Stilbene oxide ring opening

Potassium permanganate does not oxidize the sulfur atom in dibenzo[ft,/]thiepin (1), rather attacking the C —C double bond, which is very similar in reactivity to that in (Z)-stilbene, to provide various oxidation products, formed from ring opening or ring contraction, depending upon the acetone or water reaction medium.20... 

Reactions of 1 with epoxides involve some cycloaddition products, and thus will be treated here. Such reactions are quite complicated and have been studied in some depth.84,92 With cyclohexene oxide, 1 yields the disilaoxirane 48, cyclohexene, and the silyl enol ether 56 (Eq. 29). With ( )- and (Z)-stilbene oxides (Eq. 30) the products include 48, ( > and (Z)-stilbenes, the E- and Z-isomers of silyl enol ether 57, and only one (trans) stereoisomer of the five-membered ring compound 58. The products have been rationalized in terms of the mechanism detailed in Scheme 14, involving a ring-opened zwitterionic intermediate, allowing for carbon-carbon bond rotation and the observed stereochemistry. 

Table 10. Products Yield and ee s of asymmetric ring opening reaction of ieso-stilbene oxide with aniline using complexes 70-72...

Table 11. Asymmetric catalytic ring opening of meso stilbene oxide with aniline using...

The same catalyst 73 also worked efficiently for catalytic asymmetric ring opening of cyelohexene oxide and meso-stilbene oxide [34] with anilines under mierowave irradiation to afford P-amino alcohols in high yield (up to 95%) and good enantioseleetivities (ee up to 55%). The reaction under mierowave was found to be 10 times faster than traditional oil-bath heating without compromising enantioselectivity. Also the ee for the produet P-amino aleohols was comparable with the values obtained at room temperature (Table 12). 

Cis-stilbene (Zl) also undergoes a conrotatory cyclization reaction into trans-4a,4b-dihydrophenanthrene (DHP, 2), a short living, not isolated product with an absorption in the visible spectrum at 450 nm. In the absence of an oxidizing agent DHP will return to the starting material, by both a thermal and a photochemical ring opening reaction. 

Fig. 5a) does not correlate with the betaine HOMO (Fig. 5b), and it can be shown, therefore, that the thermal reactions are symmetry forbidden whereas photochemical reactions are symmetry allowed. Mechanistically, these reactions are analogous to the disrotatory ring opening of a-cyano-c/.s-stilbene oxide (450 - 451) which Huisgen and Markowski have demonstrated takes place thermally with a free energy of activation of 35.6 kcal mol at Clearly, although forbidden, these reactions are not... 

An investigation of various catalysts, the solvent, and the additive used in the meso-stilbene oxide enantioselective ring opening with anilines at room temperature has been carried out.29 The best catalyst, solvent, and additive proved to be (17), toluene and triphenylphosphane, respectively. The reaction gave 95% of the syn-fi-wm m alcohol with 78% ee. A single recrystallization increased the enantiomeric excess to 98%. The catalyst is recyclable. 

Ammonia and amines open oxide rings to form amino alcoholthe yields are markedly higher when amines are employed (55-90% vs. 18-40%).The ready availability of ethylene and propylene oxides makes this procedure attractive for preparing 2 dialkylaminoethanols and l-dialkylamino-2-propanols. Thus /3-diethylaminoethanol is conveniently prepared by the addition of ethylene oxide to diethylamine in methanol at 45° to 60° or by a combination of the two reactants in an autoclave at 100° (81%). Isopropylamine reacts with ethylene oxide in the presence of water and a small amount of hydrochloric acid to form /3-isopropylaminoethanol (76%). The reaction is general and is shown by higher oxides like isobutylene oxide/ styrene oxide/ and stilbene oxide/ ... 

Stiituner and Messwarb have reported addition of aniline, 1 -naphthylamine, and others to eis- and Iraaa-stilbene oxide to afford tireo and ers/tkro adducts respectively, as shown in Eq. (627). Their observations are consistent with the premise that ring-opening ia accompanied by Walden inveimon at the site t nudeophUic attack. 

