Totally Synthetic by Paul H. Docherty, 29 August 2010
Total Synthesis of Solandelactone E
A. Robinson, V. K. Aggarwal, Angew. Chem. Int. Ed. 2010, 49, 6673-6675.
Aggarwal’s work with lithiation-borylation-allylation sequences continues to impress me, along with the targets he chooses to make with it. His heritage (working for Stuart Warren at Cambridge) shows up early in the synthesis, with a really neat enantioselective cyclopropane formation. Working from an allylic alcohol derivative, an SAE gave them an epoxide, which after treatment with base and acetonitrile, gave them the cyclopropane. A really nice step, leaving some interesting functionality in the product.
A second fragment, required for the lithiation-borylation-allylboration sequence, also required installation of a stereodefined stannyl group - something I’d have thought would be quite tricky, but was actually done with ease. Using a commercially available starting material, the group formed a carbamate and then did a substrate directed lithiation and capture of the stannyl group. The yield might not be high, but the diastereoselectivity is impressive, so I’m sure they were very happy with this…
With this fragment set to go, they were ready to try out their new methodology. However, the chemistry didn’t go quite to plan - as they only achieved a 15% yield. Analysis of the reaction mixture showed that the lithiated carbamate was attacking the aldehyde directly, and that the reaction with the vinyl borane was incomplete. However, by reordering the addition of TMEDA, they were able to get past this problem and boost the yield to 73%, and in a very respectable diastereoselectivity.
To complete the molecule, they still had a few steps to go; firstly they did a selective epoxidation of the allyl silane using Ti(OiPr)4/t-BuOOH. Treatment of this fragile intermediate with acid promoted a rearrangement and delivery of the target. Top work - and I’m looking forward to seeing this methodology again!