Template-directed epoxidations

The regioselective intramolecular epoxidation of the peroxy ester (163), which can be prepared from famesol, has been effected by treating it with Cu(OCOCT3)2 (equation S ). This reaction provides a convenient route for the preparation of the 6,7-epoxide (164), which cannot be synthesized from farnesol by conventional methods or even by template-directed epoxidation using Mo(C(5)6/rBHP. 

Remote Photochemical Functionalization 13 3.2 Template-directed Epoxidation... 

This template directed functionalization of double bonds within reach was also used to determine the conformational preferences of some terpene polyenes [62], As in the previously described work with attached benzophenone reagents, the selective template-directed epoxidations revealed the details of chain folding in flexible molecules. 

Although epoxidation reactions are treated in detail elsewhere in these volumes, it should be mentioned here that a template ester attached to a steroid alkene can direct epoxidation to remote double bonds using the general concepts of remote functionalization. Steroidal diene (5) underwent the epoxidation shown (Scheme 13) with excellent regiochemical and stereochemical control. The product was formed in quantitative yield, although the reaction was carried through to only 25% conversion. 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

In agreement with catalytic results it is clear that upon direct grafting, a very high dispersion of isolated tetrahedral centres may be generated on the walls of mesoporous MCM-41 and MCM-48. This in turn allows for the possible tuning to improve the catalytic activity while preserving the mesoporous framework intact. Epoxidation with samples where template was removed by solvent extraction proceeds at better rate than with other mesoporous samples. 

Peters et al. [143] used a valine-based chiral selector as the template molecule to prepare monolithic capillaries. These capillaries were used to successfully separate enantiomers of N-(3,5-dinitrobenzoyl)leucine. However, they found that the hydrophobicity of the monomers had a direct effect on the resolution and efficiency of the capillaries. The peaks tailed drastically due to reverse-phase interactions between the enantiomers and the monolith. They found that increasing the hydrophilicity of the monolith by the hydrolysis of the epoxide functionalities of the glycidyl methacrylate moieties resulted in a much more efficient separation. 

Oxazolin-2-ylidenes can also be used directly for coordination to metal atoms [109], but then they are not functionalised and therefore not covered by this book. However, it may be interesting to note that their generation is possible as a template synthesis in the coordination sphere of transition metal complexes using a functionalised hydroxyisocyanide [110,111] or the reaction of an epoxide with a hydrogen isocyanide complex [112]. 

Remote epoxidation. Breslow and Maresca have found that Sharpless epoxidation of olefins bearing hydroxyl groups (5, 76 77) is amenable to direction by a template (for an example of this method of control see 5, 352-353). Thus the steroid ester 1 can be epoxidized in high yield by t-butyl hydroperoxide with catalysis by Mo(CO)e. In contrast the ester 2 under the same conditions is recovered unchanged. Another striking example of the template effect is that only the 17,20-double bond of the ester 3 can be epoxidized by the Sharplcss method. Ordinarily A - stenols are epoxidized readily. In these examples both the template and the steroid are rigid molecules. 

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