Rran Elimination

The enzyme fumarase catalyses the stereospecific iram -addition of water to fumaric acid giving (5)-malic acid, and the reverse reaction, the rrans-elimination of water from (S )-malic acid ... 

When Pt-olefin complexes such as PtCl3(olefin) are treated with a second olefin, replacement of the coordinated olefin by the incoming olefin does not result in either double bond migration or (Z), ( ) isomerization of the displaced olefin. However, the olefin complexes when treated at low temperature with a nucleophile such as pyridine (py) or a secondary amine undergo conversion to a cr-complex by a stereospecific trans-process i.e., rranj-addition and rrans-elimination. Treatment of frans-[PtCl2(Z)-ethylene-l,2-d2)](py)] with excess py at - 15°C gives reversible formation of the carbon bonded complex with slow release of pure (Z)-ethylene-l,2-[Pg.384]

Carbohydrate derivs. A soln. of 2-acetamido-3,4,6-tri-0-acetyl-2-deoxy-a-D-gluco-pyranosyl chloride in isopropenyl acetate containing a catalytic amount of p-toluenesulfonic acid monohydrate refluxed 24 hrs. 3,4,6-tri-0-acetyl-2-(N,N-diacetylamino)-l,5-anhydro-2-deoxy-D-arabino-hex-l-enitol. Y 80-86%. - The diacylamino group significantly facilitates the rran -elimination. N. Pravdic, I. Franjic-Mihalic, and B. Danilov, Carboh. Res. 45, 302 (1975). 

If alkyl groups are attached to the ylide carbon atom, cis-olefins are formed at low temperatures with stereoselectivity up to 98Vo. Sodium bis(trimethylsilyl)amide is a recommended base for this purpose. Electron withdrawing groups at the ylide carbon atom give rise to trans-stereoselectivity. If the carbon atom is connected with a polyene, mixtures of cis- and rrans-alkenes are formed. The trans-olefin is also stereoseiectively produced when phosphonate diester a-carbanions are used, because the elimination of a phosphate ester anion is slow (W.S. Wadsworth, 1977). 

The cycloaddition of benzonitrile oxide to cis- and rrans-l,2-dichloroethylene produced the appropriate cis- and trans-4,5-dichloro-3-phenyl-2-isoxazoline diastereomers. Base elimination produced only one compound, 4-chloro-3-phenyloxazole (Scheme 103) (70CJC3753). 

Data are extant in the literature for two other sets of ethynylene compounds. The sets studied are reported in Table XXXIII. Results of correlation are set forth in Table XXXIV and values of Pr in Table XXXV. The ionization constants of 3-substituted propiolic acids, which were first correlated with the Hammett equation by Charton (18), gave an excellent correlation with eq. (2) (set 22-27-1). The results are very much improved by the elimination of the value for X = H (set 33-27-2). The composition of the electrical effect corresponds approximately to that of the Op constants. The magnitude of the electrical effect is significantly less than that observed for the rrans-3-substituted acrylic acids. 

Table 7. Protected rran.s-Cyclopentendiols from Enantiosclcctive Elimination of 5...

Compounds containing carboxyl groups on adjacent carbons (succinic acid derivatives) can be bisdecarboxylated with lead tetraacetate in the presence of 02.231 The reaction is of wide scope. The elimination is stereoselective, but not stereospecific (both meso- and dl-2,3-diphenylsuccinic acid gave rrans-stilbene) 24" a concerted mechanism is thus unlikely. The... 

The stereoselectivity of elimination of 5-bromononane on treatment with potassium ethoxide was described in Section 5.14. Draw Newman projections or make molecular models of 5-bromononane showing the conformations that lead to cw-4-nonene and rrans-4-nonene, respectively. Identify the proton that is lost in each case, and suggest a mechanistic explanation for the observed stereoselectivity. 

Vic-diols can thus be easily converted to alkenes through their reaction with A, A -thiocarbonyldimidazule. The reported synthesis of trans-cyclooctene is illustrative of the method [219]. It should be noted that continuous elimination of rrans-cyclooctene by a stream of argon was necessary to avoid isomerization to the cis isomer. The conversion of cis-cyclooctene to ww-cyclooctene through a trithiocarbonate is described in the same paper. 

