Cycloartanes, Euphanes, and Tirucallanes

The driving force behind this biogenetic-like backbone rearrangement is evidently conformational in nature. In protosterol (311) ring B is necessarily constrained to a boat conformation while a more stable allchair conformation can be adopted by dihydrolanosterol (309-OAc). 

Consistent with an inherently greater thermodynamic stability of the lanostane skeleton is the finding that treatment of dihydrolanosterol acetate (309-OAc) with hydrogen chloride in chloroform (2 hr.) effects simply partial isomerization of the double bond into the position (Le.,io 314) (389). 

Despite the greather stability of the lanostane nucleus, it is apparently possible to reverse the protostane- lanostane rearrangement. It has been recently reported that the reaction of the epoxide (317) of dihydroparkeol acetate with Lewis acids affords, protostadienol acetate 318 (45%), presumably by secondary dehydration of protostene-3p,l la-diol 

The observation that dihydroapoeuphyl acetate (326) is converted to dihydroisoeuphyl acetate (324-OAc) under conditions (hydrogen chloride in chloroform, 0°, 2 hr.) which leave dihydroeuphyl acetate (321-OAc) 

When the euphenyl- isoeuphenyl rearrangement (321-0Ac- 324-OAc) is carried out in deuterated acetic acid at low conversion, the iso-euphenyl acetate obtained (7.5%) is extensively deuterated (8% 39% dz, 

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