Presqualene diphosphate rearrangement

Evidence has been obtained467 for the involvement of a tertiary cyclopropylcarbinyl cationic intermediate in the rearrangement of presqualene diphosphate to squalene. 16-Oximino-17a-benzyl-17//-hydroxy derivatives in the androstane and estrane series have been converted into 16-oxo-17//-benzyl-17a-hydroxy derivatives with inversed configuration at C(17), on treatment with titanium trichloride. It has been suggested468 that the rearrangement occurs through the key intermediate (401) (see Scheme 97). 

Evidence has been obtained467 for the involvement of a tertiary cyclopropylcarbinyl cationic intermediate in the rearrangement of presqualene diphosphate to squalene. 

The second step in the squalene biosynthesis is the reductive rearrangement of presqualene diphosphate (36a) to squalene. Initial diphosphate abstraction formally gives a primary cyclopropylcarbinyl catimi 37a that rearranges via a secondary cyclobutylium intermediate 38a to the tertiary cyclopropylcarbinyl cation 39a (Scheme 87.12) [176-178]. Hydride attack from NADPH at C3 terminates the squalene biosynthesis. Evidence for the existence of the tertiary... 

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