Easy aryl reductions
The Birch reduction has been widely used for more than half a century to achieve partial reduction of aryl rings by alkali metals at just two diametrically opposed carbon sites. However, the conditions require condensation of caustic gaseous ammonia. A variation developed soon afterward by Benkeser used safer liquid ethylene diamine but was prone to overreduction. By diluting ethylene diamine in tetrahydrofuran solvent, Burrows
et al
. now obtain selectivities comparable to Birch conditions but without the need for condensed ammonia. —JSY
Herein, we describe the first total synthesis of cochlearol B, a meroterpenoid natural product featuring a 4/5/ 6/6/6-fused pentacyclic structure. Key steps, oxidative cyclization and subsequent intramolecular [2+2] photocycloaddition, which constructed the pentacyclic structure in highly stereoselective manner, allowed efficient access to cochlearol B with the longest linear sequence of 16 steps, and in 9 % overall yield. Single-crystal X-ray crystallographic analysis clearly confirmed the stereochemistry of cochlearol B.
Total syntheses of
pyocyanin, lavanducyanin, and marinocyanins
A and B have been accomplished. The N-substituted
phenazin-1-one skeleton, a common framework of these natural products,
was constructed through the oxidative condensation of pyrogallol with N-substituted benzene-1,2-diamine under an oxygen atmosphere
in a single step. Regioselective bromination with N-bromosuccinimide at the C-2 position of N-alkylated
phenazin-1-ones afforded brominated natural products.
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