Oxiranes neighboring group

The direction of oxirane ring-opening in the stereoisomeric compounds 289 depends on several factors for example, favored axial attack of the nucleophile, anchimeric assistance of neighboring groups, and conformational control of the approaching nucleophile. The examples shown in Scheme 2 provide support for the importance of each factor. There are, however, exceptions to these rules (see, for example, Ref. 184). Therefore, the results of oxirane ring-opening in 289 cannot always be foreseen. 

The hydroxy function may originate from the reduction of an a-halogenated carbonyl compound or a Grignard reaction. The oxirane oxygen is not always incorporated directly from a hydroxy group for instance, in the case of a carboxylic acid ester, it is derived with neighboring-group participation from an ortho-monoester intermediate. 

At temperatures around 200°, a-hydroxyacetates are transformed to oxiranes via a tautomeric ortho-monoester with neighboring-group participation. By the action of heat. CO2 is split off the carbonic acid esters of 1,2-diols and oxiranes are formed. Disecondary or ditertiary 1,2-diols react with diaryldialkoxysulfurane 66 by antiperiplanar intramolecular nucloephilic displacement via a (3-hydroxy-alkoxysulfonium ion intermediate 67 (Eq. 56). ... 

The effect of a heteroatom 0 to the oxirane ring has been studied in the reaction of monosubstituted oxiranes 131 and benzylamine. If X = 0, the reaction occurs via the transition state 132 with neighbouring-group participation. Neighboring-group participation has similarly been observed in the reactions of mono- and tri-substituted oxiranes and )3-substituted primary amines. The product ratio of the aminoalcohols formed from the reactions of a-vinyloxiranes and primary amines depends on the geometry of the oxirane.Base-catalyzed addition of oxiranes to oximes leads not only to 0-alkyl but to A -alkyl derivatives (Eq, 319). ... 

Keywords SN2 and SN2 reactions Baldwin mles Oxirane opening Activation of a cyclopropane 1,2-rearrangement Ring contraction Ring expansion Solvolysis Neighboring group participation Oxirane rearrangement Classical and nonclassical carbocations 5-exo- vis-a-vis 5-endo cyclization Addition and elimination reactions... 

The regiochemistry of hydrolytic oxirane ring opening in alkyl 3,4-anhydro-DL-glyco-pyranosides is dependent on several factors, three of them playing a particularly important role preference for the axial attack of the nucleophile, neighboring group participation, and conformational control. 

In its simplest form, the study of neighboring group effects notes that a heteroatom with a pair of nonbonding electrons can act as a nucleophile in an intramolecular displacement reaction, just as external nucleophiles can participate in intermolecu-lar S]v2 reactions. In an example we have seen (p. 317), oxiranes can be formed from halohydrins in base (Fig. 21.2). 

FIGURE 21.2 Formation of an oxirane by treatment of a bromohydrin with hydroxide involves an alkoxide acting as a neighboring group. 

First, base removes the hydroxyl hydrogen to make the alkoxide. Intramolecular Sn2 displacement of the bromide generates the oxirane. Here the alkoxide is the neighboring group, and the whole process couldn t be simpler. 

Oxiranes of terminal monosubstituted and internal disubstituted olefins do not undergo the isomerization under standard conditions, but give aldehydes at elevated temperature. For the special substrates described in Sch. 35, different modes of reaction originated from intermediary carbocationic species, involve neighboring functional group participation, oxidation, etc. An improvement employing other silicon Lewis acids, for example Mc3SiI and McsSiBr, was developed by Kraus, Detty, and Sakurai [17,19f,62]. 

Oxiranes with a rigid conformation, containing a neighboring OH group, are opened regioselectively (Eq. 159), whereas flexible systems give a mixture of diols. 

In all of the examples so far described (except the isomerization), the oxirane ring is formed by way of internal, bimolecular replacement of the sulfonyloxy group by an alkoxide ion resulting from the neighboring hydroxyl group under alkaline conditions (see Scheme 10). An energetically advantageous situation for the transition state... 

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