Cis-frans-Isomers

This diene, prepared according to A. E. Montagna and D. H. Hirsch fUS Patent 2,902,722 (1959)], is practically a pure nms isomer, whereas the commercially available reagent is a cis-frans-isomer mixture. 

Unfortunately, this reaction does not appear to be useful for synthesis of isoprenoid polyenes. Thus the ylide (6), prepared as shown, reacts with isobutyr-aldehyde to give four isomers in comparable amounts. Apparently both E- and Z-isomers of (6) are formed and each isomer affords a pair of cis, frans-isomers in the reaction with the aldehyde. 

Ring-opening of the frans-cyclobutene isomer shown takes place at much lower temperature than a similar ring-opening of the cis-cyclouiitciie isomer. Explain the temperature effect, and identify the stereochemistry of each reaction as either conrotatory or disrotatory. 

Where X is phenyl, the result of irradiation (sunlight, mercury lamp) is the formation of Ru(NO)X3(PPh3)(OPPh3) (X = Cl, Br) in the case of the diethyl-phenylphosphine complex, irradiation causes isomerization to the cis,mer-isomer. The frans,mer-isomer is the usual synthetic product, but in the case of dimethylphenylphosphine the/ac-isomer was obtained using short reaction times it isomerized to the usual mer,trans-isomer on heating [123],... 

Osmium forms a 6-coordinate acetylacetonate, Os(acac)3, isomorphous with the ruthenium analogue unlike ruthenium, however, the osmium(IV) complexes Os(acac)2X2 (X = Cl, Br, I) can be made (cis- and frans-isomers exist) from OsXg- and Hacac, as can Os(acac)X [176]. 

The structures of the cis- and frans-isomers of Pt(l,4-thioxane)2Cl2 have been determined. The Pt—S distance (2.298 A) is longer in the frans-isomer than in the cis-form (2.273 A) showing the frans-influence of thioxane to be greater than that of chloride [119],... 

Easily available copper(II) tartrate has also been used for an enantioselective cyclopropanation. From 3-methoxystyrene and 4-bromo-l-diazo-2-butanone, the cyclopropanes cis/trans-204 were obtained the mainly formed frans-isomer displayed an enantiomeric excess of 46% i99>. This reaction constituted the opening step of asymmetric total syntheses of equilenin and estrone. 

The complexes [RhCl(CO)(PR3)]2 can exist as cis- and frans-isomers (Figure 2.26). 

Pt(acac)2 has the expected square planar coordination by oxygen (Pt-O 1.979-2.008 A) with bidentate diketonates this has also been confirmed for Pd(PhCOCHCOMe)2, which is obtainable as cis- and frans-isomers that can be crystallized and separated manually (Figure 3.23). 

The cis- and frans-isomers of Ptpy2X2 can be made by various routes, for example that shown in Figure 3.32. 

The presence of electron-donating substituents in the diene enables it to react with simple aldehydes thus both acetaldehyde and benzaldehyde add to 1-methoxy-1,3-butadiene at 50-65 °C under high pressure (20 Kbar) to give dihydropyrans as 70 30 mixtures of cis- and frans-isomers (equation 5)4. The combination of electron-rich diene/electron-poor dienophile makes it possible to perform the reaction under milder conditions. 2-Alkyl-l-ethoxy-1,3-butadienes and diethyl mesoxalate afford dihydropyrans almost quantitatively (equation 6)5. 

The regiochemistry of the acid-catalyzed water addition to cis- (8c) and trans- (8t) 1-ethoxy-l,3-butadienes leading to 9c and 9t, respectively33, has been investigated in deuterium incorporation experiments (equations 5 and 6). The c/s-isomer incorporated deuterium at the 2-position as well as the 4-position whereas deuterium was added to the fraws-isomer exclusively at the 4-position. This result has been interpreted in terms of equations 7 and 8 y-protonation in the frans-isomer was assumed to be controlled mainly by thermodynamic factors whereas a-protonation was assumed to arise from charge control... 

H and 13C chemical shifts have been reported for many 1,2-disubstituted oxiranes432 436-439. Similar to cyclopropanes, oxirane ring carbon chemical shifts are smaller in the cis- than in the frans-isomers, and an increment rule using pairwise substituent interaction cross-terms has also been developed439. 

Reaction of methyl bicyck>[1.1 O]butane-l-carboxylate with dinitrogen tetroxidc gave methyl 1,3-dinitrocyclobutane-l-carboxylate (18a) as a mixture of the cis- and frans-isomers in only 17% yield. Similarly, the reaction of dinitrogen tetroxide with the corresponding ethyl ester gave the corresponding isomeric ethyl ester 18b in only 40% yield.33... 

Scheme 4.26 Lipase PS-catalyzed acylation for resolution of the racemic cis- and frans-isomers of 4-acetoxy-3-pentyltetrahydropyran (Jasmal).

Dichloroethylene M 96.9, b 60 (cis), d 1.284, b 48 (trans), d 1.257. Shaken successively with cone H2SO4, water, aqueous NaHCO3 and water. Dried with MgSO4 and distn separated the cis- and frans-isomers. 

The cis- and frans-isomers are both dangerously explosive compounds. See other AMMINEMETAL OXOSALTS... 

The result of acid-catalyzed isomerisation of F-dienes depends on several factors structure of substrate, catalyst, and temperature. Action of SbF5 on terminal dienes under mild conditions causes a 1,3 fluorine shift occurring stereoselectively to give trans-, frans-, and cis-, trans- isomers of the corresponding internal dienes [160] ... 

