Sequential epoxidation

An endo-Selective Sequential Epoxidation-Oxyallyl Cycloaddition and the First Nitrogen-Stabilized Oxyallyl Cations. 

In the special case in which the substrate is already enantiomerically pure (as in entry 5), it should be clear from Figure 6A.2 that asymmetric epoxidation will be successful (with regard to diastereomeric purity) only when the choice of catalyst directs delivery of oxygen to the face of the olefin opposite that of the C-1 substituent. Such choice of catalyst is further illustrated in Scheme 6A.2, wherein the two sequential epoxidations each proceed with better than 97% diastereoselectivity. The bisepoxide is obtained in an overall yield of 80% [130c],... 

The aim of this section is to demonstrate how reaction calorimetry in combination with IR-ATR spectroscopy can be used for the determination of kinetic and thermodynamic parameters. Several examples of chemical reactions will be discussed, each highlighting a different aspect in the application of reaction calorimetry. The reactions considered are the hydrolysis of acetic anhydride, the sequential epoxidation of 2,5-di-ferf-butyl-l,4-benzoquinone and the hydrogenation of nitrobenzene. The results discussed in this section were obtained using a new calorimetric principle presented below. 

Example 2 sequential epoxidation of 2,5-di-tert-butyl-1,4-benzoquinone... 

The sequential epoxidation of 2,5-di-ferf-butyl-1,4-benzoquinone with ferf-butyl hydroperoxide is shown in Scheme 8.2. In the experiments discussed below, Triton-B was added to the mixture as a catalyst, and the basic reaction model is written as Equations 8.24 ... 

Fig. 8.7 Application of protocol A1 for the sequential epoxidation of 2,5-di-ferf-butyM, 4-benzoquinone at 17 and 30°C using the combined evaluation algorithm [20], Mean values from all experiments at each temperature are shown. Absorbance in the lower plots corresponds to a single wave number (1687 cm"1) from the reaction spectrum.

Table 8.1 Reaction parameters Ari, /o, f a, i and Ea,2 for the sequential epoxidation of 2,5-di-ferf-butyl-1,4-benzoquinone determined by the protocols A1, A2, A3 and B1, B2, B3 [20].

Table 8.3 Kinetic (k, k 2, ordjb, ord p) and thermodynamic (ArH, Ar hfc) parameters for the sequential epoxidation of 2,5-di-ferf-butyM,4-benzoquinone based on six measurements at 30°C at different hydroperoxide concentrations using the protocols C1, C2, C3 [20], The reaction model in Equations 8.25 was applied.

The examples presented in Section 8.3 demonstrate this synergy in an approach using calorimetry and IR-ATR spectroscopy. For the hydrolysis of acetic anhydride, the combination of the two analytical techniques enabled a differentiation between the heat effect due to the chemical reaction and that due to a physical phenomenon - in this case, mixing. Due to this separation of the physical heat effect, a more reliable value for the chemical heat effect was obtained. For the sequential epoxidation of 2,5-di-fert-butyl-l,4-benzoquinone, the importance of selection of an appropriate kinetic model has been demonstrated. For complex reaction systems, several models can be postulated. The appropriateness of these models can then be tested on the basis of experimental data. Combined analytical techniques provide an enriched data set for this purpose as has been demonstrated for this example. After the selection of the most appropriate model, the corresponding parameters can be used... 

A new example of the aza-Payne rearrangement has been used to prepare a-hydroxyaziridines <2005OL3267>. The epoxy imine 673 is prepared by a sequential epoxidation and imination. Reaction of 673 with a series of alkyllithium reagents initially adds to the imine which then does an aza-Payne rearrangement to form the hydroxy-aziridine 674 (Scheme 165). While the method generally suffers from poor yields, the one-step nature of the transformation lends greatly to its appeal. 

Parrain and co-workers examined the epoxidation and subsequent reactivity of 1,4,7,10-cyclododecatetrene 32 <2003JOC3319> fourfold oxidation with either MCPBA (75% yield) or DMDO (98% yield) gave exclusively the co,a o, o,r [Pg.244]

Sequential epoxidation of a /f,/f-hexano-bridged oxepin (65) leads to the interesting mono- and bis-epoxides (66) and (67), respectively (Scheme 11) <82CB1162>. The double bonds adjacent to the oxepin oxygen proved to be the preferred sites of attack in both cases. 

Allenes react with DDO by sequential epoxidation of the two double bonds to give the previously inaccessible, highly reactive allene diepoxides. In the case of the f-butyl-substituted allene shown in eq 8, a single diastereomer of the diepoxide is generated, owing to steric control of the f-butyl group on reagent attack. 

In a total synthesis of the poly-ether antibiotic lasalocid A, the key intermediate (5) (Scheme 11) was prepared by sequential epoxidation of the keto-diene (4) subsequent transformations led to synthetic lasalocid A (6). 

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