Substituted epoxides ring opening

The experimental observations combine with the principles of nucleophilic substitution to give the picture of epoxide ring opening shown in Figure 16.5. The nucleophile attacks the less crowded carbon from the side opposite the carbon-oxygen bond. Bond... 

Cir—R S Nucleophilic substitution, conjugate addition and epoxide ring-opening h... 

Although some substitution patterns -as the 2-substitution- can favour the nucleophilic opening of the resulting epoxide, the use of Ti(OBu )4 in place of Ti(OPr )4 has been proposed as a means to reduce this problem. However, the catalytic version of the reaction usually used nowadays precludes greatly the amount of epoxide ring opening. 

Energies for various possible arenium ions and regioisomeric benzylic cations were computed at the DFT B3LYP/6-31G(d) level or by AMI for comparison with the experimental results. These findings provided further evidence in support of the stability sequence 1-pyrenyl > 4-pyrenyl > 2-pyrenyl in a-pyrene-substituted carbocations as models for the intermediates arising from BaP-epoxide ring opening (Fig. 10). 

Fig. 10 a-BaP-substituted carbenium ions as simplified models for BaP-epoxide ring opening. 

New diterpenoids of the dolabellane class have been reported, such as clavinflol B (592) from the Taiwanese soft coral Clavularia inflata (706). This metabolite has comparable cytotoxicity against the KB cell line to doxorubicin. A sea whip of the genus Eunicea has yielded the cembrane 593 (707). Both 592 and 593 are considered to be natural since no chlorinated solvents were used in the isolation process, and in both metabolites the chlorine is attached to the less substituted carbon, opposite to what is expected for acid-induced epoxide ring opening. A Kenyan soft coral, Sinu-laria erecta, contains the norcembrane sinularectin (594) (708). 

Rough guidelines for the prediction of regioselectivity in epoxide ring openings are summarized in Scheme 4.60. Under neutral or basic reaction conditions alkyl-or aryl-substituted epoxides react with most nucleophiles at the less substituted carbon atom [248-253]. Under acidic reaction conditions, however, product mixtures or preferential attack at the most substituted carbon atom can be observed. Acids can usually be used to enhance the reactivity of epoxides and to promote substitution at the site of an epoxide which forms a carbocation more readily. 

Step 5a Under basic conditions, the epoxide ring opening proceeds at the less substituted carbon (compare to Problem 3 a). 

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