Towards the total synthesis of a novel diarylheptanoid.
Diarylheptanoids are a family of plant metabolites with a characteristic structure of two hydroxylated aromatic rings attached by a linear seven-carbon chain. Diarylheptanoids have mostly been isolated from plants belonging to the Zingiberaceae family. The South African medicinal plant Siphonochilus aethiopicus, more commonly known as ‘wild ginger’, also belongs to the Zingiberaceae family. One of the compounds isolated from this plant it a novel diarylheptanoid. In this study, the synthesis of this novel diarylheptanoid will be investigated. The targeted diarylheptanoid has two substituted phenyl rings attached by a seven-carbon aliphatic chain with two sterogenic centers and a carbon-carbon double bond. Osmiumcatalysed asymmetric dihydroxylation was used to generate the two stereogenic centres. The Horner Wadsworth-Emmons (HWE) reaction was investigated, in order to generate the trans-double bond on the seven carbon aliphatic chain. HWE reaction is a transselective reaction leading to the formation of only the desired isomer. The synthetic strategy used for the synthesis of the targeted diarylheptanoid is the C2-moiety + C5-moiety strategy. The C2-moiety is the phosphonate ester and the C5-moiety is the aldehyde for the HWE reaction. In this investigation we were able to successfully synthesis the required C2-moiety and the C5-moiety. Both the precursors were synthesised from commercially available starting materials, utilising functional group transformation reactions. However, modifications to the C5-moiety were made due to its instability under the HWE reaction conditions. When the C5-moiety was an aldehyde, decomposition was seen under HWE reaction conditions. Thus the C5-moiety was converted to the corresponding lactol and then subjected to the HWE reaction. Nevertheless, this reaction was not successful, thus we were not able to couple the two precursors to form the desired seven-carbon aliphatic chain. Even though the targeted diarylheptanoid was not successfully synthesised, the synthetic route developed in this investigation is not only viable to the target compound but is also versatile enough to allow the synthesis of its analogues.