Lutnaes Bjart Frode, Bruås Liv, Kildahl-Andersen Geir, Krane Jostein, Liaaen-Jensen Synnøve
Department of Chemistry, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway.
Org Biomol Chem. 2003 Nov 21;1(22):4064-72. doi: 10.1039/b307531a.
The reaction between beta,beta-carotene and BF3-etherates has been investigated, leading to structural elucidation of the blue product, formed in appropriate organic solvents, as a symmetrical charge delocalised dication (lambda(max) 985 nm at room temperature in CHCl3) with considerable stability. The reaction, monitored by EPR studies at -25 degrees C, occurred via free radical intermediates. A C40H56BF3 intermediate was captured by EIMS. The detailed structure of the dication was established by COSY, HSQC, HMBC and 1D and 2D ROESY NMR techniques (600 MHz, CDCl3, -20 degrees C) leading to complete assignments of 1H and 13C chemical shifts and 3J(H,H) coupling constants. The effects of the two delocalised charges on chemical shift (charge distribution) and bond distance (3J(H,H)) were considered. The results are consistent with charge delocalisation mainly in the C-5-C-9 and C-5'-C-9' regions and with bond inversion to retro shifted double bonds in the central C-13-C-13' region. A convention for denoting the charge delocalisation and bond types is presented. The experimental results are discussed relative to previous theoretical calculations of the beta,beta-carotene dication structure. (All-E) and (15-Z)-beta,beta-carotene provided the same dication. The NIR spectra and stability of dications prepared in the same manner from the related carotenes 20,20'-dinor-beta,beta-carotene, heptapreno-beta,beta-carotene and nonapreno-beta,beta-carotene were examined for comparison. Reactions of the beta,beta-carotene dication with selected nucleophiles provided products including isocryptoxanthin, isocarotene and mutatochrome with H2O as nucleophile, and isocryptoxanthin methyl ether, 8-methoxy-7,8-dihydro-beta,beta-carotene and isocarotene with CH3ONa as nucleophile. The formation of these products is rationalised from the structure assigned to the dication.
对β,β-胡萝卜素与三氟化硼醚化物之间的反应进行了研究，从而阐明了在合适的有机溶剂中形成的蓝色产物的结构，该产物是一种具有相当稳定性的对称电荷离域双阳离子（在室温下于CHCl₃中λ(max)为985 nm）。通过在-25℃下的电子顺磁共振（EPR）研究监测该反应，反应通过自由基中间体进行。通过电子轰击质谱（EIMS）捕获到了C₄₀H₅₆BF₃中间体。利用COSY、HSQC、HMBC以及一维和二维ROESY核磁共振技术（600 MHz，CDCl₃，-20℃）确定了双阳离子的详细结构，从而完成了¹H和¹³C化学位移以及³J(H,H)耦合常数的全归属。考虑了两个离域电荷对化学位移（电荷分布）和键距（³J(H,H)）的影响。结果表明电荷主要在C-5-C-9和C-5'-C-9'区域离域，并且在中心C-13-C-13'区域存在键反转形成回移双键。提出了一种表示电荷离域和键类型的惯例。相对于之前对β,β-胡萝卜素双阳离子结构的理论计算，讨论了实验结果。（全-E）和（15-Z）-β,β-胡萝卜素产生相同的双阳离子。为作比较，研究了由相关胡萝卜素20,20'-二降-β,β-胡萝卜素、七异戊二烯基-β,β-胡萝卜素和九异戊二烯基-β,β-胡萝卜素以相同方式制备的双阳离子的近红外光谱和稳定性。β,β-胡萝卜素双阳离子与选定亲核试剂的反应产生了多种产物，以H₂O作为亲核试剂时生成异隐黄质、异胡萝卜素和变色素，以CH₃ONa作为亲核试剂时生成异隐黄质甲醚、8-甲氧基-7,8-二氢-β,β-胡萝卜素和异胡萝卜素。这些产物的形成可根据双阳离子的结构进行合理说明。
