Oxirane reactions ring basicity

The oxirane ring-opening reaction requires the presence of a basic catalyst. An acidic catalyst also works, but the polymerization of the oxirane limits its usehilness. In the case of 2-mercaptoethanol (eq. 8), the product has been found to be autocatalytic, ie, the product is a catalyst for the reaction. 

When 1,2-diols are subjected to the same reaction conditions required for the formation of sulphonic esters, oxiranes are produced [27]. Presumably, the mono ester is initially formed and, under the basic conditions, intramolecular elimination occurs to produce the oxirane. Partial hydrolysis and ring-closure of a,p-di(tosyloxy) compounds under basic phase-transfer catalytic conditions provides a convenient route to carbohydrate oxiranes [e.g. 28, 29]. Oxiranes have been produced by an analogous method via carbonate esters from partially protected carbohydrates [30],... 

Few solid-phase syntheses of oxazoles have been reported (Table 15.17). The most general strategy is the dehydration of a-(acylamino) ketones (Entry 2, Table 15.17) or 2-(acylamino)phenols (Entry 1, Table 15.17). Oxazolidin-2-ones have been prepared by intramolecular nucleophilic cleavage of carbamates from insoluble supports (Entries 5 and 6, Table 15.17). Resin-bound 2-aminoethanols, which are accessible by nucleophilic ring-opening of oxiranes with amines, undergo cyclocondensation with aldehydes to yield oxazolidines [220,221]. These compounds are unstable towards acids, and can be released from the support only under neutral or basic reaction conditions. 

The oxirane ring is probably broken by both the SN1 and SN2 processes, depending on the reaction conditions [25-28], In acid-initiated reactions of mutually competing heterocycles of the ether type (e.g in copolymerization), the reactivity of epoxides is lower than would be expected from their high internal strain [29], and it depends more on their basicity [30],... 

Three-membered cyclic compounds can generally be prepared from difunctional compounds by 1,3-elimination. In the formation of oxiranes from 2-substituted alkanols, the alkoxide produced in the basic medium participates in an internal nucleophilic attack, which promotes the departure of the substituent on the adjacent carbon atom leading to ring-closure. A stereochemical condition for the reaction is that the reacting groups should be in an antiperiplanar conformation relative to each other (Eq. 51). 

In the reaction with methyloxirane in H2O at 20 C, the reactivity of the amines decreases in the order piperazine > dimethylamine > pyrrolidine > piperidine > morpholine > diethylamine > di-n-butylamine > di-n-propylamine. ° Under neutral or basic conditions, nucleophiles generally attack at the sterically less-hindered carbon atom of the oxirane ring. 

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