<i>meso</i>-Aryl-3-alkyl-2-oxachlorins

Abstract

The formal replacement of a pyrrole moiety of <i>meso</i>-tetraarylporphyrin <b>1</b> by an oxazole moiety is described. The key step is the conversion of porpholactones <b>4</b> (prepared by a known two-step oxidation procedure from <b>1</b>) by addition of alkyl Grignard reagent to form <i>meso</i>-tetraaryl-3-alkyl-2-oxachlorins <b>9</b> (alkyloxazolochlorins; alkyl = Me, Et, <i>i</i>Pr). Hemiacetal <b>9</b> can be converted to an acetal, reduced to an ether, or converted to bis-alkyloxazolochlorins <b>11</b>. The optical properties (UV–visible and fluorescence spectroscopy) are described. The chlorin-like optical properties of the alkyloxazolochlorins are compared to regular chlorins, such as 2,3-dihydroxychlorins and nonalkylated oxazolochlorins made by reduction from porpholactone <b>4</b>. The conformations of the mono- and bis-alkylated 2-oxachlorins, as determined by single crystal X-ray diffractometry, are essentially planar, thus proving that their optical properties are largely due to their intrinsic electronic properties and not affected by conformational effects. The mono- and bis-3-alkyl-2-oxachlorins are a class of readily prepared and oxidatively stable chlorins