Alkoxides cleavage

However, even planar carbanions need not give racemization. Cram found that retention and even inversion can occur in the alkoxide cleavage reaction (12-39) ... 

The following mechanism for the reaction has been suggested. It postulates formation of an intermediate paraconic ester (A) the irreversible alkoxide cleavage of this cyclic ester drives the reaction to completion ... 

See G. Mehta, R. V. Venkateswaran, Tetrahedron 2000, 56, 1399. In this updated Review, the authors indicate the inclusion of alkoxide cleavages as Haller-Bauer. March describes these as "Hydro-de-Acylation" reactions. See also M. B. Smith, J. March in March s Advanced Organic Chemistry, 5th ed., John Wiley and Sons, Inc., New York, 2001, p. 814 K. E. Hamlin, A. W. Weston, Organic Reactions 9, 1. 

In tetraarylantimony alkoxides, cleavage of the one aryl group has been observed in the presence of sodium alkoxide and it has been postulated that these reactions proceed via an ionic reaction mechanism in which ate complexes are formed as intermediate279,281. ... 

The synthetic utility of the Haloform reaction is revealed further by a synthesis of isotopically pure (99.2%) CDCb by the reaction of trichloroacetophenone with NaOD in D20. The alkoxide-cleavage of trichloromethyl ketones has been employed to synthesize dihydropyridine esters (e.g., 24) in which ester exchange does not occur.59... 

The applicability of these methods is restricted to polyhalogenated compounds, since the inductive effect of the three halogen atoms is necessary both for easy decarboxylation and for elimination of a carbanion from the tetrahedral intermediate generated in the alkoxide-cleavage procedure. 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

More useful than the preceding methods is cleavage of alkoxides by acetyl chloride or bromide. One, two, three, or four alkoxyls can be replaced by chloride or bromide. Benzoyl chloride gives poor yields, however. The tri- and tetrachlorides, which are stronger Lewis acids than mono- and dichlorides, coordinate with the alkyl acetate formed and yield distillable complexes (46,55,56). 

The following cleavage proceeds via intramolecular assistance from the alkoxide formed on base treatment. ... 

These reactions proceed by alkoxide or fluoride attack at silicon which results in C—Si bond cleavage and elimination of the leaving group from the fi carbon. These reactions are stereospecific anti eliminations. 

In this case, the magnesium alkoxide protects the ketal from cleavage. " ... 

Alkylberyllium alkoxides (RBeOR ) can be prepared from BeR2 by a variety of routes such as alcoholysis with R OH, addition to carbonyls, cleavage of peroxides R OOR or redistribution with the appropriate dialkoxide Be OR )i, e.g. ... 

The retro-Claisen reaction occurs by initial nucleophilic addition of a cysteine -SH group on the enzyme to the keto group of the /3-ketoacyl CoA to yield an alkoxide ion intermediate. Cleavage of the C2-C3 bond then follows, with expulsion of an acetyl CoA enolate ion. Protonation of the enolate ion gives acetyl CoA, and the enzyme-bound acyl group undergoes nucleophilic acyl substitution by reaction with a molecule of coenzyme A. The chain-shortened acyl CoA that results then enters another round of tire /3-oxidation pathway for further degradation. 

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