Remarks on the sequence of quasi-conformal mappings

Monomeric Dihydroanthraquinones: A Chemoenzymatic Approach and its (Bio)synthetic Implications for Bisanthraquinones

“…[15c] Accordingly,w ep ropose an alternative pathway for the conversion of 5 into 6,i nw hich aM ichael-type addition in the presence of ab ase gives 16,w hich subsequently oxidizes in air to form 17.A nother intramolecular Michael addition in 17 finally results in the formation of the thirdC ÀCb ond to give (À)-rugulosin (6). Similarly,t reatment of (À)-lunaskyrin (14) with pyridine under oxygen atmosphere resulted in the formation of (À)-dianhydrocytoskyrin (18)a nd (À)-2,2'-epi-cytoskyrin A( 21), in 12 and 60 %y ield, respectively (Scheme 1). The spectroscopicd ata of 21 match those of (+ +)-2,2'-epi-cytoskyrin A isolated from the endophytic fungus Diaporthe sp., [22] except for the opticalr otation, which was expected based on the (R)configuration of 11 (see the Supporting Information).T he same reaction under argon atmosphere led to the isolation of 19;h owever,a ll attempts to isolate intermediate 20 failed because it undergoes facile Michael addition to form 21.The chemoenzymatic synthesis of (À)-lunaskyrin (14)a nd (À)-2,2'-epicytoskyrin A( 21)d escribed here points towardt heir putative existence as naturalp roducts which have yet to be isolated (Scheme 1).…”

“…[13] Another related biosynthetic intermediate, dihydroemodin (4a), has been proposeda sb eing involved in the biosynthesis of (À)-flavoskyrin (5)a nd (À)-rugulosin (6), isolated from P. islandicum,a nd (+ +)-rugulosin (7), isolated from P. rugulosum. [14,15] The proposal was based on as eries of monomeric anthraquinones synthesized by Zahn and Koch almost 80 years ago, which tend to dimerize through ah etero-Diels-Alder reaction into flavoskyrin-type dimericc ompounds. [15b, 16] This has been reflectedi nt he biomimetic synthetic strategies to bisanthraquinones,e videntf rom the cascade syntheses of (+ +)-rugulosin (7)a nd its analogues developed independently by the groups of Nicolaou [17] andS nider, respectively.…”