39 Syntheses and Stereochemistry of the Hydrofluorenes derived from Pine Tree Rosin : New Type of Compound having Strong Sweetness
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Since a decade ago, studies on chemical conversion of pine tree rosin to biogenetically active compounds have been carried out in our laboratory. Recently, four stereoisomers (1, 14, 15 and 16) of 1, 2, 3, 4, 5, 10-hexahydro-4β, 10α-dimethyl-fluorene-4α, 6-dicarboxylic acid were synthesized from the rosin and their structures regarded as a basic skeleton of gibberellin were elucidated. During the course of this research, it was discovered that one isomer (1_a and 1_b) alone had a remarkable sweet taste and while the others (14, 15 and 16) had no taste. This taste has some bitterness besides the sweetness and (1_b) is 1600-2000 times sweeter than sucrose. The new type of sweet compound has completely different structure from the other sweet substances. Many kinds of derivatives of the hydrofluorene (1) were synthesized and correlation between sweetness and structure were studied. In order to increase the yield of (1) from the rosin, the preparative process was simplified and shortened. A skeletal conversion to the hydrofluorene from resin acid had been completed by the rearrangement of diketone (11) to (12_a). The stereochemistry of the important intermediate (12) was reinvestigated. As the result, acetoxy anhydride (46'), Grove's key compound for elucidation of (12), should be revised to (46) and an interesting epimerization at C-6 was newly found during the acetylation (12_a→46).
