Germanicanes, Taraxastanes, Oleananes, and Ursanes

The stereospecificity of the uranediol rearrangement is further exemplified by predominant formation of different products in the formolysis of the epimeric 20a-tosylate (359-OTs) (443, 444). That olefin (360) is formed by 1,2-hydrogen and methyl rearrangements (rather than elimination and secondary acid-catalyzed rearrangement) was demonstrated by introduction of deuterium and tritium label at C20. A substantial 

It is relevant to note that 2- C-mevalonate is incorporated specifically into the methyl group of the isopropenyl side chain of the lupane triterpene betulinic acid (348, R = C02H) (447). Hence, in the biosynthesis the two methyl groups in postulated intermediate (347) are not randomized. A related chemical observation is the acid-catalyzed equilibration of two C20 epimeric dammarane diol derivatives (12p-hydroxy-24,25-dihydro version of 320 and 345) (448). 

A D-homoannulation rearrangement of possible biogenetic significance has been observed in the acid-catalyzed cyclization of diene acetate (362) (31). One possible mechanism for the formation of perhydro-chrysene-type product (363) includes a cyclopentylcarbinyl- cyclohexyl rearrangement analogous to that (343 346) depicted in biogenetic scheme 30.  

In accord with an a f/-parallel relationship between the migrating carbon (C20) and the entering nucleophile, the carbon chlorine bond in 364-Cl is found to be in the equatorial (a) configuration. On the other hand, the acid-catalyzed cyclization of methyl betulonate (367), and related diol occurs so as to form an axial carbon-oxygen bond to C19, despite the ostensibly unfavorable stereoelectronics 453, 454). It is possible, however, that this transformation is more complex than a simple ring expansion followed by lactonization. 

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