Oxirane Ring-Opening Functionalization

Crotti and co-workers extensively studied the ring-opening functionalization of oxi-ranes using a variety of alkali-metal salts. Several oxiranes were reacted with ammonium halides [119], KCN [120], NaNa [121], lithium acetyhde [122], amines [123], and ketone enolates [124] in the presence of alkali-metal salts to afford the formation of the corresponding 8-functionalized alcohols and some of the results are listed in Table 1. 

Unfortunately, NaC104 and KCIO4 totally lack catalytic activity in the azidolysis and cyanidolysis that utilize relatively weak nucleophiles (entries 7, 8, 18, and 19) whereas the former led to modest activation of oxirane when used with acet-ylides (entry 10), the latter was totally ineffective. 

KCIO4 and KBF4, although readily soluble in MeCN, did not seem to exert any catalytic effect on the aminolysis of oxiranes. 

In some instances, metal ion-catalyzed aminolysis could be performed with toluene, acetone, or Et20 as solvent although all of these led to lower reaction rates than the analogous reactions in MeCN. 

The reactions are highly regioselective, with the attack of the nucleophiles on the less substituted carbon, except for styrene oxide. 

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After conversion to aldehyde 683 (acetonide hydrolysis, tosylation, cyclization with K2CO3), oxirane ring opening with a mixed cuprate derived from 674 as in the previous synthesis cleanly gives 684. Acid-catalyzed hydrolysis of 684 forms the spiroacetal framework, and functional group manipulation leads to the desired aldehyde 685. 

Oxirane ring opening reaction with a nucleophile mediated by alkali metal salts has been studied extensively. Several oxiranes react with ammonium halide [51], KCN [52], NaNs [53], lithium acetylide [54], amines [55], and ketone enolate [56] in the presence of alkali metal salts providing corresponding -functionalized alcohols. 

Azomethine ylides are also frequently obtained by ring opening of aziridines, and the analogous carbonyl ylides from oxiranes. These aspects are dealt with in Section 3.03.5.1. A variety of five-membered heterocycles can also function as masked 1,3-dipoles and this aspect is considered in Section 3.03.5.2. 

Some examples of ring opening reactions with carbanions leading to five-membered heterocyclic ring formation are shown in Scheme 85. Pyrrole syntheses from functionally substituted oxiranes and amines are often described and typical examples are shown in Scheme 86. 

It may be concluded that the conversion of functionalized oxiranes into the corresponding aziridines by an azide ring opening followed by a Staudinger ring closure with triphenylphosphine constitutes a general method for the preparation of aziridines with high enantiopurity. 

Hudlicky and Barbieri (178) developed a simple two-step strategy to generate vinyl oxiranes, and then rearrange the vinyl oxirane to form functionalized dihydrofurans in a formal [2- -3]-annulation process. Both pyrolysis and ring opening with TMSI have been used to initiate the rearrangement process (Scheme 4.90). 

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