B.R.S.M. Help! I'm trapped in a molecule factory!


Schindilactone A

Here's an impressive total synthesis of schindilactone A by Tang, Chen, Yang,[1] and 14 coworkers. At 29 steps in the longest linear sequence that's comfortably fewer than two per author. Still, the route is entirely linear and it's a fairly heroic effort, as we'll see.

Work began sometime ago as the group published syntheses of the ABC (Org. Lett., 2006, 8, 107) and FGH (Org. Lett., 2005, 7,885) fragments of the slightly more complex micrandilactone A some time ago. Apparently that unique, ketal spanned, 7-8 carbocyclic system in the middle took some time to work out. I'll cover the older work on those fragments as well, as it shows the origins of some of the key steps in the schindilactone A total synthesis.

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Named Reactions: The Hugershoff Reaction

Now over a hundred years old,[1] the venerable Hugershoff reaction is a great way to convert aryl thioureas to 2-aminobenzothiazoles.[2] Classically, an aryl thiourea in chloroform is treated with an equivalent of bromine at around room temperature then the product is just filtered off. The first step is believed to be bromination on sulfur, followed by electrophilic aromatic substition in the usual manner. Yields are generally good (in my experience), but it's important to get the electronics of the ring right - some activation is usually required, but if it's too electron rich then bromination on the aromatic ring can compete with bromination of the thiourea.

There's actually very little information on the reaction available through casual googling - it has no wikipedia page and I don't believe it's even been reviewed.[3] You're not limited to elemental bromine, with NBS and other halogenating reagents reported to work. A similar transformation (via a different mechanism) can be accomplished by treating aryl thioureas with iodine(III) reagents, or even DDQ.

Filed under: Named reactions, Serious | 6,507 views | 3 comments Continue reading