3 research outputs found
11-Step Total Synthesis of Araiosamines
A concise route to
a small family of exotic marine alkaloids known as the araiosamines
has been developed, and their absolute configuration has been assigned.
The dense array of functionality, high polarity, and rich stereochemistry
coupled with equilibrating topologies present an unusual challenge
for chemical synthesis and an opportunity for innovation. Key steps
involve the use of a new reagent for guanidine installation, a remarkably
selective C–H functionalization, and a surprisingly simple
final step that intersects a presumed biosynthetic intermediate. Synthetic
araiosamines were shown to exhibit potency against Gram-positive and
-negative bacteria despite a contrary report of no activity
Switching Off the Charge Transfer and Closing the S<sub>1</sub>–T<sub>1</sub> ISC Channel in Excited States of Quinolizinium Derivatives: A Theoretical Study
Excited states of benzoÂ[<i>b</i>]Âquinolizinium (<b>BQ</b>) derivatives that show efficient
pH-responsive fluorescence
switching properties were studied quantum-chemically by employing
the CASSCF/CASPT2 and TD-DFT methods. Protonation of aminophenyl-<b>BQ</b> at the electron-donor amine moiety converts the nitrogen
lone pair into a σ bond and the HOMO into a lower-lying orbital
that is no longer involved in the excitation, thereby rationalizing
the suppression of the charge transfer. An S<sub>1</sub>–T<sub>1</sub> seam between the vertically excited Franck–Condon
(FC) point and the S<sub>1</sub> equilibrium geometry favors intersystem
crossing (ISC). The T<sub>1</sub> state of the protonated form remains
well below S<sub>1</sub> (1.5 eV) because of favorable exchange interactions,
whereas the T<sub>1</sub> state of the unprotonated form does not
experience any analogous stabilization because of the difference in
the spatial domains of the singly occupied orbitals in the S<sub>1</sub> and T<sub>1</sub> states. The S<sub>1</sub> surface from the FC
point until the equilibrium geometry for the protonated species is
energetically downhill. Calculations on models and available experimental
data suggest design principles for similarly functioning pH-responsive
species, namely, an amine lone pair as the electron donor and a cationic
ring of moderate size as the electron acceptor that are structurally
separated by virtue of a spacer
Stereochemical Determination of the Leupyrrins and Total Synthesis of Leupyrrin A<sub>1</sub>
The
stereochemical determination of the potent antifungal agents
leupyrrin A<sub>1</sub> and B<sub>1</sub> and the total synthesis
of leupyrrin A<sub>1</sub> are reported. The relative and absolute
configuration was determined by a combination of high field NMR studies,
molecular modeling, and chemical derivatization. The expedient total
synthesis involves a one-pot sequential Zr-mediated oxidative diyne-cyclization/regioselective
opening sequence for preparation of the unique dihydrofuran ring,
a highly stereoselective one-pot approach to the butyrolactone, a
challenging sp<sup>2</sup>–sp<sup>3</sup> Suzuki coupling and
a high-yielding Shiina macrolactonization