2 research outputs found
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 ZnÂ(II) bisporphyrin <i>tweezer</i> in which two
achiral ZnÂ(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 ZnÂ(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