Epoxy dehydration

Borohydrides reduce a-substituted ketones to the corresponding a-substituted alcohols, and such products can be further reduced to olefins (see section VIII). Other reagents serve, through participation of the carbonyl group, to remove the substituent while leaving the ketone intact. The zinc or chromous ion reduction of a-halo ketones is an example of this second type, which is not normally useful for double bond introduction. However, when the derivative being reduced is an a,jS-epoxy ketone, the primary product is a -hydroxy ketone which readily dehydrates to the a,jS-unsaturated ketone. Since... 

Diketo steroids have been prepared by air oxidation of 3-keto-5i5-steroids in potassium /-butoxide-t-butanoF or by hydrolysis of 4,5-epoxy-3-ketones followed by dehydration. However, the most general synthesis is that used by Camerino et al. to prepare Vketo-A-norandrostanes and pregnanes. Hydroxylation of 17a,20.20,21-bismethylenedioxypregn-... 

Embed the dehydrated samples in epoxy resin (Quetol 653), cut into thin sections, stain with 4% uranyl acetate and 0.4% lead citrate, and examine with a Jeol 1200EXS electron microscope. 

This reduction of epoxy ketones has been used to prepare a number of santan-olides from the diepoxide (1) of a-santonin.2 Thus reduction of 1 is accompanied by dehydration of the intermediate tertiary alcohol to give dehydroisoerivanin (2) in 80% yield. 

Tissue processing tissue specimen (0.5 1.0 mm3) are fixed in 4% buffered formalin for 30 60 min, post-fixed in 1% osmium tetroxide in cacodylate or phosphate buffer, pH 7.2 7.4, stained en bloc for 30 min with 2% aqueous uranyl acetate, then dehydrated in ethanol and embedded in epoxy or acrylic resin. 

The full paper on the synthesis of onikulactone and mitsugashiwalactone (Vol. 7, p. 24) has been published.Whitesell reports two further useful sequences (cf. Vol. 7, p. 26) from accessible bicyclo[3,3,0]octanes which may lead to iridoids (123 X=H2, Y = H) may be converted into (124) via (123 X = H2, Y = C02Me), the product of ester enolate Claisen rearrangement of the derived allylic alcohol and oxidative decarboxylation/ whereas (123 X = 0, Y = H) readily leads to (125), a known derivative of antirride (126) via an alkylation-dehydration-epoxi-dation-rearrangement sequence. Aucubigenin (121 X = OH, R = H), which is stable at —20°C and readily obtained by enzymic hydrolysis of aucubin (121 X = OH, R = j8-Glu), is converted by mild acid into (127) ° with no dialdehyde detected sodium borohydride reduction of aucubigenin yields the non-naturally occurring isoeucommiol (128 X=H,OH) probably via the aldehyde (128 X = O). ... 

Other possible reactions, such as homopolymerization (epoxide+epoxide) and epox-ide+hydroxyl group (in the latter stages of cure), can be neglected when the ratio of epoxide to amine is stoichiometric and in the absence of catalyst or accelerator [194], For TGDDM/DDS resins, the homopolymerization reaction may be neglected at cure temperature below 180°C [84], At temperatures between 177°C and 300°C, dehydration and/or network oxidation occur, which results in formation of ether cross-linkings with loss of water. Decomposition of the epoxy-OH cure reaction can also take place, which results in propenal... 

Fig. 22. The reduction in the initial tensile modulus of an epoxy after saturation with moisture and after dehydration under reversible conditions at 20, 70 and...

Drzal et al. 90) have investigated the effect of interphase modification on interfacial moisture absorption. The fibers used were a surface treated and a surface treated and finished type A carbon fiber in the same epoxy matrix studied previously. Three equilibrium exposure conditions were investigated. 20 °C, 70 °C and 120 °C were selected for moisture equilibration of single fiber samples and for the neat epoxy resin. The interfacial shear strength was measured both in the saturated and the dehydrated cases and compared to the initial dry values. 

Figure 22 is a plot of the initial tensile modulus of the epoxy matrix after equilibrium moisture exposure and dehydration. At both 20 °C and 70 °C, the effect of moisture absorption on the matrix is reversible as evidenced by the reattainment of dry properties. The exposure at 125 °C is not completely reversible as shown by the data. 

Exposure to 70 °C gives similar results for the surface treated fiber (Fig. 24). That is, a complete reversibility in noted. The finished fiber (i.e. the fiber with the interphase consisting of the amine deficient brittle interlayer) experiences a nonrecovery of interfacial shear strength after moisture exposure and dehydration. Parallel surface spectroscopic investigation of the fiber surfaces show that under these conditions the fiber surface chemistry is not permanently altered by this exposure. Model studies of epoxies with the amine deficient composition of the interphase show that, the wet Tb of this material is about 70 °C. Therefore, the interphase is at or above its wet Tg and therefore because of the compliant nature of this material, stresses cannot be transfered efficiently and the interface is permanently distorted. 

Exposure at 125 °C is very severe for this epoxy matrix (Fig. 25). Permanent changes in the matrix are noted. The interphase layer, however, acts to mitigate some of the deleterious interfacial effects and allows that system to regain a larger portion of its interfacial shear strength after moisture exposure and dehydration. The fiber without the finish layer has lost almost all of its interfacial shear strength and recovers very little after dehydration. 

Table 2. Application of Fourier Transform IR to Dehydration Studies of Epoxy Systems. Tentative Infrared Absorption Assignments for the Three Cured Epoxy Resins and the Absorption Variations During Degradation 18)... 

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