[Shinza]

My problem is the following synthesis:
butadiene to 1-cyclohexylethanol

I want to start with a diels-alder reaction using 1-butyne and and butadiene.

How would you finish the synthesis?

[macman104]

Use 3-butyne-2-one instead of 1-butyne?  Then just reduce the ketone to an alcohol?

[sjb]

Might you not get the dihydropyran then (from D/A with the carbonyl, rather than the alkyne). I don't have my books or easy access to the literature at the mo', so there's probably something very obviously wrong with my thought.

Maybe homo Diels Alder coupling of 1,3-butadiene, careful oxidation of the monosubstituted alkene to the aldehyde without touching the disubstituted alkene (not sure if this is doable), methyllithium (or equivalent), then reduction of the alkene to the alkane.

By my reckoning, Diels Alder of 1-butyne and 1,3-butadiene would make 1-ethylcyclohexa-1,4-diene which seems a bit of a dead end at the minute...

S

[macman104]

Might you not get the dihydropyran then (from D/A with the carbonyl, rather than the alkyne). I don't have my books or easy access to the literature at the mo', so there's probably something very obviously wrong with my thought.

I believe if the not in the presence of the triple bond, you may get such a reaction.  But in the presence of the triple bond, the carbonyl acts to enhance the triple bond's dienophile "character" through resonance.  There is such a thing as a hetero diels alder, I just don't know if it will occur in this situation.  Glad someone is at least checking what I write, still learning myself so there's always the chance for mistakes .

By my reckoning, Diels Alder of 1-butyne and 1,3-butadiene would make 1-ethylcyclohexa-1,4-diene which seems a bit of a dead end at the minute...

That is what I saw as well, and I too thought we were sort of at a dead end there...

[Shinza]

Would the synthesis in the image I attached work?

There is one main synthesis (the solid arrow) and 2 possible variations/methods (the dotted arrows).

Along with the reactions being used properly, I am worried about the structure falling apart or the bromine falling off between reactions.


Also, if I were substituting HO- for Br, is it bad to just write HO-, or does the HO- have to come from something?

Note: I meant to use butene, not butyne.

[azmanam]

What's wrong with methyl vinyl ketone?  It's a very common starting material.  I think you're making too much work for yourself.

As far as your 3-proposals... bromination with the double bond still in the molecule will target the double bond, and bromination of the saturated material will lead to very un-selective bromination.

[Shinza]

Wow, if I wrote the ketone to alcohol reaction using NaBH4 properly, I see what you mean about too much work.

[sjb]

By my reckoning, Diels Alder of 1-butyne and 1,3-butadiene would make 1-ethylcyclohexa-1,4-diene which seems a bit of a dead end at the minute...

S

You seem to be missing a double bond after the Diels Alder in your initial sketch (and also after the one with MVK...)

S

[Shinza]

I see, I was thinking about about an older problem that used a triple bond, I meant to start with butene in my initial attempt (not 1-butyne).

So if my third drawing was done properly with a double bond, would NaBH4 also act on the c=c bond to give the desired product, or would I have to hydrogenate somewere before or after using NaBH4?

[agrobert]

You must hydrogenate alkenes with H₂and Pd/C.  NaBH₄ is not strong enough.