Next, we considered the activation of 13 towards hydrolysis by K-complexation of a cationic metal unit to the electron-rich diene system. On the basis of the well-known palladium-mediated addition of nucleophiles to alkenamines, it was anticipated that the enol ether function in 13 would add H2O in the presence of Pd(II).21 Interestingly, exposure of 13 to a slight excess of Pd(OAc)2 led to the isolation of 14 (Scheme 8). This material suggested the exploitation of the existing Pd-C linkage for carbon-carbon bond formation with an appropriate A-side chain. In particular, the intramolecular syn insertion of the allylic double bond in the rrans-butenyl substituent in 15b and subsequent syn (3-hydride elimination would give the desired E-alkene 17. This proposal was examined using alkene 15a as a model system, synthesized in a manner similar to 13. Upon exposure to Pd(OAc)2 under the conditions... 

Mono- and 1,2-di-substituted alkenes react with PhSeCl/Hg(SCN)2 in benzene (0.5-% h, at 20 C), giving P-rrans-phenylselenoalkyl isothiocyanates in 70-94% yields. Terminal alkenes generally give the product with the selenium terminal (an exception is the product from Bu CH=CH2) internal alkenes show the expected stereochemistry cis to threo, trans to erythro). Oxidation to selenoxides could be achieved cleanly only with ozone, and the products cis eliminate in the usual manner to give predominantly the vinylic isothiocyanates (Scheme 73). 

Photochemical generation of the bent metallocene (39) has turned out to be very useful for the preparation of 3. Thus, (s-trans-r -butadiene)ZrCp2 (3a) is obtained in good yield from the photochemically induced reductive elimination of biphenyl from diphenylzirconocene (10) at low temperatures in the presence of 1,3-butadiene. This is probably the most versatile and most widely applicable method to prepare ( -rran.r-T -diene)zirconocenes. A wide variety of examples of this class of complex [and of (s-cis-diene)zirconocenes 5, as well] has been prepared by this route using various substituted conjugated dienes (22, 23) (Scheme 2). 

A study carried out by Kocienski and Lythgoe flrst demonstrated the trans selectivity of the Julia coupling process. The authors found the i uctive elimination could best be carried out with the acet-oxy or benzoyloxy sulfones. If the lithio sulfone derivative is used for addition to the carbonyl, the reaction can be worked up with acetic anhydride or benzoyl chloride to obtain the alkene precursor. In cases where enolization of the carbonyl is a complication, the magnesium derivative can frequently be used successfully. A modification of the reductive elimination was found to be most effective. Methanol, ethyl acetate/methanol or THF/methanol were the solvents of choice and a temperature of -20 C was effective at suppressing the undesired elimination of the acetoxy group to produce the vinyl sulfone. With these modifications of the original procedure, the ability of the reaction to produce dienes as well as rran.r-disubstituted alkenes was demonstrated, llie diastereoisomeric erythro- and threo-acetoxy sulfones could be separated and it was demonstrated that both isomers were converted to the rrans-alkene. It... 

An alternate radical-induced addition-fragmentation sequence relies on the 1,4-elimination of a leaving group to drive the reaction. In this sequence, because the fragmentation is accompanied by the expulsion of a tiibutylstannyl radical, the requirement for the initial tributyltin hydride is only catalytic (Scheme 79). The stereospecificity of the reaction course has been rationalize in terms of a conformational preference for the elimination step, as depicted in Scheme 79 where the rran.r-substituted cyclohexanone forms only one double bond geometry in the product. As shown in equations (40) and (41), the ci.r-substituted cyclohexanones undergo the four- and three-carbon expansions stereospecifically to give the (Z)-alkenes. 

In contrast to the reactivity behavior observed with alkenylalanes, halogenation of alkenyldialkylboranes takes a different course. Thus, bromination of alkenyl-dialkylboranes proceeds via an initial rran -addition of the bromine to the double bond. rran5-elimination of dialkylboron bromide during hydrolytic workup leads to cis-1 -alkenylbromides... 

The Julia olefination reaction is highly regioselective and ( )-stereoselective, providing a valuable alternative to the Schlosser reaction for making rrans -disubstituted olefins. The reaction involves condensation of a metalated alkyl phenyl sulfone with an aldehyde to yield a P-hydroxysulfone, which is then subjected to a reductive elimination to produce the alkene. There are limitations to the preparation of tri- and tetra-substituted alkenes via the sulfone route because the P-alkoxy sulfones derived from addition of the sulfone anion to ketones may be difficult to trap and isolate or they may revert back to their ketone and sulfone precursors. 

