文摘
From a complex mixture of mono- and di-2-anthracenecarboxylic acid (AC) esters of cyclic nigerosylnigerose (CNN), two monoesters (2B and 6A) and four diesters in which AC was introduced on the transannular B/D (2B2D), adjacent A/B and A/D (6A2B and 6A2D), and same B/B (2B3B) nigerose rings were isolated. Possessing two ACs at distant positions, 2B2D and 6A2D showed negative Cotton effects for the 1Bb band, the intensities of which were stronger than that of 6A. 2B2D and 6A2D slowly photocyclodimerized to give HH dimers 3* and 4 with 57% and 81% HH selectivity, respectively, which were appreciably higher than that for 6A (34%), while the enantiomeric excesses (ee鈥檚) of anti-HH dimer 3* were 2% and 鈭?8%, respectively. In contrast, 6A2B and 2B3B carrying two ACs on adjacent A and B rings or at vicinal positions on the B ring, respectively, exhibited strong positive CD couplets, the amplitudes of which amounted to 97 and 409 M鈥? cm鈥?, respectively. Upon irradiation, 6A2B afforded 3* with 鈭?2% ee and 4 in 96% combined yield, whereas 2B3B gave almost exclusively 3* with 鈭?9% ee in 96% yield, likely as a result of the introduction of two ACs at the vicinal positions of the rigid CNN scaffold.