Question 9 of 13: Position of the Double Bond After Dehydration/Hydrolysis — Enantiopurity

Question 9: Position of the Double Bond After Dehydration/Hydrolysis — Enantiopurity

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Shared underlying principle. Both reactions are acid-catalysed dehydrations/ tautomerisations of a species with more than one possible alkene-forming direction. In both cases the regiochemistry is decided not by simple Zaitsev substitution-count alone, but by which resulting alkene position achieves extended conjugation with a remaining (or newly forming) carbonyl — the thermodynamically preferred outcome.

1. Reaction 1 — the alkene forms toward the carbonyl that gives a conjugated enone, not merely toward the more substituted alkene.
   reaction 1: hydroxy-diketone (enantiopure)

   conjugated ene-dione product
   The tertiary, doubly-allylic-type alcohol at the ring fusion ionises readily under acid catalysis (protonation of –OH, loss of water) because the resulting cation is tertiary. That cation could in principle lose a β-H toward either neighbouring ring, but only one choice places the new C=C directly conjugated with the other ring's ketone, generating an extended, cross-ring-fusion enone system — substantially more stable than the alternative, non-conjugated alkene. Elimination proceeds toward that thermodynamically preferred, conjugated outcome (Zaitsev's rule extended: maximise conjugative stabilisation of the product, not merely alkene substitution count).
2. Reaction 1 — enantiopurity of the product: RETAINED. The stereocentre that is destroyed by this reaction is exactly the one bearing the leaving –OH (it becomes part of the new, planar sp² alkene, so its configuration is simply erased, not scrambled). The other ring-fusion stereocentre — the quaternary carbon bearing the angular methyl group — is never touched by the elimination chemistry and has no acidic α-H available to enolise away (it is fully substituted). With no mechanism available to epimerise it, this centre retains exactly the configuration set in the (enantiopure) starting material. The product is enantiomerically pure — one real stereocentre is destroyed (converted to an achiral sp² centre), but the one that survives is untouched.
3. Reaction 2 — the double bond ends up conjugated with the surviving ketone (vinylogous hydrolysis).
   reaction 2: dienol/enol-ether diacid (model)

   cyclohexenone diacid product
   This substrate is a cross-conjugated dienol/methyl-enol-ether: one ring carbon carries the (dienol) –OH directly on an sp², ring-alkene carbon (i.e., it is itself the enol tautomer of a ketone), while a second, vinylogously-conjugated ring carbon carries the methyl enol ether. Aqueous acid hydrolyses the methyl enol ether (protonation β to the OMe-bearing carbon, loss of methanol from the resulting oxocarbenium, exactly analogous to Question 7(b)'s stabilised cation), unmasking a ketone at that position; the dienol end simply tautomerises to keto/enol equilibrium. The net, thermodynamically preferred outcome places the remaining ring alkene directly conjugated with the new ketone (an ordinary cyclohexenone), exactly the same "unsaturation seeks the carbonyl it can conjugate with" principle as Reaction 1.
4. Reaction 2 — enantiopurity of the product: RETAINED. The two CO₂H-bearing ring carbons are spectators throughout — neither the dienol carbon nor the enol-ether carbon chemistry touches them, so their original (wedge-defined, enantiopure) configurations pass through unchanged. The one position whose stereochemistry is genuinely ambiguous in the starting material (the dienol carbon, drawn with a wavy bond precisely because it is fated to become part of a planar, non-stereogenic alkene/carbonyl system) carries no stereochemical information that could have been lost. The product is enantiomerically pure, by the same logic as Reaction 1: destroying a stereocentre by flattening it to sp² is not the same as racemising a surviving one.