Ring opening base-catalysed

A system of parallel reactions as shown in Fig. 5.3-9 was studied by Paul et at. (1992). The reactions are an acid-base neutralization and a base-catalysed hydrolysis of product (C). The labile compound (Q is in solution in an organic solvent, and aqueous NaOH is added to raise the pH from 2 to 7. Enolization occurs under basic conditions and is accompanied by irreversible decomposition (ring opening), which is not shown in the figure. The system was studied in the laboratory using the 6-Iitre reactor shown in Fig. 5.3-10. 

Hydro ythiophenols react with dibromomethane to produce 1,3-benzoxathioIes [27] (Table 4.6). 1,3-Benzoxathioles have also been obtained by the one-pot base-catalysed ring opening of 1,3-benzoxathiol-2-ones and subsequent reaction with dibromomethane in the presence of Aliquat [28]. 

Dioxo-l,3-dioxanes ring-open under basic conditions. Cleavage of the 5,5-disubstituted derivatives in the presence of quininium or quinidinium alkoxides produces chiral malonic hemi-esters (ee 30-40%) in high yield [11]. The addition of cetyltrimethylammonium bromide promotes the base-catalysed cleavage of p-keto esters to form ketones under sonication [12]. 

A. De Munno, V. Bertini, F. Lucchesini, On the Base Catalysed Ring Opening of 3-Un-substituted Isoxazoles. Derivatives of 4- and 5-Phenylisoxazole , J. Chem. Soc., Perkin Trans. 2 1977, 1121 - 1124. 

Epoxidation reactions are important because epoxides are useful intermediates -ring opening allows subsequent reactions at one or both of the carbon atoms, and this is particularly attractive if asymmetric epoxidations can be accomplished (3.1.1.3). This is an increasingly important area for Ru-catalysed oxidations, and a review [2] emphasises in particular mechanistic aspects of such processes. There are also earlier brief reviews of the topic [4, 7,18], in particular by porphyrin-based systems [17-22],... 

Base-catalysed ring fission of 3,4-diphenylcyclobut-3-ene-l,2-diones (103) in 50% (v/v) aqueous DMSO proceeds by rapid reversible addition of hydroxide ion followed by rate-determining benzilic acid-type rearrangement to form an intermediate 1-hydroxycyclopropane-1-carboxylic acid which ring opens to the corresponding (Z)-2-oxo-3,4-diphenylbut-3-enoic acid (Scheme 8).173 This is supported by the value of Hammett p = 1.3 (for variation of substituents on one or both rings), the kinetic solvent effects, and the three-oxygen enrichment of (107) from reaction of (103) in 50% H2 180-DMSO. 

A similar investigation of the base-catalysed ring opening of 3,4-diphenylcyclobut-3-ene-l,2-diones (77) to give (Z)-2-oxo-3,4-diphcnylbut-3-cnoatcs (78) has been carried out in aqueous DMSO.108 The evidence points towards a rapid, reversible addition of hydroxide to one carbonyl, followed by a benzilic acid-type rearrangement to give a cyclopropene intermediate (79), which ring opens. 

The synthetic method (c), is a base-catalysed ring opening of an oxirane.246 Since the oxirane may be formed by the epoxidation of an olefin (Section 8.1.3, p. 1132), or by a methylene insertion reaction into a carbonyl group (Section 8.1.2, p. 1131), this method is of some versatility. 

A review of the metal-catalysed ring opening of achiral epoxides by achiral carbon-, sulfur-, nitrogen- and halogen-containing nucleophiles and kinetic resolution of racemic epoxides has been published.24 The review also discusses the reactions of chiral bases with epoxides that give allylic alcohols. 

The catalyst derived from chlorine displacement was effective in the base catalysed ring-opening of epoxides to form monoglycerides.[16] The other TBD-derived catalyst (from epoxide ring-opening) was also found to be active in the Knoevenagel reaction, the Michael addition and the base catalysed epoxidation of alkenones.[15] Results in the epoxidation reaction were of particular interest, as this leads to highly labile and... 

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