Syntheses of Dammarane Sapogenins

After Stork etaL 123) had carried out the total synthesis of a a-onocerin (68), Tsuda and Hattori 124) in 1967 succeeded in converting (68) to hydroxyhopanone (69) via gammaceran-3-on-21-ol (70) (see Chart 16). 

A synthesis of dammarenediol-II (15), a constituent of dammar resin 125), was carried out by FujiMOXoand Tanaka 126) in 1970 as follows (see Chart 17). Acid treatment of (69) 127) afforded hopenone-I (71) which was reduced with LiAlH4, to afford hop-17(21)-en-3P-ol (72). The acetate (73) 

Reaction of (78) with the Grignard reagent prepared from 4-methyl-3-pentenyl bromide (82) furnished synthetic (15) (725). Although formation to some extent of the 20-epimer dammarenediol-I (14) was also expected, it could not be detected. 

In an attempt toward syntheses of betulafolienetriol (13) and 20(5)-protopanaxadiol (26), the genuine sapogenin, introduction of an oxygen function at the unactivated methylene at c-12 of the dammarane skeleton was achieved 128, 129) by means of remote oxidation (130) using a photoexcited aromatic nitro group (131, 132, 133) (see Chart 18). Irradiation of the p-nitrophenyl-acetate (83) of 3-epidammaranediol-II (84) 

Because irradiation of the / -nitrophenylacetate of 3-epidammarenediol-II resulted in intermolecular addition of the photoexcited nitro group to the side chain double bond in preference to intramolecular hydroxylation of c- 

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