Epoxides nucleophilic ring-opening

Me o-epoxides, nucleophilic ring-opening reactions by aryllithium/(—)-sparteine (11) have been realized by Alexakis and coworkers with very good differentiation between the enantiotopic positions . Eliminative desymmetrization reactions of me o-oxacycles such as 144 or 147, which are triggered by an enantiotopos-differentiating deprotonation... 

Scheme 5.2 Domino epoxidation-nucleophilic ring opening...

C4C im][BF4] Re03Me UHP Domino epoxidation-nucleophilic ring opening of glycols with dibutylphosphate product [17]... 

DOMINO MTO-CATALYZED EPOXIDATION-NUCLEOPHILIC RING OPENING OF GLYCALS... 

G. Soldaini, F. Cardona, A. Goti, Methyltrioxorhenium ctalyzed domino epoxidation-nucleophilic ring opening of glycals. Tetrahedron Lett. 44, 5589-5592 (2003). 

Epoxides provide another useful a -synthon. Nucleophilic ring opening with dianions of carboxylic acids (P.L. Creger, 1972) leads to y-hydroxy carboxylic acids or y-lactones. Addition of imidoester anions to epoxides yields y-hydroxyaldehyde derivatives after reduction (H.W. Adickes, 1969). 

We saw an example of nucleophilic ring opening of epoxides in Section 15 4 where the reaction of Grignard reagents with ethylene oxide was described as a synthetic route to primary alcohols... 

Nucleophilic ring opening of epoxides has many of the features of an 8 2 reac tion Inversion of configuration is observed at the carbon at which substitution occurs... 

As we ve just seen nucleophilic ring opening of ethylene oxide yields 2 substituted derivatives of ethanol Those reactions involved nucleophilic attack on the carbon of the ring under neutral or basic conditions Other nucleophilic ring openings of epoxides like wise give 2 substituted derivatives of ethanol but either involve an acid as a reactant or occur under conditions of acid catalysis... 

The reactivity of epoxides toward nucleophilic ring opening is responsible for one of the biological roles they play Squalene 2 3 epoxide for example is the biological... 

Nucleophilic ring opening of epoxides by ammonia (Section 16 12) The strained ring of an epoxide is opened on nucleo philic attack by ammonia and amines to give 3 ammo alcohols Azide ion also re acts with epoxides the products are p azido alcohols... 

The 2,3-epoxy alcohols are often obtained in high optical purity (90% enantiomeric excess or higher), and are useful intermediates for further transformations. For example by nucleophilic ring opening the epoxide unit may be converted into an alcohol, a /3-hydroxy ether or a vicinal diol. 

The suggested reaction mechanism involves a nucleophilic attack of the imine nitrogen at the activated triple bond, followed by a proton exchange, to give a benzimidazolinium system which, by intramolecular attack at the carbonyl group, leads to an epoxide that ring opens to the observed product. For the ethyl derivative (R = Et) a tub conformation could be established by X-ray crystallographic analysis.33... 

Metalated epoxides are a special class of a-alkoxy organometallic reagent. Unstabilized oxiranyl anions, however, tend to undergo a-elimination. On the other hand, attempts to metalate simple unfunctionalized epoxides may lead to nucleophilic ring opening. The anion-stabilizing capability of a trimethylsilyl substituent overcomes these problems. Epoxysilanes 22 were... 

A variety of reaction conditions have been developed for nucleophilic ring opening by cyanide.123 Heating an epoxide with acetone cyanohydrin (which serves as the cyanide source) and triethylamine leads to ring opening at the less-substituted position. 

Scheme 12.15 gives some examples of both acid-catalyzed and nucleophilic ring openings of epoxides. Entries 1 and 2 are cases in which epoxidation and solvolysis are carried out without isolation of the epoxide. Both cases also illustrate the preference for anti stereochemistry. The regioselectivity in Entry 3 is indicative of dominant bond cleavage in the TS. The reaction in Entry 4 was studied in a number of solvents. The product results from net syn addition as a result of phenonium ion participation. The ds-epoxide also gives mainly the syn product, presumably via isomerization to the... 

The key to the success of the vinyl epoxide route would reside in a nucleophilic ring-opening of vinyl epoxide 75. If this ring-opening proceeds regioselectively at C5 via a SN2 pathway, it would have been the most welcome at this stage of our efforts (Scheme 8.21). However, an array of conditions, including Pd(0)-mediated and Lewis acidic conditions, were screened over a period of almost 2 years, but we never observed the desired 1,4-diol 80. Instead, we could at various times see small... 

The primary type of epoxide reaction remains the nucleophilic ring-opening reaction. Research on the development of novel catalysts or catalytic systems for epoxide opening continues to be a highly active area of study. 

Key steps, as shown in Scheme 4-15, involve the formation of a urethane intermediate 37 by treating epoxide 36 with methyl isocyanate in the presence of sodium hydride. Intramolecular A-nucleophilic ring opening of oxirane affords oxazolidine 38. Subsequent treatment furnishes product 34. 

Sasidharan et al. (258) reported the formation of pinacols from alkenes catalyzed by various titanosilicates. Aluminum-free Ti-beta exhibited better activity and pinacol selectivity than TS-1, TS-2, Ti-MCM-22, or mesoporous Ti-MCM-41 (Table XLVIII). The side products included the pinacolone, alcohol, and oligomers. The epoxide was the initial product, which underwent acid-catalyzed nucleophilic ring-opening by H2Q molecules to give the pinacol (Scheme 25). 

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