This trans-selectivity ultimately results from the fact that trans-alkenes are more stable than their cis-isomers. This energy difference is especially pronounced for the alkenes in Figure 4.3 because they are styrene derivatives. Styrenes with one alkyl group in the trans-position on the alkenyl C=C double bond enjoy the approximately 3 kcal/mol styrene resonance stabilization. This is lost in cis-styrenes because in that case the phenyl ring is rotated out of the plane of the alkenic C=C double bond to avoid the cis-alkyl substituent. However, the transselectivity documented in Figure 4.3 is not a consequence of thermodynamic control. This could occur only for a reversible elimination or if the alkenes could interconvert under the reaction conditions in some other way. Under the conditions of Figure 4.3, alkenes are almost always produced irreversibly and without the possibility of a subsequent cis/frans-isomeriza-tion. Therefore, the observed trans-selectivity is the result of kinetic control. 

It does not matter whether it is the cis- or the frans-isomer of the allyl alcohol that is more easily accessible. According to Figure 11.46, by selecting the appropriate solvent enolate formation can be directed to convert both the cis- and the /ram-allyI alcohols into rearranged products that contain either a syn- or an ow/i-arrangement of the vicinal alkyl groups. 

Acyclic azo compounds are obtained by usual routes only as frans-isomers. Photochemical isomerisation provides an easy and effective way to the cis-isomers. The cfs-azo compounds are however thermally quite labile and decompose with elimination of nitrogen 41>, some of them even at —120 °C 5>. Activation energies for the thermal decomposition of cfs-azo-alkanes are of the order of 20... 

The procedure reported here (parts B and C) has been applied with minor modifications to the syntheses of the cis,cis,cis isomers of the 1,3,2-dioxaborole and perhydro-9b-phenalenol.7 The two other stereoisomers, cis,trans,trans and trans,trans,trans, have been prepared from cis, cis, fran,v-perhydro-9b-phenalenol via the cis and trans isomers of A3a,9b-perhydrophenalene.5 In addition, a few isomers of perhydrophenalenol and of perhydrophenalene and cis, cis, frans,-9b-chloroperhydrophenalene have also been prepared from cis, cis, fraw.s-perhydro-9b-phenalenol.5 Some of the representative transformations are summarized in Scheme 1. (The numbers in parentheses refer to references.)... 

It has recently been shown that H NMR spectra can distinguish between cis- and fran.s-isomers of this type. The /(Pt-H) coupling constants between platinum and the o-hydrogen of the pyridines are slightly higher for the cw-isomers therefore for cw-Ptpy2Cl2 /(Pt-H) is 42 Hz while /(Pt-H) is 34 Hz for the fran -isomer [83]. 

Stahl, W. Schwarz, W. von Laar,J. Sies,H. 1995. All-frans P-carotene preferentially aeeumulates in human ehylomierons and very low density lipoproteins compared with the 9-cis geometrieal isomer. J. Nutr. 125 2128-2133. 

Cycloaddition of orf/io-quinone 162 with ferf-butyldimethylsilyl ether of sinapyl alcohol (163) yielded the 1,4-dioxane-linked adduct 164, as shown in Scheme 10b, but no regioisomer was detected. The ketal group in 164 was removed under acidic conditions and the derived ketone, a trans-cis mixmre in the ratio of 2 1, on treatment with alkali, yielded the frans-isomer, 165, in a good yield. The ketone 165 was converted to the key intermediate aldehyde 166 through the iodoform reaction, esterification, LAH reduction, and oxidation with Dess-Martin periodinane after protection of the hydroxy groups by MOM groups (Scheme 10b). 

The absorption of light by rhodopsin results in a change in the configuration of the retinaldehyde from the 11-cis to the all-frans isomer, together with a conformational change in opsin. This results in both the release of retinaldehyde from the Schiff base and the iiutiation of a nerve impulse. The overall process is known as bleaching, because it results in the loss of the color of rhodopsin. 

Chiroptical Methods. The c.d. spectra of some mono-cw-carotenoids have been studied. All showed opposite Cotton effects relative to the all-frans-isomers. The intensity of c.d. bands in the cw-region was enhanced in the cis-isomers, especially if the cis-double bond was near to the centre of the chromophore. It was confirmed that in c-ring carotenoids, chiral centres at C-2 and C-3 do not contribute significantly to the c.d. spectra, but a 19-hydroxy-group does influence chiroptical properties. The c.d. properties of squid and octopus rhodopsin and metarhodopsin have been discussed in relation to the conformation of the retinal chromophore.The o.r.d. spectra of retinal and retinylidene phos-phorylethanolamine in a micelle have been obtained. 

Polyterpenoids.—The presence of dolichols (202) with up to 23 isoprene units, in various sources, has been reported.In addition to these dolichols, which have their terminal isoprene unit saturated, fully unsaturated polyprenols have been found. Bovine pituitary contains a mixture of cis-trans-isomers of decaprenol (203), a Cioo prenol from bovine thyroid has been shown by n.m.r. to have eighteen cis- and two frans-isoprene residues, and 2,3-dehydrodoli-chyl pyrophosphate (204) has been identified as a product of a chick enzyme system and isopentenyl pyrophosphate.The structure of ulmoprenol from Eucommia ulmoides has been elucidated (207). ... 

Dieb-AUer catalyst. Aluminum chloride catalyzes the reaction of 2-cyclo-alkenones (1) with 1,3-butadiene to give bicyclic ketones in satisfactory yields. The rate of the addition and the yields are strongly dependent on the quantity of catalyst. When less than 0.3 equiv. of AICI3 is used, the rate and yield are low when more than 1 equiv. is used, the desired reaction fails. When R = H, the initial cis-adducts isomerize to the frans-isomers. ... 

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