Frans-elimination

Alcohols with organoelement groups listed in Table 13 gave with one exception only small amounts of olefine. But with hydroxyalkyl-selenides (35a, G = -SePh or -SeCHs) stereospecific frans-elimination can be achieved in acidic (for instance excess of perchloric acid ether at room temperature) or basic media to give olefines in good yield So we can state that preparatively useful carbonyl olefination reactions in which epoxides are not a by-product, are allowed not only with phosphorus and silicium containing regents but are possible in the wide area of the periodical table marked in Scheme 55c with little lines. 

The Brown group developed a multi-step procedure including sy -hydroboration of terminal alkynes and frans-bromination followed by frans-elimination to access Z-afkenyl bromides (Fig. 16) [39]. 

Labeled compounds. A soln. of codeinone dimethyl ketal[6-O GH3] in toluene mixed with a suspension of POGI3 in pyridine, and heated 6 hrs. at 115-120° under Ng -> thebaine[6-O GH3]. Y 70-75% retention of radioactivity 90%. -By this method, the quasi-axial methoxy group is czs-eliminated, whereas K-ethoxide in xylene frans-eliminates the quasi-equatorial labeled methoxy group causing 85% loss of radioactivity. U. Eppenberger, M. E. Warren, and H. Rapo-port, Helv. 51, 381 (1968). 

FIGURE 22.4 New-man projections showing the conformations leading to (a) 1-butene, and (b) frans-2-butene by Hofmann elimination of sec-butyl-trimethylammonium hydroxide. The major product is 1-butene. 

Kira and coworkers25 found that in deaerated DMSO solution of frans-stilbene both the solute cation and anion are produced and the anions are eliminated by aeration. Since they found26 that the absorption spectra of the anthracene cation and anion are quite similar, they suggested25 that the absorption spectrum observed by Hayon for anthracene solution in DMSO is a superposition of the spectra of the solute cation and anion. This observation casts a serious question on the yield of solvated electrons found by Hayon23. 

The possibility that 34 and 35 were formed via 33 was eliminated and hence it must be concluded that, in contradistinction to the reaction with 2,3-dibromocyclo-pentyl hydroperoxide 38), the Ag02CCF3-induced dioxabicyclization of 3,4-dibromo-cyclopentyl hydroperoxide involves preferential displacement of the ciy-3-bromine. It seems highly probable that this process is assisted by the vicinal bromine, i.e. that the frans-bromonium ion 36 is an intermediate. Failure to observe the analogous mechanism with 2,3-dibromocyclopentyl hydroperoxide presumably reflects the disfavoured nature of the mode of ring closure needed in the corresponding species 37. 

The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the trans elimination from a boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). 

Reaction of the TMS-nitro compound 1496 and the indole carboxaldehyde 1497 with a catalytic amount of TBAF led to the desired alcohol 1498, which, on further treatment with TFAA, followed by elimination of the corresponding trifluoroacetate with DBU, afforded the frans-stilbene 1499. Reductive cyclization of 1499 under Cadogan-Sundberg conditions afforded the bisindole 1500. Finally, condensation of 1500 with (dimethylamino)acetaldehyde diethyl acetal led to tjipanazole D (359) in 71% yield (796) (Scheme 5.255). 

Of the synthetically more useful substrates studied in such asymmetric eliminations, protected cis- and frans-3,4-epoxycyclopentanol are converted in excellent yield and with modest to high enantioselectivity to (lS /O-l -dihydroxycyclopentene derivatives, which are useful intermediates in the synthesis of prostaglandins66 (Tables 6 and 7)42 44 66a. 

Rhodium-catalyzed, silane-initiated cascade cyclization of 1,6,11-triynes 83 was proposed to occur via a silane-initiated cascade carbocyclization to form the silylated bicyclic triene (Z,Z)-In. / -Migratory insertion of the silylated G=G bond into the Rh-G bond of (Z,Z)-In followed by / -hydride elimination from frans-lln could then form 84a. Alternatively, cisitrans isomerization of (Z,Z)-In followed by / -migratory insertion of the silylated G=G bond into the Rh-G bond of resulting isomer ( ,Z)-In could form cis-Wn. Subsequent / -silyl elimination from m-IIn would form unsilylated tricycle 84b (Scheme 21). 

The rate of dehydrobromination was found to be three times faster than dehydrochlorination and the reaction followed anti elimination.7 Thus, anti elimination of hydrogen chloride from as.a.s,fran. -l,2,3,4-tetrachlorocyclobutane (16) with potassium hydroxide affords rram-1,3.4-trichlorocyclobutene (17) in 48% yield. However, anti elimination of hydrogen bromide from... 

Thermolysis of indenone oxides (151) is an equally useful route to the betaines (150). Dimethyl acetylenedicarboxylate and compound 151 (R = R = Ph) at 175°C give adduct 153, and cyclohexanone at 150 C gives adduct 154. - Similarly, 1,3-dipolar adducts (e.g., 155) have been obtained using a wide variety of olefins—including cis- and [rans-, 2-dichloroethylene, dimethyl maleate, dimethyl fumarate, maleic anhydride, cis- and tran -stilbene, fran -dibenzoylethylene, tra .y-l,2-dicyanoethylene, A -phenylmaleimide, vinylene carbonate, acenaphthylene, and norbor-nadiene. With cis olefins the endo adduct (155) is usually the predominant isomer. Diphenylcyclopropenone gives compound 156 by spontaneous elimination of carbon monoxide from the initial adduct (157). Adduct 156... 

Catabolism of histidine in most organisms proceeds via an initial elimination of NH3 to form urocanic acid (Eq. 14-44). The absence of the enzyme L-histidine ammonia-lyase (histidase) causes the genetic disease histidinemia 284/285 A similar reaction is catalyzed by the important plant enzyme L-phenylalanine ammonia-lyase. It eliminates -NH3+ along with the pro-S hydrogen in the (3 position of phenylalanine to form frans-cinnamate (Eq. 14-45). Tyrosine is converted to p-coumarate by the same enzyme. Cinnamate and coumarate are formed in higher plants and are converted into a vast array of derivatives (Box 21-E,... 

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