2 research outputs found
Stereoselective Total Synthesis of the Putative Structure of Nitraraine
After
the structure originally proposed for nitraraine was shown
to be incorrect by total synthesis, the alternative structure <b>5</b> was recently suggested for the alkaloid on biosynthetic
grounds and by comparison with the <sup>1</sup>H NMR data of tangutorine.
The unambiguous synthesis of <b>5</b> is reported from tryptophanol
and ketodiester <b>6</b>, via oxazoloquinolone lactam <b>7</b>. However, the melting point and <sup>1</sup>H NMR data of <b>5</b> did not match those reported for the natural product
Carbon Nanotubes as Activating Tyrosinase Supports for the Selective Synthesis of Catechols
A series
of redox catalysts based on the immobilization of tyrosinase
on multiwalled carbon nanotubes has been prepared by applying the
layer-by-layer principle. The oxidized nanotubes (ox-MWCNTs) were
treated with poly(diallyl dimethylammonium chloride) (PDDA) and tyrosinase
to yield ox-MWCNTs/PDDA/tyrosinase <b>I</b>. Catalysts <b>II</b> and <b>III</b> have been prepared by increasing the
number of layers of PDDA and enzyme, while <b>IV</b> was obtained
by co-immobilization of tyrosinase with bovine serum albumin (ox-MWCNTs/PDDA/BSA-tyrosinase).
Attempts to covalently bind tyrosinase provided weakly active systems.
The coating of the enzyme based on the simple layer-by-layer principle
has afforded catalysts <b>I–III</b>, with a range of
activity from 21 units/mg (multilayer, <b>II</b>) to 66 units/mg
(monolayer, <b>I</b>), the best system being catalyst <b>IV</b> (80 units/mg). The novel catalysts were fully characterized
by scanning electron microscopy and atomic force microscopy, showing
increased activity with respect to that of the native enzyme. These
catalysts were used in the selective synthesis of catechols by oxidation
of <i>meta</i>- and <i>para</i>-substituted phenols
in an organic solvent (CH<sub>2</sub>Cl<sub>2</sub>) as the reaction
medium. It is worth noting that immobilized tyrosinase was able to
catalyze the oxidation of very hindered phenol derivatives that are
slightly reactive with the native enzyme. The increased reactivity
can be ascribed to a stabilization of the immobilized tyrosinase.
The novel catalysts <b>I</b> and <b>IV</b> retained their
activity for five subsequent reactions, showing a higher stability
in organic solvent than under traditional buffer conditions