[g_orbital]

In the following reaction the hemiacetal is oxidized into lactone (by Jone's reagent), and then the C=C bond is reduced selectively by Zinc + Acetic acid. My question is - why is only this C=C bond reduced, while the carbonyl (C=O) groups remains unchanged? Furthermore, these conditions (Zn + HOAc) are similar to those in Clemmensen rxn (Zn + HCl) in which carbonyl reduces into methylene (C=O ---> CH₂). What is the reason for this chemical behaviour?

Thank you for your kind assistance!

[lavoisier]

HOAc+Zn is 'similar' to HCl+Zn, but it's not the same! Just look at the pH... And actually, Hg salts are added to form a zinc amalgam, which again has a different reactivity.
Anyway, March says that under Clemmensen conditions, alpha,beta unsaturated ketones are either completely reduced or, if partially, only the C=C bond is reduced (but there is a counter-example: Tetrahedron 1986, 42, 6615).

Concerning the other question, if you look at methods for selectively reducing C=C in a C=C-C=O system, you generally find 'soft' transition metals (copper, rhodium, palladium...), i.e. species that prefer to coordinate the C=C moiety rather than the oxygen in C=O, together with hydride donors or even H2.
On the other hand, if you look at methods for selectively reducing the C=O moiety, you find harder, strongly oxygen-coordinating species, like AlH3, LiAlH4, NaBH4, often in the presence of lanthanide salts (quite hard, trivalent cations).

In a way it's like when you want to add carbon nucleophiles to these systems: cuprates add 1,4, whereas simple lithium alkyls tend to add 1,2 (to the carbonyl).

In your example, therefore, it appears that zinc behaves as a 'soft' cation. Or maybe there is some other, specific effect due to the structure of your substrate.

Hope it makes sense.