Totally Synthetic by Paul H. Docherty, 28 January 2009
Total Synthesis of FR901483
C. A. Carson, M. A. Kerr, Org. Lett. 2009, 11, 777-779.
FR901483 has a pretty decent biological profile, and with a phosphate group, no surprises that it interacts with the DNA machinary. I’m not going to discuss this, but it’s certainly a worthy target; with syntheses by Snider, Sorensen and Fukuyama (amongst others), that are already out there.
So it’s going to take quite a bit of work to bring something new to the table, but Kerr makes a pretty good start with this enantioselective deprotonation. Again, it’s worth checking out Peter O’Brien’s work in this area, and probably this Publication in Tetrahedron Lett. by EJ Corey. This proceedure established asymmetry at a quaternary centre with apparent ease.
The silyl-enol ether was then quenched with NIS to give the α-iodo ketone (as a mixture of diastereomers, but this isn’t relevant), which participated in a pretty complex Reformatsky coupling. I say this as there are several factors - the aldehyde partner is chiral, so has a bearing, the iodide is also chiral, and the conformation of the cyclohexanone is flexible too. Kerr doesn’t try to rationalise the reaction too much - he was just happy to get a 4:1 mixture of diasteroisomers.
A few more steps - a substrate controlled reduction and protecting group manipulations - and it was time to crack that cyclopropane open. Kerr’s been doing work on this kind of chemistry for years, with several publications on it, so there’s no real surprise that this works. A bit of formaldehyde gives the N-methylene intermediate, which does a stepwise opening of the cyclopropane (nice, stabilised malonate helping out a bit here). The malonyl anion then shuts down on the imine, building that pyrolidine nicely.
Seven more steps took the synthesis to fruition, but there were a few
issues along the way. Mono-decarboxylation of malonate was a bit dissapointing,
as no asymmetry could be induced at the common centre. This was saponified,
underwent a Curtius
reaction, protection with a Boc group in-situ, and methylated, giving them an
intermediate that was close enough to those previously prepared that ‘formal’
methods were use to complete the synthesis.
Nice work on a compact target.