ChemInform Abstract: A New Practical Synthesis of Silyl Enol Ethers. Part 2. From α,β-Unsaturated Aldehydes and Ketones.

It is shown that the cyanoethylation of 3β-acetoxy-21-methyl-5α-pregn-17-en-21-one results in part in the addition of acrylonitrile to the γ-position, with concomitant cyclization, and that therefore the cyclo-γ-cyanoethylations of α,β-unsaturated carbonyl compounds reported previously were not confined to aldehydes. However, the main reaction products of the conjugated ketone were α-cyanoethylated derivatives. A variation of the reaction time did not affect the proportion of α- and γ-cyanoethylated products; the implications of this finding on mechanistic considerations are discussed; in particular, an explanation based on the assumption of an equilibrium between reactants and α- and γ-cyanoethylated products is ruled out. It is shown that the γ-additions observed, particularly in the case of α,β-unsaturated aldehydes, are not due to steric hindrance, neither to an abnormal charge distribution in the anion. It is further shown that in the presence of base the propionitrile moiety of oxygen-cyanoethylated α,β-unsaturated aldehydes with a suitable geometry is transferred to the γ-position of the original aldehyde and that cyclization occurs so that the same products are obtained from such cyano-enol ethers as in the direct cyanoethylation of the free aldehydes. On the basis of this finding, a mechanism for the cyclo-γ-cyanoethylations of α,β-unsaturated carbonyl compounds, involving such a transfer reaction, can be tentatively proposed.

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Best Synthetic Methods: Oxidation and Reduction

Organic Synthesis ◽

10.1093/oso/9780199965724.003.0010 ◽

2013 ◽

Author(s):

Douglass F. Taber

Keyword(s):

Synthetic Methods ◽

Terminal Alkyne ◽

University Of Kansas ◽

University Of Mississippi ◽

Enol Ethers ◽

Silyl Enol Ethers ◽

Aldehydes And Ketones ◽

The University ◽

Technological University ◽

Trisubstituted Alkenes

Johannes G. de Vries of DSM Pharmaceuticals prepared (Chem. Commun. 2009, 3747) Fe nanoparticles that selectively mediated the hydrogenation of Z alkenes and not trisubstituted alkenes. This should allow the conversion of 1 to 2. In the course of a synthesis (Tetrahedron Lett. 2009, 50, 4368) of centrolobine, Teck-Peng Loh of Nanyang Technological University employed an elegant protocol for the reduction of the secondary bromide 3. István Markó of the Université catholique de Louvain observed (Tetrahedron 2009, 65, 10930) that toluates such as 5 can be reduced smoothly with SmI2 to the corresponding C-H. Dan Yang of the University of Hong Kong devised (Organic Lett. 2009, 11, 3302) a triethylsilane-based procedure for the reductive amination of aldehydes and ketones such as 7. Jon A. Tunge of the University of Kansas developed (J. Am. Chem. Soc. 2009, 131, 16626) a complementary protocol for the conversion of an aldehyde or ketone to the protected amine 12. Mark T. Hamann of the University of Mississippi established (Tetrahedron Lett. 2009, 50, 3901) that a nitroaromatic 13 could be reduced in the presence of an acid chloride 14 to deliver the amide 15 directly. Matthias Beller of the Universität Rostock (Angew. Chem. Int. Ed. 2009, 48, 9507) and Hideo Nagashima of Kyushu University (Angew. Chem. Int. Ed. 2009, 48, 9511; J. Am. Chem. Soc. 2009, 131, 15032) reported parallel investigations of the silane-based reduction of an amide 16 to the amine 17. Xue-Long Hou of the Shanghai Institute of Organic Chemistry demonstrated (Tetrahedron Lett. 2009, 50, 5578) that a terminal alkyne 18 could be oxidized to the α-acetoxy ketone 19. Philippe Renaud of the Universität Bern and Armido Studer of West fälische-Wilhelms-Universität established (Angew. Chem. Int. Ed. 2009, 48, 6037) that both zinc enolates and silyl enol ethers could combine with chlorocatechol borane followed by TEMPO to give the α-oxygenated ketone. Stephen P. Marsden of the University of Leeds devised (Tetrahedron Lett. 2009, 50, 6106) a protocol for oxidizing a primary amine 18 to the benzoxazole 24, which has the oxidation state of the carboxylic acid.

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