Synthesis of geranyl PDglycopyranoside20

Monoterpene glycosides are a group of water-soluble natural products widely distributed in the plant kingdom. Among them, two kinds of naturally occurring geranyl 6-O-glucosyl-P-D-glucopyranoside congeners, geranyl 6-O-P-d-xylopyranosyl-P-D-glucopyranoside (82) and geranyl 6-O-a-L-arabinopyranosyl-P-D-glucopyranoside (Kenposide A, 83) were isolated from a methanol extract of Camellia sinensis var. sinensis cv Shuixian and an ethanol extract of Hovenia dulsis var. tomentella, respectively. The biological activity of some compounds has been investigated and 83 is reported to indicate immunomodulatory properties (Fig. 15). These P-D-glucopyranosides were synthesized from geranyl P-D-glucopyranoside (51) as shown in Fig. 16.

3.6.1. Synthesis of geranyl 6-O-P-D-xylopyranosyl-fi-D-glucopyranoside (82) Teri-butyldimethylsilylation of 51 gave a silyl ether (84, 63% yield), which was subjected to benzoylation to give a benzoate (85) in 71% yield. Desilylation of 85

Geranyl O-p-D-glucopyranoside (51) R1 = H

Geranyl 6-O-p-o-xylopyranosyl-p-D-glucopyranoside (82) K "

Geranyl 6-O-a-L-arabinopyranosyl-p-o-glucopyranoside (Kenposide A, 83) OH

Geranyl O-p-D-glucopyranoside (51) R1 = H

Geranyl 6-O-p-o-xylopyranosyl-p-D-glucopyranoside (82) K "

Geranyl 6-O-a-L-arabinopyranosyl-p-o-glucopyranoside (Kenposide A, 83) OH

Figure 16: Synthesis of naturally occurring geranyl ^-D-glucopyranosides.

using 1 M HCl provided the desired 86 in 99% yield. By applying the reported procedure, coupling reaction of geranyl ^-D-glucopyranoside congener (86) with 2,3,4-tri-O-benzoyl-a-D-xylopyranosyl bromide in the presence of AgOTf and TMU gave the coupled product (87) in 92% yield. Finally, treatment of 87 with NaOMe in MeOH provided the synthetic geranyl 6-O-^-D-xylopyranosyl-P-D-glucopyranoside (82) in 85% yield.

3.6.2. Synthesis of geranyl 6-O-a-L-arabinopyranosyl-fi-D-glucopyranoside (83) (Kenposide A)

By following the reported procedure, coupling reaction of 86 with 2,3,4-tri-O-benzoyl-a-L-arabinopyranosyl bromide in the presence of AgOTf and 2,4,6-collidine gave the coupled product (88) in 57% yield. Finally, treatment of 88 with NaOMe in MeOH provided the synthetic geranyl 6-O-a-L-arabinopyranosyl-^-D-glucopyranoside (Kenposide A, 83) in 85% yield.

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