Totally Synthetic by Paul H. Docherty, 6 February 2010
Total Synthesis of Phostriecin & Sultriecin
C. P. Burke, N. Haq, D. L. Boger, J. Am. Chem. Soc. 2010, 132, 2157-2159.
Figured it out already? I bet this caused some headache in Dale Boger’s group when having made the published structure for Sultriecin and things didn’t work out. We’ve discussed quite a few reassignments here over the years, but the majority are antipodal stereoclusters, or isomerisations at worst. In this case, even the molecular formula was wrong! The group may well have been suspicious before this point, though, as the related fostriecin, cytostatin and phospholine all contain the phosphate-mono ester that eventually replace the sulfate group initially assigned. Their journey in reassigning this centre is best read in the pdf, but the synthesis is pretty good too!
First up is a fairly simple coupling of an acetylene and an acyl chloride. Now, I would have tried out BuLi in this case - but not Boger, who used palladium catalysis. This certainly has an advantage from a ‘mildness’ point of view, but I'm not sure why it was necessary. Perhaps the acetylide could have reacted with the furan? BTW, it’ll come as no surprise to those readers who know their coupling reactions that the reference given is a Sonogashira paper.
I’m only really mentioning that coupling in passing, as the interesting reaction is one that follows after a pair of reductions. The freshly installed hydroxy-furan underwent ring expansion using just a base and NBS. No explanations or further information here, but it is a nice method! With the left-hand-side looking ready, it was time to tackle the other half, containing a rather precarious looking triene. However, before you try to combine a whole load of Suzuki/Stille methods for it’s construction, have a look at Richard Taylor’s approach - opening of pyrilium tetrafluoroborate with an alkyl lithium to generate the dienal. Dibromoolefination using carbon tetrabromide and triphenyl phosphene completed the triene, a reduction furnished the desired bromo-coupling partner. What a sweet way to configure your olefins.
Then, predictably, came the coupling itself, using a rather surprising approach of lithium/halogen exchange, and then addition into the aldehyde partner. This had the advantage of conferring a pretty decent diastereomeric ratio on the new hydroxyl group, something unachievable in the Suzuki disconnection I’d have been planning. It’s pretty brave, though, as the lithiate looks prone to rearrangement. At least it’s not skipped-conjugation…
A really neat piece of synthesis coupled to a remarkable reassignment. I’m amazed that this error was made by the isolation team - the fact that sulphate and phophate-mono-esters are equal in mass is well known. 31P NMR would have been helpful!