the selective dithioketalization of the Wieland-Miescher ketone on the unsaturated ke

[Neil9327]

How do you explain the selective dithioketalization of the Wieland-Miescher ketone on the unsaturated ketone?

Similary the saturated ketone can be selectively protected by 1,3-propanediol. Almost similar conditions can be used, pTsOH in AcOH for the ketalization or in benzene for the other one?

Is it Hard and Soft postulate? If yes or maybe why the thiol does not add onto the beta carbone of the unsaturated positon?

Thanks for your suggestions.

[Tartaglia]

I think this can probably be explained by a combination of steric, electronic and entropic effects, but there is a lot of competing effects here.

 

1,4 Micheal addition is certainly favoured by soft nucleophiles such as thiols, but the methyl on the quarternary carbon will probably hinder attack from above and the other ring will probably hinder attack from below. The same methyl also hinders 1,2 attack at the simple ketone. Ring closure is certainly going to be favoured by the second law of thermodynamics as there is another water molecule eliminated.

The use of a 1,2 diol to attack the simple ketone is probably electronically driven as I am reasonably sure the alpha beta unsaturated ketone is less favourably inclined to 1,2 attack by hard nucleophiles than a simple ketone. ( I can at least partially justify this to myself by drawing other resonant forms.) Also oxygen is considerably smaller than sulphur and so steric hinderance may be less of a problem.

 

Hope this helps