ChemInform Abstract: A Novel Route to Isoannulated Heteroaromatic Compounds. Part 2. The Dipole Route to Furo/Thieno Pyrroles and α‐Pyridones, and Their Benzo Derivatives.

Abstract

A Novel Route to Isoannulated Heteroaromatic Compounds. Part 2. The Dipole Route to Furo/Thieno Pyrroles and α-Pyridones, and Their Benzo Derivatives.-A new methodology, involving 1,7-dipolar cyclizations of conjugated dipoles followed by a multistep rearrangement affording ring-contracted products is employed for the synthesis of various examples of the title compounds. As demonstrated for (IV) and (V) the reaction starts with the alkynylcarbaldehydes (I) which are converted into nitrones such as (III). In some cases beside the Z isomers 3-10% of the E isomers are obtained. Under different thermolysis conditions the nitrones are converted into the pyrroles and α-pyridones. The product distribution is strongly influenced by the nature of the terminal alkynyl substituent. The derivatives (V) are not stable. They react with oxygen to the imides (VI). The benzo-analogues are more stable, some Diels-Alder reactions of them are demonstrated. -(BUSSENIUS, J.; LABER, N.; MUELLER, T.; EBERBACH, W.; Chem.

Generation and Reactions of a Benzodehydrotropylium Ion–Co2(CO)6 Complex

“…We envisioned that a benzodehydrotropylium cation–Co 2 (CO) 6 complex ( 3 ), specifically as methyl-substituted 3b , would be most readily obtained through benzocycloheptynol complex 4 , which in turn would be prepared in several steps from 2-ethynylbenzaldehyde ( 5a ) (Scheme ). 2-Ethynylbenzaldehyde ( 5a ), itself available by a literature Sonogashira-desilylation protocol from 2-bromobenzaldehyde via 6a , , was subjected to a Sonogashira reaction with 2-bromoallyltrimethylsilane to give enyne 7a (71% yield, Table ). Upon treatment with Co 2 (CO) 8 , complex 8a formed in 80% yield.…”

“…With the goal of ultimate generation of the dehydrotropylium ion−Co 2 (CO) 6 complex, compound 9a was selected for further manipulation. A Swern oxidation 17 afforded ketone 10 with minimal decomposition (89% yield) (Scheme 3), as evidenced by the appearance of a shifted CH 2 singlet at 3.76 ppm in the 1 H NMR spectrum and ketone carbonyl 13 C NMR spectral resonance at 198.3 ppm. While this material could also be obtained with MnO 2 oxidation, the yields were inferior (28%, 43% by recovered starting material).…”

“…Coupling constants are reported in Hertz (Hz). 13 C NMR data were obtained at either 75 or 125 MHz. Infrared spectra (IR) were recorded on a FT-IR spectrophotometer using KBr plates.…”

“…An aqueous solution of NH 4 Cl (sat) was added, and the mixture was subjected to a conventional extractive workup using diethyl ether. The crude material was subjected to flash chromatography (25:1 petroleum ether/Et 2 O) to give product 7a (0.719 g, 71%) as a yellow oil; IR (KBr) ν max 2927, 1700, 1599 cm −1 ; 1 H NMR (500 MHz, CDCl 3 ) δ 10.53 (d, J = 0.5 Hz, 1H), 7.91 (m, 1H), 7.53 (m, 2H), 7.41 (m, 1H), 5.37 (d, J = 2.0 Hz, 1H), 5.18 (d, J = 1.5 Hz, 1H), 1.79 (d, J = 0.5 Hz, 2H), 0.11 (s, 9H); 13 C NMR (75 MHz, CDCl 3 ) δ 192.0, 135. 9, 133.9, 133.2, 128.6, 128.4, 127.3, 127.2, 120.8, 98.7, 83.8, 28.1, −1.5; HRMS m/e for C 15 H 18 OSi [M + ] calcd 242.1127, found 242.1130.…”

“…Infrared spectra (IR) were recorded on a FT-IR spectrophotometer using KBr plates. 2-Bromo-3-(trimethylsilyl)-1-propene and alkynes 6a , 5a , 6b , 5b , 6c , 5c , 6d , 5d , and 6e were prepared by literature methods.…”