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Sordarin   

6 May 2006 6,161 views 2 Comments

Sordarin_2.jpg

Narasaka, Chiba and Kitamura. JACS, 2006, ASAP, DOI: 10.1021/ja060408h

Tot. Syn. loves oxidative radical cyclisations, so Narasaka’s synthesis of Sodarin was a welcome read this week. Also containing a ß(1,2-cis)-selective glycosidation and a Tsuji-Trost allylation to construct the bicyclic system, this is a particularly ambitious route. First to the radical chemistry:

Sordarin.jpg

Using diethylzinc and diiodomethane to generate the cyclopropane, exposure to catalytic quantities of silver nitrate opened the cyclopropane homolytically, oxidising the hydroxyl and leaving a beta radical. This then closed onto the terminal olefin in a 5-exo-trig manner to leave the fused 7,5 system in 85%. Of course, a stoichiometric oxidant was needed, and in this case, was (NH4)2S2O8.

To construct the bicycle they used an impressive Tsuji-Trost allylation:

Sordarin_3.jpg

With the end now in sight, they screened conditions for the glycosidation using neopentyl alcohol as a model for their system, and settled upon Mukaiyama conditions (SnCl2, AgClO4 in Et2O), which gave them a 79% yield of the ß-anomer when coupling with their fluoro-sugar (made in 15 steps from D-mannose).

Phew! What an effort – and a fine read too!

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2 Comments

  • secret milkshake says:

    FYI, Ag(+) in combination with ammonium peroxodisulphate
    generates Ag(2+), a pretty good one-electron oxidant. This oxidant is strong enough to oxidize Mn(2+) all the way up to MnO4(-).

    One can do a radical alkylation of quinones or some electron-deficient heterocycles (pyridines, I think). The alkyl radicals are generated by oxidative decarboxylation of carboxylic acids with the Ag(+)/S2O8(2-) system.

    The above synthesis is absolutely gorgeous – and I am glad that I do not have to work on molecules like this one!

  • NoName says:

    nice ‘catalytic’ oxidative radical generation it is; however, radical termination is reductive (i’m just trying to be difficult today). i guess Bu3SnH would do the same job in a nasty way (it’s getting fuzzy).

    behind such awesome IM Tsuji-Trost allylation (possible because 7-membered ring has so many degenerate ground states), there is a real-deal in alpha alkylation of ketone (difficult for the same reason above). Smartly enough (narasaka, Os to B), they used local geometry to get high selectivity from hydrazone substrate; the choice of KCN’s paper today alludes it? if so, nice shot!