Robinson annulation steroid synthesis

Summary

  • Under acidic reaction conditions, selective monohalogenation at the a -postion is observed.
  • Chlorination, bromination and iodination all occur at the same rate.
  • The reaction can be catalyzed by addition of acid, but it is actually autocatalytic as the by-product is the hydrogen halide.
  • The reaction proceeds via the enol tautomer.
Related Reactions
  • a -halogenation under basic conditions
  • The Haloform reaction
MECHANISM OF a- HALOGENATION OF ALDEHYDES AND KETONES  Step 1:
First, tautomerize the carbonyl to its enol tautomer (mechanism not shown here : review) Step 2:
Here the electrons from the oxygen are used as they enhance the nucleophilicity of the alkene, as a result we end up with an oxonium ion and a bromide ion. Step 3:
An acid / base reaction. Here the bromide ion is used to deprotonate the oxonium ion.  

When performing an imine synthesis it is important to ensure the pH remains around 4 or 5 and that conditions don’t get too acidic due to Hydrazine’s susceptibility to protonation from strong acids; however, when forming 2,4-Dinitrophenylhydrazone such care for maintaining the pH is not as necessary because 2,4-Dinitrophenylhydrazine is not as susceptible. Why? I’m assuming it is because the unshared pair of electrons of the Hydrazine portion of the compound are not actually unshared but are participating in resonance with the benzene ring via pi bonding (I’m also assuming that the nitro groups being so electron withdrawing is further pulling hydrazine’s electron pairs inward. Is this correct? Does resonance with the benzene ring coupled with the withdrawing effect of the NO2s make it difficult for a strong acid to protonate 2,4-Dinitrophenylhydrazine as opposed to just Hydrazine?

Acyclic alkenones are one of the most important classes of conjugate acceptors. Much of the early work on these additions focussed on developing the annulation reaction invented by Robinson. The need to develop an enatioselective version of this reaction led to the investigations of numerous combinations of chiral ligands and catalysts. 
An annulation is a reaction in which a new ring is formed. The Robinson annulation is named for English chemist Sir Robert Robinson (1886-1975). It consists of a series of two reactions that use the same catalyst. This (Robinson Annulation) reaction is also called as ring formation reaction.

Robinson annulation steroid synthesis

robinson annulation steroid synthesis

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