Snapper, Williams and Deak. JACS, 2007, 129, 486. DOI: 10.1021/ja0674340.
An extraordinary synthesis of this simple-looking guaiane natural product (along with it’s exo-methylene analogue, pleocarpenene), Marc Snapper has once again used deft control of pericyclic reactions to produce some extremely odd looking intermediates in this synthesis. As regular readers will know, I tend to use a disconnection-based retrosynthesis to explain the synthetic plan, but as you can see, Snapper’s approach makes this a little tricky!
From this retro, it’s clear that the double ring-expansion (is that the correct term?) is key to this synthesis, but that 3,4,4,5 ring system looks just a little strained to me, and to my eyes, it’s construction is far from trivial. However, Snapper used some incredibly funky iron chemistry to build those cyclobutanes, performing a photochemical cyclisation to build the first ring (a cyclobutadiene), and then inserting iron to stabilise it. Functionalisation of the exocyclic ester using fairly standard chemistry (and a oxidation/CBS reduction sequence to introduce asymmetry) left them with a free alkene, which they were able to add to the freed butadiene in one step. This generated the second four-member, along with the cyclopentane, in good yield, and with an reasonable excess of the preferred diastereoisomer.
The final elaboration to produce the rearrangement substrate was cyclopropanation of the butene ring; I thought that this might be a tricky step, but it really didn’t faze them at all:
Copper catalysed elimination of the diazo group left the carbene, which added very efficiently to the butene, generating that amazing 3,4,4,5 ring system, and after oxidation and treatment with methyl grignard, the target substrate. A bit of (somewhat less than gentle) thermal persuasion then snapped those strained rings open, inverted the stereocentre at C1, completed the carbocyclic skeleton. More mundane chemistry was then applied over a couple of steps, completing the synthesis of the target, and my favourite paper of 2007 so far.