Intensely sweet saponin osladin: synthetic and structural study.

Abstract

Saccharin, dulcin, sodium cyclamate, acesulfame-K, and aspartame are well-known artificial sweeteners that have been discovered accidentally during some synthetic experiments. Intensely sweet tasting substances are also widely distributed in nature particularly in the plant kingdom. These materials have been an exciting research area for natural product chemists and a variety of sweet-tasting natural products have been characterized.1,2 Many of them are glycosides of terpenoids or steroids. A sweet principle of the Chinese drug Glycyrrhiza glabra (Fabaceae) is the well-known glycyrrhizin (1); it has been used as an auxiliary food additive.3 This glycoside is 150 times sweeter than sucrose. A saponin, mogroside V (2), is 250 times sweeter than sucrose and found in the fruit of a Cucurbitaceaeous plant, Momordica grosvenorii.4 The triterpene glycoside abrusoside B (3) was recently characterized as the sweet principle of Abrus precatorius (Leguminosae),5 and is 100 times sweeter than sucrose. The sweet diterpene glycoside stevioside (4) has been isolated from Paraguayan Composite plant Stevia rebaudiana.6 It is 300 times sweeter than sucrose. Baiyunoside (5) is the sweet principle of a Chinese drug Phlomis betonicoides (Labiatae),7,8 and the labdane-type diterpene glycoside 5 is 250 times sweeter than sucrose. A fern metabolite osladin (6) is the intensely sweet steroidal glycoside isolated from the fern Polypodium vulgare.9 Since these sweet-tasting natural glycosides have not been the subject of organic synthesis, we have been interested in developing synthetic approaches to these natural products. We have developed an effective olefin cyclization agent mercury triflate amine complex, Hg(OS02CF3)2·C6H5NMe2,10–14 that is useful to construct labdane-type diterpenoids.