Question

The reaction(s) that produce(s) the following compound is(are)

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Ketenes give [2+2], not [4+2], with dienes; look for "masked ketone" dienophiles (nitroalkene reduced by TiCl3, or acetoxynitrile hydrolyzed by NaOH) instead.

Answer 1

The Correct Option is B, C

The fastest way to solve this is to recognize that plain ketenes ($\mathrm{R_2C{=}C{=}O}$) are notoriously bad Diels-Alder partners: they almost always react with dienes through a $[2{+}2]$ pathway instead of the wanted $[4{+}2]$, so organic chemists use "ketone equivalent" dienophiles that behave like a normal alkene in the Diels-Alder step and only reveal the carbonyl afterwards, on workup.

The target is the norbornenone skeleton, bicyclo[2.2.1]hept-5-en-2-one, i.e. cyclopentadiene's Diels-Alder adduct with a dienophile that eventually becomes a plain $\mathrm{C{=}O}$ group.

1. (A): dichloroacetyl chloride plus base generates dichloroketene in situ. Real ketenes give $[2{+}2]$ adducts (four-membered rings), not the bridged $[2.2.1]$ skeleton, so even after removing the chlorines with Zn/AcOH the ring size is wrong. Rejected.
2. (B): nitroethylene is a normal, very reactive dienophile (the $\mathrm{NO_2}$ group activates it), so the Diels-Alder step is clean $[4{+}2]$. $\mathrm{TiCl_3/H_2O}$ is a standard reagent for turning a secondary nitro group into a ketone (a Nef-type reduction). This delivers the target directly. Accepted.
3. (C): 2-acetoxyacrylonitrile is the textbook masked-ketene dienophile, an acetylated cyanohydrin of a ketene. It undergoes normal $[4{+}2]$ cycloaddition, and base hydrolysis afterwards strips off the acetate and then the cyanide (reverse cyanohydrin formation), unmasking the same carbonyl. Accepted.
4. (D): the dienophile here is a chloro-substituted acryloyl chloride; the Diels-Alder step works, but base hydrolysis converts the acid chloride into a carboxylic acid/carboxylate group, not a ring ketone, so the final functional group is wrong even though the ring skeleton is right. Rejected.

Let's summarize:

• A dienophile has to look like a normal alkene during the cycloaddition and only reveal the carbonyl on workup, since ketenes themselves give $[2{+}2]$, not $[4{+}2]$.
• Nitroethylene (with TiCl3/H2O) and 2-acetoxyacrylonitrile (with NaOH/H2O) are both classic ketone-equivalent dienophiles; an acid chloride dienophile gives an acid, not a ketone.

So the reactions that give the target compound are (B) and (C).