Cellular screening of various synthetic triterpenoid compounds formally derived from lupane has identified a number of analogues as potential anticancer drug candidates. Here we describe the synthesis and structure-activity relationships of betulin and betulinic acid derivatives containing an E-ring modified with different oxygen functions. Thus compounds containing the lup-18-en-21-one, lup-18-ene-21,22-dione, 18,19-secolupane, and the highly oxygenated 18,19-secolupane systems, as well as des-E-lupane derivatives, were prepared from the readily available natural pentacyclic triterpene betulin using oxidative procedures. These compounds were named betulinines. We demonstrate that only selected compounds, particularly those containing a lupane E-ring-derived unsaturated ketone or diketone function, possessed in vitro cytotoxic activity against tumor cell lines, suggesting a structure-activity relationship.

A series of 3- and 28-glucosides and glucosyl esters of betulinic acid (1a), 28-hydroxy-3,4-secolupa-4(23),20(29)-dien-3-oic acid (22a), dimethyl ester of 28-hydroxy-2,3-secolup-20(29)-en-2,3-dioic acid (43a), their 20(29)-dihydro derivatives (1b, 22b, 43b) and several other triterpenes of the lupane (12a, 12b) and 3,4-secolupane series (18a, 18b, 32a) has been prepared by reaction of tetra-O-acetyl-α-D-glucopyranosyl bromide in acetonitrile in the presence of mercury(II) cyanide and subsequent deacetylation of the obtained tetra-O-acetyl-β-D-glucopyranosyl derivatives. In several cases attempted glucosylation in the presence of silver silicate afforded predominantly the corresponding 1,2-orthoacetates of α-D-glucopyranose.

Depending on the conditions and the acid employed, 18β,19β-epoxy-28-hydroxy- 21-oxolupan-3β-yl acetate (2a) and 18β,19β-epoxy-21-oxolupane-3β,28-diyl diacetate (2b) on treatment with acid gave three types of products: (i) 28-nor derivatives: 21-oxo-28-norlupa-16,18-dien-3β-yl acetate (6), 19β-hydroxy-21-oxo-28-norlup-17-en-3β-yl acetate (7) and 17ξ-hydroxy-21-oxo-28-norlup-18-en-3β-yl acetate (8), (ii) lupa-12,18-dien-21-ones 4a and 4b, and (iii) 22β-substituted lup-18-en-21-one derivatives of the type 5. The formation of 22β-substituted derivatives of the type 5 probably proceeds in the enol form of epoxy ketone 2. Opening of the epoxide ring with shift of the double bond to position 19(21) and attack by nucleophilic species at C-22 followed by elimination of water and re-formation of the 22-oxo group leads to derivatives of the type 5.

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