- Stereoselective synthesis of 2-aminocyclobutanols via photocyclization of α-amido alkylaryl ketones: Mechanistic implications for the Norrish/Yang reaction
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A series of chiral N-acylated α-amino p-methylbutyrophenone derivatives 1a-1h was synthesized from α-amino acids via a three-step procedure. These substrates were photolyzed in benzene and gave Norrish II and Norrish I cleavage products as well as the N-acylated 2-aminocyclobutanols that derive from γ-hydrogen abstraction and 1,4-triplet biradical combination (Yang cyclization). The products were formed with characteristic Yang/cleavage ratios. The quantum yields for the photodecomposition of the N-acetyl-protected substrates 1b,e,f were moderate (12-26%); the diastereoselectivities of the cyclobutanol formation were remarkably high for all substrates. High diastereospecificity was observed for the isoleucine derivatives (2S,3S)-1g and allo-(2S,3R)-1g; the Yang reaction dominated the photochemistry of allo-1g, whereas 1g gave preferentially Norrish II cleavage. The role of hydrogen bonding as one of the stereo-directing effects was demonstrated by comparison of the cyclization efficiency of the valine derivative 1e with 1h,i,j. Also, aromatic β-keto esters gave the Yang cyclization products in low yields. The diastereoselectivity of the cyclobutanol formation was rationalized by a three-step mechanism where every step is connected with one distinct stereochemical induction mechanism: (a) diastereoselective hydrogen abstraction, (b) conformational equilibration of the 1,4-tetramethylene biradicals (as calculated by semiempiric methods) controlled by hydrogen bonding, and (c) diastereoselective biradical combination (versus cleavage) influenced by spin-orbit coupling controlled intersystem crossing geometries.
- Griesbeck, Axel G.,Heckroth, Heike
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p. 396 - 403
(2007/10/03)
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- Aminoketone, oxazole and thiazole synthesis. Part XIII. 1 new 2-aryl-6-phenyloxazoles with 4-methyl or 4-isopropyl groups
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Benzoylation of (±)-α-alanine and (±)-valine followed by cyclization afforded the corresponding azlactones, which reacted with benzene, toluene, o- and m-xylene yielding substituted N-benzoyl-phenacylamines. Their cyclization in the presence of phosphorus oxychloride led to the title compounds whose electronic and NMR spectra (1H and 13C) are presented.
- Schiketanz, Iosif,Istrati, Daniela,Drǎghici, Constantin,Balaban, Alexandru T.
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p. 137 - 142
(2007/10/03)
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