Ring-opened dianion

Cleavage of the S-C(2) bond occurs when thiochroman is treated with the radical anion 4,4 -di-/-butylbiphenylide (LDBB). The resulting ring-opened dianion 317 has good synthetric potential. For example, the alcohols arising from reaction with carbonyl compounds can be cyclized under acidic conditions to benzothiepines (Scheme 56) < 1995TL4459>. 

In the laboratory of D.R. Williams, a carbanion methodology for the alkylations and acylations of substituted oxazoles was investigated. The study showed that the monoalkylation of the dianion generated from 2-(5-oxazolyl)-1,3-dithiane exclusively led to the substitution of the carbon adjacent to sulfur. However, acylation reactions of the dianion afforded 4,5-disubstituted oxazoles. These new products presumably arose from carbonyinitrile ylide intermediates, which were generated by the selective C-acylation of a ring-opened dianion tautomer. This is the first example of a base-induced, low-temperature Cornforth rearrangement. 

Epoxides provide another useful a -synthon. Nucleophilic ring opening with dianions of carboxylic acids (P.L. Creger, 1972) leads to y-hydroxy carboxylic acids or y-lactones. Addition of imidoester anions to epoxides yields y-hydroxyaldehyde derivatives after reduction (H.W. Adickes, 1969). 

In aqueous alkaline conditions with chloroacetic acid the pyrido[4,3- f]pyrimidinethione (80) undergoes facile ring opening, attributed to the resonance stabilization of a delocalized covalent hydrate dianion intermediate (81) (82). Pyrido[2,3- f]pyrimidine-4-thiones (and... 

Benzo[c]thiophene is a source of 3-substituted derivatives of 3,4-dihydro-1//-2-benzothiopyrans via ring opening to the dianion and subsequent reaction with electrophiles <96JOC1859>. 

Group 4 metal complexes of the dianion [ BuNP( -N Bu)2PN Bu] polymerize ethylene in the presence of a co-catalyst, but they are readily deactivated [10,14]. This behaviour is attributed to coordination of the lone-pair electrons on the phosphorus(III) centers to Lewis acid sites, which initiates ring opening of the ligand [15]. 

A domino process based on the twofold addition of alkenyl anions to a squarate ester was used by Paquette and coworkers [96] for the total synthesis of the triqui-nane sesquiterpene hypnophilin (4-284). The three-component reaction of 4-281,4-282 and vinyl lithium gave primarily the trans- and cis-adducts A and B, which furnished D either by an electrocyclic ring opening/ring closure via C or a dianionic oxy-Cope rearrangement (Scheme 4.60). Further transformations led to E and F, which resulted in the formation of 4-283 on treatment with acid. 

Dianion 201, generated by treatment of alcohol 200 with 2 molar equivalents of BunLi at —78 °C, undergoes ring opening with extrusion of N2 and formation of new anion 202. Cyclization to oxirane 203 and hydrogen shift creates anion 204 that is hydrolyzed during work-up to aniline derivative 205 (Scheme 24) <1998H(48)187>. 

The corresponding endoperoxides of furans usually ring-open to afford 1,4-diketones. Such a sequence, followed by an intramolecular Diels Alder reaction was utilized in the synthesis of the tts-indacenyl dianion (6.7) 613>. 

The formation of butanes by reduction of arylethenes may arise by radical-radical coupling of two radical-anions giving a dianion, which is then protonated. An alternative route is by nucleophilic addition onto one neutral molecule of the radical-anion, followed by further reduction and protonation. In support of this alternative, cyclobutanes have been isolated from electrochemical reduction of phenylvinylsulphones [21] and vinylpyridines [22], A mechanism for the latter process is illustrated for the case of 2-vinylpyridine 7. Nucleophilic attack of a radical-anion on the substrate gives an intermediate and this disproportionates to form the cyclobutane and a 1,4-diary Ibutane. Cyclobutanes are themselves reduced with ring opening to form the 1,4-diarylbulane. 

The method described is that of Miles, Harris, and Hauser2 and is an improvement over the earlier procedure of Hauser and co-workers.3,4 In the earlier method the dianion of benzoylacetone, formed by the action of alkali amide in liquid ammonia, was treated with methyl anisate to yield 1 -(/>-methoxyphenyl)-5-phenyl-l,3,5-pentanetrione (61% based on the ester). This compo ir d has also been prepared by the base-catalyzed ring opening of 2 - f -mcthoxyphenyl) -6-phenyl -4 pyrone however, no yield is reported.5... 

Epoxy-l,3-dioxepanes are readily converted into ring-opening products by reaction with nucleophiles. Thus, addition of 124 with dianions 123 generated from thioamides 122 and 2 equiv of -butyllithium yielded a mixture of diastereomers 125 and 126, which were separated by column chromatography (Scheme 31) <2005JOC8148>. 

Substituted 3,4-dihydro-l //-2-benzothiopyrans 499 are formed from benzo[c]thiophenes by ring opening to the dianion and reaction with electrophiles (Scheme 184) <1996JOC1859>. 

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