[Radu]

    Hello,
 
    Can anyone explain me why treating 1,5 hexadiin-3-tosylate with DBN leads to cis-1,5heaxdiin 3 ene?  I mean, I have written the most stable conformations with anti-perriplanar elimination, but -C≡CH groups are sin-intercalated in both cases.....so both trans and cis should be obtained...

    Thanks!

[discodermolide]

I wonder if you are getting firstly elimination of the tosylate to produce an allene type intermediate followed by H abstraction to regenerate the acetylene and give the cis olefin. I have drawn it out in the picture:

[Radu]

  Thank you, now it makes sense, DBN is a bulky base and it reacts from the less hindered side \