Sudden Tamiflu

Hey there, JLC fans! Long time, no post.

Today's inspiration comes from a molecule I've enjoyed watching the synthetic, process, and educational chemistry communities go to town on for the past 18 years: (-)-oseltamivir, also known by the trade name Tamiflu. See that cyclohexene in the middle? This molecule mimics the transition state of a mostly-flat oxonium cation derived from sialic acid, so well that it interrupts the flu virus's ability to release further infectious particles, allowing the body to "catch up" and reduce overall time spent hovered over a steaming bowl of chicken noodle soup.

But enough about the Why, let's get to the How; as the authors of this paper pitch in their opening line: Time is Money!

Hayashi and Ogasawara, no strangers to Tamiflu themselves (syntheses in 2009, 2010, and 2013), report in Org. Lett. ASAP a synthetic economy we don't always consider: time. They claim a one-pot Tamiflu synthesis, five steps, average step yield around 70%...finished in just 1 hour.*

Compare that against the benchmark of the previous 1-pot reaction - 57 hours. Wow!

Several interesting modifications to their previous syntheses have enabled this savings. First, addition of a hydrogen-bond donor catalyst to a nitro-Michael addition, which accelerates the reaction 3x. Second, swap of a base in an HWE reaction: 3.5 hours in cesium carbonate becomes 20 mins in tBuOK. Finally, a sacrificial move - rapid epimerization of the penultimate nitrohexene, knowing that only 50% of the product will successfully reduce to the desired diastereomer.** A short silica column completes the rapid realization of this antiviral drug.

Lest you believe that I'm hornswoggled by a synthetic sprint, a quick glance at the Supporting Info*** provides a cold shower. There's some lingering impurities in those 1H NMRs, enough to make me believe that the yield, even at gram-scale, isn't really 16%. And the reagent equivalencies used to drive these nitro-boosters aren't pretty: 30 parts Zn, 3 parts phosphonate, 15 aliquots of TMSCl. To touch upon an allusion the authors themselves make - perhaps a Jamison-style flow reactor is the next logical step for this speed-demon of a route.

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*As I wrote this line, I couldn't stop comparing this to other "just 1 hour!" claims from consumer products: teeth whitening, photo development, tax preparation, LensCrafters, oil changes, and pizza delivery can all refer to the 2016 Hayashi Tamiflu synthesis as a spiritual brother of sorts.

**I don't believe that there's a spontaneous kinetic resolution here, but if any sharp-eyed reader can prove differently, I'm all ears.

***Counter-counterpoint: The one-pot is so streamlined that it now takes only 281 words to synthesize Tamiflu. 
Damn.

Bruceollines: Short and Sweet

You can't put your finger on it, but sometimes you just feel compelled to read a paper. This one, from Org. Lett. ASAP, scratched all the usual itches: protecting-group free total synthesis (check), traditional medicine (check), tropical diseases (check), and cool off-the-shelf reagents (check).

I must admit, Gordon Gribble's name at the top caught my attention, but his co-authors hail from the University of the West Indies, a school I've always been curious about.*

The goal? Fast, selective production of bruceollines, medicinal compounds isolated from the roots of a Chinese shrub. The authors initially try to assemble the common indole (the 6-5 aromatic ring) core of the bruceolline family using Fischer conditions, but the starting materials have other ideas and form non-productive intermediates. Starting over, palladium catalysis proceeds smoothly, then relatively gentle oxidation (DDQ) produces the fully oxidized bruceolline E (see picture).

To access the final compound, Gribble & Co. must reduce just one of E's two ketones. The authors attempt borane reductions, but the "usual suspects" (CBS, Alpine) fail miserably. Optimization with (+)-DIP-Cl produces the final bruceolline J in high yield and ee. To make the unnatural enantiomer, the authors turn to a personal favorite: Baker's yeast, more commonly found in breads than labs. After 14 days in a warm, sweet slurry, the wee beasties return ent-bruceolline J in 98% ee.

The synthesis, only 4 short steps, should open the door to develop new antimalarial compounds.


*First, the Hawaii paradox - recruiting high-end, serious grad students to work 4-6 years in a tropical paradise. How does that work? And how do shipping delays from the mainland impact project selection? I can imagine that protecting group-free, relatively robust chemistry would have to be the norm, to survive storms, delays, humidity, etc.

Let the Sun Shine In

Have you ever wondered why so many chemicals are shipped in amber-tinted bottles? Turns out light can do some pretty sneaky unexpected chemistry.

Reporting in from scenic Switzerland, Gademann & Co recently disclosed a sweet protecting-group free synthesis of Taiwaniaquinol F, a 6-5-6 diterpenoid with "potent cytotoxicity" against certain cancer cell lines. They finish the target in 17 steps, highlighting a Wolff ring-contraction and a neato halogen-oxygen exchange. 

The authors note that the final isolation had to strictly exclude light, or else their product slowly decomposed. Luckily, they asked themselves "Decomposed...to what?"

Source: Org. Lett. ASAP | Gademann group

Not to keep you in the dark; their product underwent a spontaneous 1,5 C-H remote functionalization. When the group tried to do it "on purpose" - placing a flask in the sun for 15 minutes - they were rewarded with a 30% yield of cyclized Taiwaniaquinol A.

The authors aren't quite sure what's so special about this system, but note a tantalizing biosynthetic possibility: since Taiwaniaquinol F comes from the bark of a certain tree, and "A" from its leaves, there's a real possibility that this process happens all the time in the plant! No enzymes, no metals, no bases. Just sunshine. 

Nature's amazing.