The Norrish Type II reaction in organic synthesis

Abstract

<p>The Norrish Type II photocyclization has been known to organic chemists for several decades. To the best of our knowledge, no effort has been made to exploit the reaction in synthesis. A study was undertaken to investigate the synthetic potential of the Norrish Type II reaction;Ultraviolet irradiation of certain [beta]-alkoxy ketones and aldehydes leads to the formation of dihydro- and tetrahydrofuranols via hydrogen atom abstraction by the excited carbonyl and cyclization of the resulting biradical. Type II photocyclization of a 2,6-disubstituted aryl ketone provided a dihydrobenzofuranol which is a precursor to aflatoxin M[subscript]2. This constitutes a direct photochemical entry into the aflatoxin family. We have also irradiated a series of differently substituted aldoses to afford a variety of sugar derivatives containing the tetrahydrofuranol moiety. The synthetic utility of this strategy was demonstrated by the six step conversion of a readily-available five-carbon sugar into a mixture of two rare six-carbon sugars. We were unable to synthesize nucleosides by this method;Synthesis of pyrrolizidine and indolizidine alkaloids by irradiation of nitrogenous ketones and aldehydes was pursued. Several amino- and amido ketones and aldehydes were synthesized and irradiated. Work continues to develop a successful photochemical route to these alkaloids.</p>