Question

The major product formed in the following reaction sequence is:

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

First spot the Heck coupling (vinyl triflate + vinyl ether, \(\mathrm{Pd(OAc)_2/Et_3N}\)) that builds a ring-conjugated 1-ethoxydiene, then apply Diels-Alder regiochemistry with methyl propiolate as the dienophile.

Answer 1

The Correct Option is D

Step 1: Identify the first transformation (Heck-type coupling).
The starting material is a cyclohexenyl triflate, \(\mathrm{OTf}\) sitting on an sp2 carbon of a cyclohexene ring. Under \(\mathrm{Pd(OAc)_2}\), \(\mathrm{Et_3N}\) (base), in DMSO with heat, \(\mathrm{Pd(0)}\) generated in situ inserts oxidatively into the vinyl \(\mathrm{C-OTf}\) bond, giving a vinyl-Pd(II) species anchored on the ring.

Step 2: Migratory insertion into the vinyl ether.
This vinyl-Pd(II) species inserts across the electron-rich alkene of ethyl vinyl ether, \(\mathrm{CH_2=CH-OEt}\). The oxygen lone pair stabilizes positive character next to it, so Pd ends up on the carbon next to the \(\mathrm{OEt}\)-bearing carbon. Syn beta-hydride elimination, away from the OEt carbon so the enol ether stays intact, then regenerates \(\mathrm{Pd(0)}\) (recycled by \(\mathrm{Et_3N}\) mopping up the acid formed) and installs a new alkene, \(\mathrm{-CH=CH-OEt}\), directly conjugated with the ring double bond that used to carry the triflate.

Step 3: Recognize the diene formed.
The ring double bond and the newly installed exocyclic \(\mathrm{-CH=CH-OEt}\) unit are now conjugated, so the whole ring-plus-appendage system behaves as a 1-ethoxy-1,3-diene, an electron-rich diene ready for a Diels-Alder cycloaddition, with the \(\mathrm{OEt}\) group sitting at diene terminus C1.

Step 4: Diels-Alder cycloaddition with methyl propiolate.
Heating this diene with methyl propiolate, \(\mathrm{HC \equiv C-CO_2Me}\), an electron-poor alkyne dienophile, drives a thermal [4+2] cycloaddition. The diene's two terminal carbons form the two new sigma bonds to the alkyne carbons, closing a second six-membered ring fused onto the original cyclohexane ring, giving an octahydronaphthalene skeleton. Because the dienophile is an alkyne rather than an alkene, one C=C double bond from the alkyne survives into the new ring.

Step 5: Apply the ortho regiochemical rule.
For a diene carrying an electron-donating group (\(\mathrm{OEt}\)) at C1 reacting with an electron-poor dienophile, the dominant pathway (favoured by the larger frontier-orbital coefficients lining up) places the \(\mathrm{OEt}\) group on the new ring-fusion double bond as a vinylogous enol ether, with the ester-bearing carbon left on the saturated part of the new ring, away from that double bond. Matched against the four drawn structures, this connectivity is option (D).

Final Answer:
The Heck coupling installs a ring-fused 1-ethoxydiene, and the Diels-Alder with methyl propiolate builds the bicyclic product with the substitution pattern of option (D). \[ \boxed{\text{(D)}} \]