Acid-catalyzed rearrangement of cyclobutanols. A novel rearrangement

Abstract

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...

Indium-Catalyzed Construction of Polycyclic Aromatic Hydrocarbon Skeletons via Dehydration

“…Dialdehyde ( 12 ) was prepared by palladium-catalyzed Buchwald–Hartwig amination between 2 equiv of 2-(2-bromophenyl)-1,3-dioxolane and 1,4-phenylenediamine. The structures of these reaction products were determined by comparing the spectrum data of these products with those of 2a , 2b , 2c , 2d , 2e , 2f , 2g , 2h , 2i , 5 , 7 , 9 , , and 11 …”

Electrochemical immobilization of a pH sensitive fluorescein derivative: synthesis and characterization of a fluorescein-derivatised polythiophene

“…5 From TAA, thiophene-3-acetyl chloride (TAC) 1 was prepared via reaction with thionyl chloride. 6 ‡ TAC was then reacted with fluoresceinamine, to yield 2. § Polymer 3 was prepared via the electrochemical polymerization on Pt coated glass in MeCN using the method reported for the preparation of poly-TAA.…”

“…Notes and References † E-mail: m.uttamlal@gcal.ac.uk ‡ Experimental procedure for 1: 6 The reaction of TAA (1.00 g, Acros, 98%) with SOCl 2 (0.8 cm 3 ) at 60 °C for 2 h in dry conditions gave an orangebrown liquid 1 (the excess SOCl 2 was evaporated in vacuo at 35 °C ) with a yield of ca. 80%.…”

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