Imagine the ring opening of m-stilbene oxide 37 by an amine in an SN2 reaction. The product is the amino alcohol 38. Redrawing this 38a so that we have the longest chain in a zig-zag is easy by application of a GSR to the right hand portion of the molecule. Note that we have not drawn any structure with a hashed or thick line between two chiral centres. In this sequence the question of absolute stereochemistry does not arise as m-stilbene oxide 37 has a plane of symmetry and is therefore achiral. 

Desymmetrization with halogen nucleophiles was effectively demonstrated with two mechanistically-divergent chiral catalysts. Denmark disclosed a Lewis-base activated delivery of chloride that was catalyzed by the enantiopuxe phos-phoramide 16. Binding of the phosphoramide was believed to induce dissociation of SiCl4 into the chiral phosphorus/silicon cation and chloride anion, which subsequently ring-opened the activated epoxide. The best enantioselectivity was observed with cfs-stilbene oxide, which was formed in 94% yield and 87% ee (Scheme 14) [28]. 

The product of the reaction with c/s-stilbene was a trichloroacetate ester which yielded the wcso-glycol upon reduction with lithium aluminium hydride the /raHs-olefin yielded the racemic glycol after the same treatment . Such reaction products are consistent with ring-opening of the epoxide by strong acid, and it was shown that (-)-rraHs-stilbene oxide reacted rapidly (relative to the rate of epoxidation) with trichloroacetic acid in benzene the overall observations are indicative of acid-catalysed epoxidation of stilbene but may admit other interpretations, one of which involves thermodynamic intervention of trichloroacetic acid dimer in a solvating capacity rather than in a truly kinetic function. 

The effect of temperature on the stereochemistry of the acid-catalysed ethanolysis of rran5-stilbene oxide has revealed two reaction mechanisms, operating concurrently. The oxiran ring may open either (a) by nucleophilic... 

Zeolites are widely used in heterogeneous catalysis. In principal, their highly controllable porous structures have great potential for use as enantioselective catalysts. A considerable research effort has been devoted to the development of chiral zeolites [32]. Only zeolite beta and titanosiUcate ETSIO exist in chiral form [33, 34], although it is very difficult to obtain zeolite in enantiopure form [32]. Zeolites are typically synthesized in the presence of surfactant templates, which are removed by high-temperature calcination, a process that invariably destroys the chiral conformation of such assemblies [32]. Low enantioselectivities attributable to the chiral zeolite framework structure have been observed by Davis and Lobo [35] for the ring opening of trans-stilbene oxide with water. 

The (/ ,/2)-[Cr(salen)]X catalyzed addition of indoles of type 75 was also applied by Umani-Ronchi (2004) to the asymmetric ring opening of mejo-stilbene oxide (76). Treatment of indoles 75a-c (1.5 equiv) and meso-stilbene oxide (1.0 equiv 76) in TBME with 5 mol% of (if )-[Cr(salen)]Cl afforded the corresponding chiral (S,./ )-P-indolyl alcohols (77a-c) in superb yield (95-98%) and enantiomeric purity (90-98% ee). Incorporation of a 2-Me moiety or a 5-MeO moiety into the framework of the indole nucleus in place of H improves or reduces the optical purity of the alkylated product [compare 77c (98% ee) and 77b (90% ee) versus 77a (93% ee), respectively]. 

Ring opening of oxiranes is catalyzed by zirconium or hafnium complexes in the presence of nucleophiles. Cp2ZrCl2 was used as a catalyst for ring opening of substituted epoxides with alcohols under mild conditions. The corresponding alkoxyalcohols were obtained in good yields. As shown in Equation 38, when trans-stilbene oxide was subjected to the reaction in methanol, a mixture of anti-and syn isomers were obtained [43]. 

The aryl meio-epoxide, c -stilbene oxide, is ring-opened by substituted anilines at 0°C in dichloromethane in the presence of 4 A molecular sieves and a (R)-(+)-BINOL-Sc(OTf)3-A-methylmorpholine complex giving )0-amino alcohols in yields ranging from 78 to 92% with 64-94% The aliphatic meso-epoxide, cyclcohexane i oxide, gave 96% of the / -amino alcohol but with only 22% ee. 

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