Systematic Strategy for the Synthesis of Cyanobacterin and Its Stereoisomers. 2. Asymmetric Total Synthesis of the 2- and 3-Epimers of Dechloro-cyanobacterin

Abstract

Stereocontrolled total syntheses of the (2S,3R)- and (2R,3S)-isomers of the non-chlorinated analog of cyanobacterin, a potent photosynthesis inhibitor, were achieved. Since both the (2R,3R)- and (2S,3S)-isomers of this compound had been previously synthesized from the same starting material, a systematic strategy for the synthesis of all stereoisomers could be established.

Synthesis and Algicidal Activity of (+)-Cyanobacterin and Its Stereoisomer

“…After refluxing for 4.5 h, the reaction mixture was cooled again to room temperature, poured into water, and extracted twice with CH 2 Cl 2 . After drying (Na 2 SO 4 ) and removing the solvent, the oily residue was chromatographed on silica gel eluted with benzene:CH 2 (9). To a cooled (À78 C) solution of selenide 7 (1.30 g, 4.50 mmol) in dry THF (20 ml) was added a 2M heptane/THF/ethylbenzene solution of LDA (2.5 ml, 5.0 mmol), and the mixture was stirred for 1 h. A solution of 6 (0.64 g, 2.10 mmol) in THF (10 ml) was added dropwise and the whole was stirred for 1 h before being quenched with a 1 M HCl solution (20 ml).…”

“…The first total synthesis of racemic 1a was achieved by Jong et al 3) in 1984, and recently Haga et al 8,9) reported an asymmetric synthesis of both enantiomers of a dechlorinated analog of cyanobacterin and formal total synthesis of (+)-1a by employing Evans's asymmetric alkylation as the key step.…”