Bridgehead positions elimination

If the leaving group of the precursor to 142 is placed at the bridgehead position (Cla), the elimination may proceed in both the desired direction to 142 or to the laW-isomeric 150. Elimination of the dimethylselenium or the dimethylsul-... 

This difference is obviously due to the difficulty in producing a double bond at a bridgehead position. Nevertheless, elimination of lithium fluoride does occur, especially at reflux temperatures the bridgehead alkene 10.8A, which probably has only a transitory existence or may even be more appropriately described as a diradical, may be trapped with furan. 

In order to introduce groups which cannot effect the elimination reaction, sterically hindered non-nucleophilic bases such as lithium diisopropylamide , lithium 2,2,4,4-tetramethylpiperidide or even t-BuO" can be used. Employing this technique, Szeimies and coworkers introduced thio and amino groups into the bridgehead position of bicyclobutane derivatives ... 

The electrophilic bromine cation can also be generated from A -bromosuccinimide (NBS). When 2-methylene-6,6-dimethylbicyclo[3.1.0]hexane with an ester function at the bridgehead position was treated with NBS in diethyl ether the electrophilic attack took place at the terminal position of the C—C double bond and the cyclopropane ring was opened to give a cyclopentene derivative. Due to the lack of a reactive nucleophile, a proton was eliminated. The reduction of the ester function did not change the course of this reaction. With tert-hvAy hypochlorite, the corresponding chloro product was obtained. ... 

Several strategies for ring opening are based on the elimination of alkoxide by the generation of a carbanion alpha to the bridgehead position. 

TpRe(CO)(MeIm)(f/ -anisole) reacts with DMAD to form an f/ -barrelene-complex (Scheme 54). Demetalation gives the barrelene and a trisubstituted benzene that is the product of retrocycloaddition involving elimination of ethyne. TpRe(CO)(MeIm)(5,6-f/ -anisole) also reacts with NMM to give a cycloadduct with the methoxy group at a bridgehead position. In addition, TpRe(CO) (MeIm)(f/ -l-methylpyrrole) undergoes a 1,3-dipolar cycloaddition reaction with dimethylfumarate to give 7-azabicyclo-[2.2.1]heptene (Scheme 54). 

When the elimination was conducted at — 35°C in the additional presence of tetramethylenediamine and diphenylisobenzofuran, an adduct of 1-dimethylamino-bicyclo[2.2.0]hexa-2,5-diene derived by addition of the deuterated solvent to the butalene was formed. In this reaction all the deuterium was found at the bridgehead position at the level of NMR detection (Scheme 7.3). 

Thioethers. Bridgehead positions in hydrocarbons are functionalized as butyl-thioethers by reaction with lead tetra-acetate and butanethiol (c/. ref. 1). Reductive sulphidation of aldehydes is accomplished by conversion to a silyl monothio-acetal and cleavage of the C-O bond as outlined in Scheme 38. Reaction of primary alcohols with aryl isothiocyanates and triphenyl phosphine produces arylalkylthioethers, presumably via the sequence of Scheme 39. In secondary and tertiary cases elimination (to ArSH and alkene) from (76) is a competing process. 

When quinamine was refluxed in dilute acetic acid for a prolonged period it afforded the ketoamine, quinamicine, which may have been the result of a direct loss of water or more likely an allylic rearrangement of the tertiary hydroxyl group in the hydroxy-methyleneindoline form then elimination (the hydroxyl cannot be lost to form the iminium ion because of the bridgehead position of the quinuclidine nitrogen). The formulation of quinamicine is the only reaction of these compounds which has any parallel in... 

Functionalization of the bridgehead 2-position in benzomorphans by standard synthetic approaches is not easy. This led Portoghese and Ramakrishnan(117) to devise a new 2-tetralone synthesis from a conventional benzomorphan Hofmann elimination product (185). Demethylation of 185 was effected by trichloroethylchloroformate to 186, which, in turn, was oxidized to the 2-tetralone (187). Removal of the trichloroethyl carbamate protecting group gave 189, presumably via 188. Treatment of 189 with KCN in weak... 

Ate-complexes of 3-borahomoadamantanes undergo a reaction of hydride elimination when treated with AcCl, giving 8-methylene-3-methyl-3-borabicyclo[4.3.1]decanes (Scheme 58). The hydride elimination occurs regioselectively only from the bridgehead j8-position of the caged compounds <86T1079>. 

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