Following a conventional [3,3] Cope rearrangement of A to B, with subsequent tautomerisation to C, a more unusual Ene reaction to form D is observed, a component of which is the trans annular ring closure. Abstraction by the carbonyl oxgyen can be of either of the two O-CH2 hydrogen atoms (Pro-R
TS leading to D (-11.2 kcal/mol) | TS leading to E (-9.2 kcal/mol) | ||
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The reaction completes with a [3,3] Claisen rearrangement,
in which new chiral centres at the carbon atoms defining the
termini of the forming C-C bond are formed stereospecifically.
Quantitative models show why this should be. The [3,3]
rearrangement proceeds via a Chair transition state (
TS leading to F (-26.6 kcal/mol) | TS leading to H (-23.8 kcal/mol) | Synplanar TS (-15.8 kcal/mol) | |||
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