- Synthetic and mechanistic investigations on the rearrangement of 2,3-unsaturated 1,4-bis(alkylidene)carbenes to enediynes
-
The synthesis of 3,4-ene-1,5-diynes, the key structural moiety present in several naturally occurring antitumor antibiotics, from 1,2-enedialdehydes under two different experimental conditions is reported. One method involves the dibromomethylenation of dialdehydes under Corey-Fuchs conditions (CBr 4, Ph3P, and Zn) and treatment of the resulting tetrabromides with nBuLi or LDA to afford enediynes. The second method involves a base-mediated reaction of enedialdehydes with diethyl (1-diazo-2-oxopropyl) phosphonate (Bestmann-Ohira reagent) and subsequent transformation of the bis(diazo) compounds generated in situ to enediynes. While the transformation of bis(diazo) compounds to enediynes could take place exclusively through alkylidene-carbenes, generated in situ by geminal elimination of N2, an alternative pathway, involving the vicinal elimination of HBr to afford an intermediate bromoalkyne and its subsequent metal-halogen exchange and protonation during workup, exists for the bis(dibromoalkylidenes). However, our deuterium-labeling experiments with a model substrate, deuterated p-methoxybenzylidene dibromide, established the predominance of the alkylidenecarbenes, generated in situ by metal-halogen exchange and elimination, for this substrate and, by analogy, for the tetrabromides as well. The scope of this novel methodology was extended to the synthesis of various heteroatom-based (S, Se, and P) enediynes by quenching the acetylides with suitable electrophiles. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.
- Sahu, Bichismita,Muruganantham, Rajendran,Namboothiri, Irishi N. N.
-
p. 2477 - 2489
(2008/03/13)
-
- Diatropicity of 3,4,7,8,9,10,13,14-octadehydro[14]annulenes: A combined experimental and theoretical investigation
-
The synthesis and study of a series of octadehydro[14]annulenes is described. The aromaticity of these annulenes was investigated through examination of experimental data from arene-fused systems as well as calculated nucleus-independent chemical shifts (NICS) and bond lengths. Benzene ring fusion to the parent system results in a stepwise loss in aromaticity as the number of fused rings is increased from one to two to three. This decrease in annulenic ring current is manifested in the alkene proton chemical shifts (0-2 benzenes) as well as the NICS (0-3 benzenes). Comparison of isomeric thiophene-fused annulenes shows further evidence of ring current competition as these allow for observation of intermittent degrees of delocalization throughout the annulenic core. A consistent relationship between the magnitude of the NICS values and the degree of benzannelation is also observed.
- Boydston, Andrew J.,Haley, Michael M.,Williams, Richard Vaughan,Armantrout, John R.
-
p. 8812 - 8819
(2007/10/03)
-
- Electronic Structure of Ethynylthiophenes
-
He I and He II photoelectron spectra of all isomeric mono- and diethynylthiophenes were recorded.The electronic structure was analyzed and spectra assigned on the basis of empirical considerations and semiempirical MO calculations.The spectra indicate various interactions taking place between thiophene ? and ethynyl ? orbitals.The relationship between deduced electronic structure and formation of novel polymer materials is discussed.
- Novak, Igor,Ng, Siu Choon,Fang, Jiye,Mok, Chup Yew,Huang, Hsing Hua
-
p. 748 - 751
(2007/10/02)
-
- Insight into the Electronic State of 1-Phenylthienoborepin by Means of Nuclear Magnetic Resonance Spectroscopy
-
In order to obtain an insight into the electronic states in connection with the condensation modes of thiophene and borepin rings, we examined 1-phenylthienoborepin (2).While 1-phenylthienoborepin is a peripherally conjugated compound, 2 was shown to be an aromatic compound which maintains characteristics in conjugation of individual rings.
- Sugihara, Yoshikazu,Miyatake, Ryuta,Yagi, Toshiyasu
-
p. 933 - 936
(2007/10/02)
-