Ethers, Acetals, and Epoxides

TABLE 4.15 Chemical Shift of Ethers, Acetals, and Epoxides (ppm from TMS) 

An alkoxy substituent causes a somewhat larger shift to the left at C-l (—11 ppm larger) than that of a hydroxy substituent. This is attributed to the C-l of the alkoxy group having the same effect as a /3-C relative to C-l. The O atom is regarded here as an a-C to C-l. 

Note also that the y effect (shift to the right) on C-2 is explainable by similar reasoning. Conversely, the ethoxy group affects the OCH3 group (compare CH3OH). Table 4.15 gives shifts of several ethers. 

The dioxygenated carbon of acetals absorbs in the range of about 88-112 ppm. Oxirane (an epoxide) absorbs at 40.6 ppm. 

The alkyl carbon atoms of arylalkyl ethers have shifts similar to those of dialkyl ethers. Note the large shift to the right of the ring ortho carbon resulting from electron delocalization as in the vinyl ethers. 

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This reaction is also a transfer dehydrogenative reaction, as two reactant hydrogen atoms are not incorporated into the enol silyl ether product but instead serve to hydrogenate another molecule of starting alkene. For example, in the reaction of vinylcyclohexane, ethylcyclohexane is obtained in equal amounts to the silylated product. Both iridium complexes effectively catalyze the reaction. Various silanes can be used, including di-ethylmethyl-, triethyl-, and dimethylphenylsilane. The reaction is successful for a range of terminal alkenes, even those bearing cyano, acetal, and epoxide functionalities. The E isomer of the product is predominantly formed. 

The ring-opening process of Equation 8.45 is, of course, simply the reverse of the process by which oxiranes (oxacyclopropanes, epoxides) are formed from halohy-drins (e.g., see item 3,Table 7.6). Further, as written, the processes shown in Schemes 8.90-8.92 are reversible and thus, at least in principle, carbonyl compounds can be converted to enol ethers, acetals (and ketals), and orthoesters. However, while acetals and ketals readily form from alcohols and acids under dehydrating conditions (Chapter 9) and esters undergo exchange reactions with alcohols in the... 

Telomerization Reactions. Butadiene can react readily with a number of chain-transfer agents to undergo telomerization reactions. The more often studied reagents are carbon dioxide (167—178), water (179—181), ammonia (182), alcohols (183—185), amines (186), acetic acid (187), water and CO2 (188), ammonia and CO2 (189), epoxide and CO2 (190), mercaptans (191), and other systems (171). These reactions have been widely studied and used in making unsaturated lactones, alcohols, amines, ethers, esters, and many other compounds. 

Dichlorodicyanoquinone (DDQ), CH2CI2, H2O, 40 min, it, 84-93% yield.This method does not cleave simple benzyl ethers. This method was found effective in the presence of a boronate. The following groups are stable to these conditions ketones, epoxides, alkenes, acetonides, to-sylates, MOM ethers, THP ethers, acetates, benzyloxymethyl (BOM) ethers, and TBDMS ethers. 

Other leaving groups are sometimes used. Sulfates, sulfonates, and epoxides give the expected products. Acetals can behave as substrates, one OR group being replaced by ZCHZ in a reaction similar to 10-101. Ortho esters behave similarly, but the product loses R OH to give an enol ether. ... 

With these many requirements, it is obvious that each group of polymers has its own plasticizers, although some types of plasticizers have been developed which are suitable for several plastics. They belong to the groups of esters, ketones, ethers, acetals, epoxides, and nitrogen containing compounds like the sulfonamides. 

The reagent utilized here is BF3OEt2 A possible mechanism for the formation of 5 has Bl VOhb first acting as a Lewis acid to cleave the acetal to enol ether 16 and then activating the epoxide for intramolecular nucleophilic attack by the alkoxide at C-20. In this way the stereochemistry ol the tetrahydrofuran ring in 17 is established. 

The photochemistry of ethers and related compounds (e.g. epoxides, acetals and hemi-acetals) is still relatively an unexplored field. Although activity in this field has increased recently, further work remains to be done and synthetic exploitation is still open to investigation. 

Recently, comprehensive World Wide Web (Internet) databases have been established on insect pheromones and semiochemicals The Pherolist , a database of chemicals identified from sex pheromone glands of female lepidopteran insects and other chemicals attractive to male moths (Am et al., 1999) and The Pherobase , a database of pheromones and semiochemicals for Lepidoptera and other insect orders (El-Sayed, 2006). These large databases on behavior modifying chemicals have extensive cross-linkages for animal taxa, indexes of compounds and source (reference) indexes. The indexes include those compounds cited in this chapter and many more with pheromone and semiochemical function acetate esters, diols, epoxides, ethers, ketones and secondary alcohols. For example, The Pherolist reports approximately 90 epoxy derivatives of C17-C23 of n-alkancs, mono-alkenes and di-alkenes as insect semiochemicals. 

Heat resistant IPN systems were obtained by simultaneous radical polymerization of divinylbenzene with benzoyl peroxide as an initiator and Zn acetate as cyclotri-merization catalyst [122], Hot-curing composition contains BPA/DC, BMI, epoxide resin, Zn acetate and divinylbenzene [123]. Crosslinked compositions consisting of BPA/DC and BPA bis(vinylbenzyl) ether show Tg values above 240 °C [124]. 

Lead tetra-acetate oxidation of the allylic alcohols (170)—(172) and (182) leads to the formation of the epoxides (183)—(186), products of a novel internal addition reaction of the electron-deficient alcohol oxygen to the allylic double bond. In some cases, (171) and (172), the formation of a new type of acetoxylated enol ether (173) and (174) is observed. Oxidation of the allylic dienols (175) and (176) gives the epoxyacetates (187) and (188). A variety of cyclization products was also isolated. Their formation requires an isomerization of the allylic trans double bond to cis.69 Lead tetra-acetate oxidation of dihydro-y-ionol (189) gives the new bicyclic ether... 

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