A series of a-aryl and diarylthioforrnarnidyl cations were observed by low-temperature nuclear magnetic resonance spectroscopy. These ions undergo efficient cyclization and deprotonation to give benzothiophenes.
E. LEE-RUFF and F. J. ABLENAS. Can. J. Chem. 67, 699 (1989). Benzylic and allylic ethers are oxidized by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to give the corresponding carbonyl containing compounds (aldehydes and ketones) and alcohols. The reaction proceeds readily with ethers bearing activating groups. Primary ethers are further oxidized with excess DDQ giving the corresponding esters. Mechanisms for these processes are proposed, based on nuclear magnetic resonance and stoichiometric analyses.
E. LEE-RUFF and FRED J. ABLENAS. Can. J. Chern. 65, 1663 (1987). The acid-catalysed dehydration-rearrangement reaction of a-thienyl cyclobutanols 3 and 4 resulted in the formation of tetrahydronaphtho[2,3-blthiophenes 5a and 6 a . The rearrangement to the linearly fused PAH system instead of the expected angularly fused system reflects a-scission of the cyclobutyl ring. A mechanism based on deuterium labelling studies is proposed to account for the product formation. Bicyclic cyclobutanones with an a-aryl substituent undergo selective p-scission under acid catalysis to give tricyclic ketones by way of intramolecular orthoannelation of an intermediate carbocation (1)(2)(3). This observation has been used as a general approach towards the synthesis of polynuclear aromatic hydrocarbons (PAH) in which non-benzenoid and heterocyclic rings are incorporated in the PAH ring system as terminal units. These studies were also extended to bicyclic cyclobutanols bearing at least one a-aryl substituent. Acid reaction of typical bicyclic cyclobutanols effects dehydration and rearrangement to give angularly fused hydroarenes that are readily dehydrogenated to PAH's (4) (Scheme 1). The intermediate carbocations are the result of selective p-scission of the cyclobutanol ring and are stabilized by positive charge delocalizing groups such as a vinyl or heteroatom group. The cyclobutanols are readily obtained from the corresponding cyclobutanones, which in turn are prepared from aryl ketene cycloadditions to cycloolefins. As an extension to these studies we were interested in the possibility of cyclobutanols with a-heteroaromatic substituents undergoing similar transformations to give heteroaromatic PAH 's. Both aand P-thienylketene precursors are commercially available and such transformations would constitute a general synthetic route to PAH's incorporating a terminal thiophene nucleus. A number of these compounds are of interest in environmental chemistry and toxicology due to their presence in fossil fuel and coal sources and their mutagenic activity (5). In this study we report a different and heretofore unreported acid-catalyzed dehydration-rearrangement of two a-thienyl bicyclic cyclobutanols in which regiospecific a-ring opening is involved. Results and discussionThe preparation of cyclobutanones 1 and 2 was accomplished by generating 2-or 3-thienylketene in situ by dehydrochlorination of the acid chloride with triethylamine in the presence of 1,3-cyclohexadiene. In each case a stereoisomeric mixture was obtained in which the endo-substituted product predominates and column chromatography on silica gel causes the equilibration to the thermodynamic exo-substituted product (6). The endo-substituted derivative is predicted to be the kinetic product based on orbital symmetry considerations of the 2 n a + 2 n s transition state (7).Reduction of ketones 1 and 2 with excess lithium aluminum hydride gave alcohols 3 and 4 (X = H, Y = H) respectively as a mixture of diastereomers in 78-80% yields. The relative distribution of the dias...
The reaction of the trimethylsilyl enol ether of 1 -thiochroman-4-one 1 ,I-dioxide with thionyl chloride gives the corresponding a-0x0-sulphine, which has been found to give a new type of [4 + 21 cycloadduct when trapped with simple electron-rich alkenes such as isobutylene and norbornene.As part of our study of the reactivity of 1-thiochroman-4-one 1,l-dioxide (1) towards a-alkylidenation,l we have examined the reactivity of the corresponding trimethylsilyl enol ether (2) towards various electrophiles. While, to our knowledge, enol ethers such as (2) have not previously been caused to react with thionyl chlorides to produce a-0x0-sulphines, the recent communication of Zwanenburg et a1.z and the earlier report by Faull and Hull3 have shown that a-sulphinylation can be used to convert suitable ketones directly into a-0x0-sulphines. We report the successful reaction of (2) with thionyl chloride to produce (3) under very mild conditions, and also the unexpected results of trapping experiments used to verify the formation of (3).When (2) was treated with thionyl chloride (CHzC12diethyl ether 1 : 1,O "C, 10 min) the expected sulphine (3) was trapped by adding an equimolar amount of 2,3-dimethylbuta-l,3-diene4 and allowing the resulting solution to remain at 25 "C for 6 h. The white, crystalline adduct (4)T (45%) was purified by chromatography on silica gel (CHZCl2-acetone 99 : 1) [m.p. 203-204 "C decomp.
Note Added in Proof. While this paper was in press, two works-one related to a dimer-catalyzed decomposition of the -intermediate (Bamkole, T. 0.; Hirst, J.; Onyido I. J. Chem. Soc., Perkin Trans. 2 1982, 889) and the other to mechanism II in the preceding paper (Banjoko, O.; Ezeani, C. Ibid. 1982, 1356)appeared. Comments on these two papers as well as on the role of the third amine molecule in the dimer mechanism and a solvent effects study have been submitted for publication in this journal.Acknowledgment. We are grateful to the National Research Council (CONICET) and to the Science and Technology Secretariat (SUBCYT), Argentina, for financial support. We also acknowledge IBM Argentina (Buenos Aires) for the computer calculations.Registry No. 2, pyridine,
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.