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Reductive Cleavage of N-O Bonds in Hydroxylamines and Hydroxamic Acid Derivatives Using Samarium Diiodide

Recently, in our study toward the total synthesis of 7-deoxypancratistatin, 4 we needed to reductively cleave the N-O bond in the product resulting from a free radical addition to an oxime ether (Scheme 1). Specifically, we needed to reductively cleave the N-O bond of the trifluoroaceto-hydroxamic acid derivative 6. Despite the large number of efficient methods for N-O bond reduction, we encountered limitations of the more common procedures. Using our previously developed method involving reduction with AI(Hg) or Na(Hg) I° the desired transformation failed. Other methods such as hydrogenolysis sa also proved to be inefficient in the transformation of the trifluoroaceto-hydroxamic derivative 6 to the trifluoroacetamide. | Pergamon Tetrahedron 55 1999 11755-11772 TETRAHEDRON Reductive Cleavage of N-O Bonds in Hydroxylamines and Hydroxamic Acid Derivatives Using Samarium Diiodide Gary E. Keck Travis T. Wager and Stanton F. McHardy Department of Chemistry University of Utah Salt Lake City Utah 84112 Received 7 April 1999 accepted 14 May 1999 Abstract An efficient process for the reductive cleavage of N-0 bonds using samarium diiodide is detailed for a variety of structural types to define the scope and limitations of the method. The reduction is shown to be compatible with base sensitive substrates such as trifluoroacetamide derivatives which cannot be reduced satisfactorily using aluminum amalgam or sodium amalgam. Direct quenching of the reduction mixture with acylating agents is demonstrated to provide high yields of protected amines in a one-pot process from the N-0 derivatives. 1999 Elsevier Science Ltd. All rights reserved. Key words Sml2 reduction nitrogen-oxygen bonds chemoselective amines INTRODUCTION Alkaloids naturally occuring amines are widely distributed in plants and animals and have been used for medicinal purposes throughout the ages.1 The medicinal value of alkaloids their sometimes limited availability from natural sources and the diversity and complexity of such structures has generated a considerable amount of interest in their total synthesis. In recent years a large number of alkaloids as well as other nitrogen containing compounds have been synthesized using electrocyclic and radical pathways via intermediates possessing N-O bonds. These N-0 bonds are then cleaved to provide nitrogen in its more common oxidation states. Such nitrogen containing intermediates may arise from 4 2 cycloadditions of acylnitroso compounds 2 ene reactions of acylnitroso compounds 3 free radical additions to oxime ethers 4 3 2 cycloadditions utilizing nitrile oxides and nitrones 5 3 2 cycloaddition reactions of O- silyl nitronates 6 and 4 2 cycloadditions of nitroalkenes7 Figure 1 . .