Naturally Occurring Sweet Glycosides

There are a few, intensely sweet, naturally occurring bisglycosides. Stevioside and rebaudoside A are found in the leaves of the Paraguayan shrub Stevia rebau diana Bertoni (yerba dulce). Both have steviol as the aglycone [16] to which there are two glycosyl groups attached, a p-D-glucopyranosyl attached to the carboxyl 

L-phenylalanine binding only two of the three complementary sites of the sweet taste protein receptors sites (NO TASTE) 

D-phenyialanine binding all three the complementary sites of the protein receptor site (SWEET TASTE) 

Osladin, a sterol bisglycoside, is found in the fern Polypodium vulgare [21]. It has a 2-0-a-L-rhamnopyranosyl-P-D-glucopyranosyl (neohesperidosyl) group at one end and an a-L-rhamnopyranosyl group at the other end (see Fig. 5.6C). It has been reported to be 3,000 times sweeter than sucrose. 

Glycyrrhizin is a glycoside with a uronic acid disaccharide, 2-0-P-D-glucopy-ranosyl uronic acid-a-D-glucopyranosyl uronic acid, attached to a polyterpenoid aglycone (see Fig. 5.6D). It is found in licorice root and is 50 times sweeter than sucrose, with a licorice taste [22]. Glycyrrhizin has medical and dental applications it has been used to treat ulcers it also has been used as an antiinflammatory 

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The structure-sweetness relationships of other naturally occurring, sweet glycosides have received very little attention. In most cases, virtually no systematic, structural modifications have been reported. 

Figure 5.6. Structures of naturally occurring sweet glycosides.

Of this class of naturally occurring, sweet compounds, the flavanone glycosides found in citrus fruits have achieved considerable interest, owing to the systematic studies of Horowitz and Gentili " (see Fig. 20). 

It is known that the naturally occurring dihydrochalcone glycosides phloridzin (75) and glycyphillin (76), which are 2-yl glycosides, possess little or no sweetness, whereas phyllodulcin (77), which is not a glycoside, has the same taste properties as the dihydrochalcones it is 400 times sweeter than sucrose. ... 

Stevioside and rebaudioside A are diterpene glycosides. The sweetness is tainted with a bitter and undesirable aftertaste. The time—intensity profile is characteristic of naturally occurring sweeteners slow onset but lingering. The aglycone moiety, steviol [471 -80-7] (10), which is the principal metaboHte, has been reported to be mutagenic (79). Wide use of stevia ia Japan for over 20 years did not produce any known deleterious side effects. However, because no food additive petition has been presented to the FDA, stevioside and related materials caimot be used ia the United States. An import alert against stevia was issued by the FDA ia 1991. In 1995, however, the FDA revised this import alert to allow the importation and use of stevia as a diet supplement (80), but not as a sweetener or an ingredient for foods. Several comprehensive reviews of stevia are available (81,82). 

The dihydroisocoumarin, 3i -phyllodulcin (3, obtained from the leaves of Hydrangea macrophylla var. thunbergii via enzymatic hydrolysis), was mentioned earlier in the chapter as having commercial use. Recently, it has been demonstrated that this sweet substance occurs naturally in unprocessed leaves of its plant of origin as a 5 1 enantiomer with the previously undescribed compound, 3S -phyllodulcin [111]. Also reported in this study were the novel 3R- and SS-phyllodulcin 3 -O-glycosides, although the presence or absence of a sweet taste in these three new phyllodulcin analogs was not disclosed [111]. 

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