The table shows that E2 eliminations, particularly in five- and seven-membered ring systems, are not completely stereospecific, but four- and six-membered rings exhibit strong preferences. Six-membered rings in particular show distinct anti selectivity, because it is very easy for such systems to reach the rrans-diaxial conformation (12) (Scheme 11 Barton s rule). This anti selectivity can be seen very clearly in the elimination reaction with the isomers of 1,2,3,4,5,6-hexachlorocyclohexane. The isomer (13), which has no Cl trans to an H, loses HCl about 7000 times more slowly than the slowest of the other isomers. ... 

Cyclic olefins preferably undergo twofold carbonylation, even without the presence of buffer compounds such as sodium acetate. With methanol serving as the solvent and under comparable reaction conditions to those mentioned above, cycloalkenes give product mixtures consisting of methyl rrans-2-methoxy-cycloalkanecarboxylates, dimethyl cw-cycloalkane-l,2-dicarboxylates and dimethyl cw-cycloalkane-1,3-dicarboxylates (eq. (3) and Table 1). The formation of 1,3-diesters may be explained by an intermediate reaction sequence which involves an oxidative elimination and a subsequent readdition of the Pd species, thus giving rise to an isomerization [9]. 

As mentioned in the previous section, the Peterson reaction proceeds by an irreversible additicxi of the silyl-substituted carbanion to a carbonyl. It has generally been assumed that an intermediate 3-oxidosi-lane is formed and then eliminated. In support of this mechanistic hypothesis, if an anion-stabilizing group is not present in the silyl anion, the 3-hydroxysilanes can be isolated from the reaction, and elimination to the alkene carried out in a separate step. Recent smdies by Hudrlik indicate that, in analogy to the Wittig reaction, an oxasiletane (304) may be formed directly by simultaneous C—C and Si—O bond formation (Scheme 43). The 3-hyc xysilanes were synthesized by addition to the silyl epoxide. When the base-induced elimination was carried out, dramatically different ratios of cis- to rrans-alkenes were obtained than from the direct Peterson alkenation. While conclusions of the mechanism in general await further smdy, the Peterson alkenation may prove to be more closely allied with the Wittig reaction than with -elimination reactions. 

Sesquiterpenoid chemistry is always full of surprises. For example, an attempt to use titanic chloride in place of aluminium chloride to produce acetylcedrene resulted in a hitherto unknown compound (Me Andrew et al., 1983). In this case, as shown in Figure 4.37, the carbocation produced by addition of the acetyl cation to cedrene, instead of eliminating a proton as normal, underwent a Wagner-Meerwein rearrangement to a second cation, which was then trapped by rran.v-annular addition of the carbonyl group. 

Dienes from epoxides. In refluxing dichloroethane in the presence of triethyl-amine, epoxides undergo a double-elimination reaction, giving conjugated dienes in good yields. Crowded epoxides remain unchanged thus, the mixture of cis- and frani-cyclododecene oxides affords a clean mixture of (IE, 3Z)-cyclododecadiene and rran -cyclododecene oxide (50% yield). 

A problem in the use of dimethylallyl pyrophosphate (3) is its instability. In a study of this problem the half-life of this substance was examined over a range of pH values and temperatures. Both cis- and trans-prenyl pyrophosphates (4 n = 0, 1, or 2) occur in Pinus radiata. Their biosynthesis from [2- C,3R,4K- H]-mevalonic acid proceeded with retention of tritium whereas with [2- C,3K,4S- HJmevalonic acid tritium was lost [except in the case of isopentenyl pyrophosphate (4 n = 0)]. The authors suggest that since they could not detect an isomerase, there may be a cis- and a trans-prenyl transferase both of which eliminate the label derived from [4S- H]mevalonic acid. However, compart-mentalization may have resulted in the isomerase not being available to the administered monoterpenoids, although it may act on geranyl pyrophosphate formed in situ. The absence of 6-cis-famesol derivatives tends to support this idea. Further work on this system again produced no evidence for isomerization or metabolism of [l- H]nerol pyrophosphate to 2-rrans-6-c(s-farnesyl pyrophosphate. 

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