Encore, Encore!

Two hot areas of research served up second helpings online this week:

C-H Azidation: Remember John Hartwig's iron-meets hypervalent iodide combination from last March? It possessed the power to insert a late-stage amine equivalent into complex natural products. John Groves has raised the stakes, disclosing a "practical and complementary" Mn-porphyrin promoted version that takes solid sodium azide as the precursor. 

Source: Groves, JACS ASAP

The group finds it can enable late-stage azidation of a variety of complex bioactive substances (sclareolide, artemisinin, estrone, papaverine). Even more surprisingly, although likely a radical-induced transformation, using a chiral salen led to a single example of 70% ee material. Groves admits they have work to do, but the fact that this reaction operates with 1% loading in wet ethyl acetate at room temperature sure sounds promising!

Synthesis Machines: Over at Nature, Kobayashi published a flow reactor approach to syntheses of either enantiomer of rolipram, an anti-inflammatory. No MIDA-boronate 'handles' here; this is classic chemistry - olefination, 1,4 addition, reduction, hydrolysis, decarboxylation, cyclization - performed over heterogeneous catalyst beds encased in stainless steel tubes. The group spices up the synthesis by including their in-house chiral PyBOX-calcium catalyst to control the 1,4 addition, and developing a Pd / polysilane-catalyzed reduction for a troublesome nitro group. 

Kobayashi claims his synthetic engine can produce a gram of 96% ee material every 24 hours, and that the system remains stable and operable for about a week's time. In a complementary Commentary, Joel Hawkins of Pfizer presents a tantalizing future, where hood-sized continuous synthesis units chug through kilo quantities of drug precursors, using commercial reagents, sans column chromatography.

Greener Nylon Synth? Just Add UV and Ozone!

"Any sufficiently-developed technology is indistinguishable from magic" - Arthur C. Clarke

Looks like we'll soon have a more straightforward way to make stockings, zip-ties, and tire belts. 

Adipic acid, a six-carbon diacid representing one of the "sixes" in Nylon 6-6, apparently takes quite a bit of industrial "elbow grease" to make. The current process, starting from cyclohexane, requires cobalt, manganese, copper, and vanadate salts, high pressures of oxygen gas, and hot nitric acid. Out the other side, its responsible for 5-8% of the nitrous oxide we humans spew into the atmosphere each year.

From Science 2014, Hwang and Sagadevan

Now, researchers Hwang and Sagadevan (National Tsing Hua University, Taiwan) believe they have a better method. Reporting in this week's Science, the two disclose a method that sounds so much simpler: flush a sample of cyclohexane with ozone and UV light, and, presto! Solid adipic acid at the bottom of your reactor. No metal salts, no nitrous oxide, no high pressures or temperatures.

Wow, that looks a lot simpler.

The researchers note that zapping ozone produces both singlet oxygen, ¹O₂, and a single singlet oxygen atom O(¹D). The highly reactive single singlet (say that three times fast!) can easily insert into C-H bonds, and, since it seems to prefer insertion next to an already-oxidized carbon, the diol, diketone, and finally diacid products are formed preferentially.

Applause, please: Look at this beautiful pictorial SI!
Twice, in two days.

Just for fun, Hwang and Sagadevan crack open some larger hydrocarbons, and check the selectivity of alkyl-functionalized rings and aromatics. There are tantalizing possibilities here that I'm sure, given the ease of this reaction setup, most organic chemists will already be trying: how do complex natural products* react under these conditions? If anyone tries it this weekend, please drop me a line.

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*For that matter, I wonder if this pathway is operative in human tissues under physiological conditions? Sunlight does have some 300 nm band, and we certainly come into contact with ozone out in the wide world. Hmm.

Friday Fun: Sweet Cardamom (Peroxide)

Rice pudding. Ginger snaps. And...malaria?

That's what'll be going through my head next time I cook with cardamom, thanks to Tom Maimone and coworkers (UC-Berkeley) and their under-the-wire JACS ASAP from yesterday afternoon. The title and abstract scratch all the Baran lab alumni itches: 1) biosynthetically inspired, 2) novel mechanisms, 3) scalable, 4) just four steps! And hey, we're making stable endoperoxides, which all the cool kids are into nowadays.



Not their actual abstract graphic...

As Maimone points out, the latent symmetry of the final product offers a really neat assembly strategy. The group McMurrys together two units of (-)-myrtenal, then hits it with singlet oxygen, initially forming a 6-membered endoperoxide they fragment / rearrange with base. A gentle oxidation (DMP) sets them up for the wild step: stitching together a 7-membered endoperoxide using Mn(III)*, a radical source, a silane reducing agent, and even more oxygen. Simple phosphine reduction knocks down the last hydroperoxide into an alcohol, and the whole target (7 stereocenters!) falls out as a single stereoisomer.

Pretty sweet.

P.S. - Since the group's made over half a gram in just this first push, I'd assume an efficacy paper against live Plasmodium parasite hot on the heels of this one...

*We're apparently already calling this the "Shenvi catalyst"...wasn't this only two months ago?