTEANNUIN H AND THE 3,2-REARRANGEMENT OF ALLYLIC HYDROPEROXIDES

Abstract The acyl endoperoxide arteannuin H, recently reported as a novel natural product from Artemtsia annua, has been obtained in two steps from the photooxidation of dihydroartemisinic acid, thereby confirming biogenetic speculation regarding its derivation from a secondary allylic hydroperoxide. The little studied 3,2-rearrangement reaction of such allylic hydroperoxides is also discussed.

AUTOXIDATION OF 4-AMORPHEN-11-OL AND THE BIOGENESIS OF NOR- AND SECO-AMORPHANE SESQUITERPENES FROM FABIANA IMBRICATA

Abstract Photooxidation of 4-amorphen-11-ol (1), recently reported as one of the major sesquiterpene natural products from the medicinal plant Fabiana imbricata, results in three allylic hydroperoxides 6, 9 and 10, which are expected from the “ene-type” reaction of molecular oxygen with the tri-substituted double bond in 1. The tertiary allylic hydroperoxide 6 undergoes carbon-carbon bond cleavage and a second autoxidation reaction to yield the more highly oxygenated seco-amorphane 11 under very mild conditions. In acid, this compound may then undergo either a second carbon-carbon bond cleavage reaction to yield nor-sesquiterpenes 2 and 3 (reported as bona fide natural products from F. imbricata, or cyclize to the sesquiterpene peroxofabianane (5), which is a presumed precursor to the natural product fabianane (4). Some mechanistic investigations concerning the two chemical processes: viz:- carbon-carbon bond cleavage and autoxidation which would account for the formation of natural products 2, 3 and 4 from 1 are reported. Tertiary allylic hydroperoxide 32, which lacks the 11-hydroxyl functional group present in 1 undergoes only C-4/C-5 carbon-carbon bond cleavage under more forcing conditions, suggesting a role for this functional group in assisting the autoxidation reactions of 4-amorphen-11-ol.

Santalane and isocampherenane sesquiterpenoids from Illicium tsangii

Abstract Six novel santalane and three novel isocampherenane sesquiterpenes have been isolated from Illicium tsangii and their structures determined by 2D-NMR spectroscopy. The santalanes may be derived from (−)-a-santalene by oxidation reactions.

SYNTHESIS OF AMORPHANE AND CADINANE SESQUITERPENES FROM FABIANA IMBRICATA

Abstract The monoterpene (−)-isopulegol (7) has been used as a starting material for the total synthesis in eight/nine steps of two sesquiterpenes which were recently isolated from the medicinal plant Fabiana imbricata. Use of this approach has shown that a structure proposed as 3,11-amorphadiene (3) should be revised to that of 4,11-cadinadiene (18) and confirmed the structure proposed for the natural product 4-amorphen-1 1-ol (1).

Allohimachalane, seco-allohimachalane and himachalane sesquiterpenes from Illicium tsangii

Abstract The dichloromethane extract of I. tsangii has yielded ten novel sesquiterpenes ( 1–4, 6–11 ) possessing or derived from the very rare allohimachalane skeleton, in addition to one new himachalane ( 12 ) and a new megastigmane ( 14 ). It is suggested that the novel skeletons possesed by seco -allohimachalanes 8–11 are the result of carbon-carbon bond cleavage reactions associated with hydroperoxide autoxidation products (e.g. 6, 7) of the tri-substituted double bond.

Autoxidation of allohimachalol

Abstract The tri-substituted (2,3)-double bond in allohimachalol ( 1 ) undergoes autoxidation in the presence of singlet molecular oxygen, resulting in tertiary and secondary allylic hydroperoxides 6 and 7 . These autoxidation products may then participate in complex rearrangement reactions to yield seco -allohimachalanes, which have undergone carbon-carbon bond cleavage at the 2,3- or 3,4-positions. The observed autoxidation/rearrangement reactions of 1 in vitro would account for the biogenesis of several allohimachalanes recently reported from the medicinal plant Illicium tsangii as natural products.

Autoxidation of α-santalene

Fifteen compounds (2 – 11) have been isolated from the spontaneous slow autoxidation of the tri-substituted double bond in the side-chain of the tricyclic sesquiterpene α-santalene; most of these compounds have also been reported as natural products.

Synthesis of sesquiterpene allylic alcohols and sesquiterpene dienes from Cupressus bakeri and Chamaecyparis obtusa

Five muurolane (1–5), one nor-seco-muurolane (6) and one nor-muurolane (7) sesquiterpene natural products recently reported from Cupressus bakeri have been obtained by chemical synthesis together with the unnatural 7-epimeric amorphane analogues of four of these sesquiterpenes (15, 16, 18 and 19). The conversion of sesquiterpene allylic tertiary alcohols 1, 2, 15 and 16 into regioisomeric sesquiterpene dienes 3–5 and 17–20 was investigated in vitro by NMR and the mechanism for such dehydrations and their relevance to the origins of sesquiterpene dienes which have been reported as natural products (such as 3–5) is discussed.

Synthesis of α-Corocalene

α-Corocalene, a constituent of hop oil, has been synthesized in four steps from the monoterpene (+)-pulegone.