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Selective Cyclization (selective + cyclization)
Selected AbstractsGold Catalysis: Alkylideneoxazolines and -oxazoles from Intramolecular Hydroamination of an Alkyne by a TrichloroacetimidateEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2006A. Stephen K. Hashmi Abstract Several propargylic trichloroacetimidates have been prepared and their reactions with gold catalysts studied. Only with the propargyl and the 1-methylpropargyl substituent was a selective cyclization observed; with gold(III) chloride in acetonitrile only the product of a fast hydroamination to 4-methylene-4,5-dihydrooxazoles was obtained, in chloroform the slower subsequent aromatization could not be prevented which delivered the oxazoles after long reaction times. Up to 3333 turnovers could be reached. With gold(I) catalysts in chloroform or dichloromethane selective hydroamination to 4-methylene-4,5-dihydrooxazoles without subsequent aromatization was exclusively observed. The gold(I) catalysts also allowed chemoselective cycloisomerization of N -propargylcarboxamides to 5-methylene-4,5-dihydrooxazoles. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] A Symmetry-Based Synthesis of the Heterobicyclic Core of the Zaragozic Acids/SqualestatinsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2009Yuzhou Wang Abstract The pseudo C2 -symmetrical diketone 22 was efficiently constructed from furan-3,4-dimethanol (7) using a two-directional route featuring a double asymmetric dihydroxylation. Acidic hydrolysis of the cyclopentylidene acetals of 22 triggered a selective cyclization of the resulting hexaol diketone to generate the 2,8-dioxabicyclo[3.2.1]octane core of the zaragozic acids/squalestatins. Chemoselective oxidative cleavage of a superfluous two-carbon appendage and further functional group manipulations yielded the enantiomerically pure triester 30, which offers itself as a general heterobicyclic building block for naturally occurring zaragozic acids/squalestatins and unnatural analogues. [source] Controlling the regiochemistry of radical cyclizationsTHE CHEMICAL RECORD, Issue 1 2006Hiroyuki Ishibashi Abstract This review describes the results of our recent studies on the control of the regiochemistry of radical cyclizations. N-vinylic ,-chloroacetamides generally cyclized in a 5- endo-trig manner to give five-membered lactams, whereas 4- exo-trig cyclization occurred when the cyclized radical intermediates were highly stabilized by an adjacent phenyl or phenylthio group to afford ,-lactams. The 5- exo or 6- exo cyclization of aryl radicals onto the alkenic bond of enamides could be shifted to the corresponding 6- endo or 7- endo mode of cyclization by a positional change of the carbonyl group of enamides. The 6- endo - and 7- endo -selective aryl radical cyclizations were applied to radical cascades for the synthesis of alkaloids such as phenanthroindolizidine, cephalotaxine skeleton, and lennoxamine. The 5- exo-trig cyclization of an alkyl radical onto the alkenyl bond of enamides could also be shifted to the 6- endo mode by a positional change of the carbonyl group of enamides. The 6- endo - selective cyclization was applied to the radical cascade to afford a cylindricine skeleton. Other examples of controlling the regiochemistry of radical cyclizations and their applications to the synthesis of natural products are also discussed. © 2006 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 6: 23,31; 2006: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20069 [source] Gold-Catalyzed 5- and 6- exo - dig Selective Cyclizations of Alkynyl Silyl Enol Ethers: Efficient Method for [3+2] and [4+2],Annulations onto ,,,-EnonesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2007Kooyeon Lee Abstract An efficient method for the annulation of five- and six-membered rings onto ,,,-enones is described via gold-catalyzed 5- and 6- exo - dig selective cyclizations of alkynyl silyl enol ethers. [source] | ||||||||||