Complexation of Chiral Zinc-Porphyrin Tweezer with Achiral Diamines: Induction and Two-Step Inversion of Interporphyrin Helicity Monitored by ECD

We report here the synthesis of a new chiral Zn­(II) bisporphyrin <i>tweezer</i> in which two achiral Zn­(II) porphyrin moieties are covalently linked by (1<i>R</i>,2<i>R</i>)-diphenylethylenediamine, which produces a strong chiral field around the porphyrin moieties. The chiral <i>tweezer</i> exhibits not only intensity modulation in UV–vis and CD exciton couplets but also a dramatic change, namely, the inversion in the sign of the interporphyrin helicity upon binding of achiral diamines of varying lengths. The stoichiometry-controlled formation of a 1:1 sandwich complex followed by a 1:2 open complex was realized with smaller achiral diamines (<i>n</i>: 2–5) at their low and high concentration regions, respectively, leading to two-step inversion of chirality. With longer achiral diamines (n: 6–8), however, only 1:1 sandwich complexes are formed with no change of sign in the CD couplet. As compared to a 1:2 open complex, a 1:1 sandwich complex shows an enhanced CD response as two porphyrin units come closer in space. Structural insights of the host–guest complexes have been obtained spectroscopically along with molecular mechanics minimizations with the newly implemented OPLS-3 force field followed by geometry optimization using density functional theory of the most stable conformer. The amide bridge in the Zn­(II) bisporphyrin has a low rotational barrier, which provides conformational flexibility to change interporphyrin helicity between 1:1 and 1:2 binding depending on the size of the achiral guests in order to minimize host–guest steric interactions

Absolute Configurations of Fungal and Plant Metabolites by Chiroptical Methods. ORD, ECD, and VCD Studies on Phyllostin, Scytolide, and Oxysporone

The absolute configuration (AC) of the bioactive metabolites phyllostin (<b>1</b>) and scytolide (<b>2</b>), two hexahydro-1,4-benzodioxines produced by <i>Phyllosticta cirsii</i>, and oxysporone (<b>3</b>), a dihydrofuropyranone recently isolated from a strain of <i>Diplodia africana</i>, has been assigned by computational analysis of their optical rotatory dispersion (ORD), electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) spectra. Computational prediction of ORD, ECD, and VCD allowed us to assign (3<i>S,</i>4a<i>R,</i>8<i>S,</i>8a<i>R</i>) AC to naturally occurring (−)-<b>1</b>, while (4a<i>R,</i>8<i>S,</i>8a<i>R</i>) AC was assigned to (−)-<b>2</b> employing only ECD and VCD, because in this case ORD analysis turned out to be unsuitable for AC assignment. Theoretical prediction of both ORD and ECD spectra of <b>3</b> led to assignment of (4<i>S,</i>5<i>R,</i>6<i>R</i>) AC to (+)-<b>3</b>. In this case a satisfactory agreement between experimental and calculated VCD spectra was obtained only after taking into account solvent effects. This study shows that in the case of flexible and complex natural products only a concerted application of more than a single chiroptical technique permits unambiguous assignment of absolute configuration