Everyone loves neat tricks in synthesis that are not as easy to spot in retrosynthesis as oxidations or reductions; one of these I came across recently is the conversion of furans to tetrahydropyrans, the Achmatowicz reaction, the seminal publication of which was in 1971 in Tetrahedron:
This reaction was used in a few total syntheses; besides those mentioned in the Wikipedia article, I particularly like O'Doherty's synthesis of the indolizidine (-)-D-Swainsonine published in Org. Lett. The four carbons that will form the five-membered ring of the bicyclic system was formed using the Achmatowicz reaction. 2-Lithiofuran opened the gamma-butyrolactone to install the alpha-oxygen; a TBS protection allowed for asymmetric Noyori reduction of the ketone, installing the necessary stereocenter for the substituent on the tetrahydropuran for the subsequent steps. The Achmatowicz was achieved using NBS.
Other than modifications to the ring's functional groups, the carbonyl was converted to an azide in several steps which was used in the second to last, pivotal step of the synthesis, closing the 5-membered ring via reductive cyclization to complete the indazolidine ring system. (The benzyl group is hydrogenated off, allowing the isomerization to the aldehyde which is then attacked by the nitrogen and the oxygen eliminated.) That makes for two rearrangement-type reactions to form the final product - I won't get into overall strategy and the greater story of the synthesis of the molecule and its enantiomer, but I just wanted to point out how the Achmatowicz was used creatively to make a pretty neat molecule.
KC Nicolaou is using this extensively to make maitotoxin building blocks. Works much faster than from sugars, gives pretty much any stereo you want.
ReplyDeleteLove this reaction..
ReplyDeleteThanks for sharing!
Interesting post, it sure seems to be a nice combination.
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