Frans-Isomers

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 frans-isomer can be obtained as a second product in the pentammine synthesis, also by... 

Pfeiffer (1913) pointed out that [Pt(PR3)X2]2 complexes can potentially exist in three isomeric forms, but only the symmetric frans-isomer has been characterized (Figure 3.40). 

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],... 

Perhalogenoaryls are more stable than the unsubstituted phenyls [176] and can be synthesized conveniently by oxidation of gold(I) complexes (demonstrating the stability of the Au-C bond). The initial product of oxidation addition seems to be the frans-isomer, which generally rearranges to the m-form ... 

Substituted 1-hydroxy cyclohexane-1-carboxyhc acids, which could be prepared from the corresponding cyanohydrins by acid hydrolysis as described above, are important as pharmaceuticals and plant-protective agents. Although the compounds derived from 2- and 3-cyclohexanones have two stereogenic centers, stereoselective syntheses of these interesting products have been published only very recently. " Completely unexpected are the results of HNL-catalyzed additions to 4-substituted cyclohexanones, which do not possess a prochiral center. The (R)-PaHNL-catalyzed addition affords almost exclusively fran -isomers, whereas with (5 )-MeHNL cA-addition is favored (Table 4). ... 

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. 

Syntheses of (l )-frans-isomers were reported by Crombie [24] and Elliott [25] starting from (1 /t Wran.v-chrysanthemic acid by means of the Wittig reaction. Their method were convenient to obtain (Z)-isomer (Scheme 10, step a) but not appropriate for the synthesis of ( )-isomer because of the (Z)-selective nature of the Wittig reaction in the case of nonstabilized ylides. It was very difficult to separate the pure ( )-isomer out of the (E)- and (Z)-mixture. This problem was overcome by use of the Takai s method (Scheme 10, step b) [26]. The ( )-selectivity of the double bond was fairly high (E Z = 89 11) (Scheme 10). 

Crystalline polyesters from CHDM and aliphatic diacids are possible, but generally of little interest because of low melting points and low glass transition temperatures. Cyclic aliphatic diacids offer some potentially attractive possibilities. For example, the polyester of CHDM with a high-frans isomer 1,4-cyclohexanedicarboxylate has a melting point ( 225°C) similar to that of PBT [53],... 

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... 

A series of p-substituted 2-fluoro-2-arylcyclopropylamines (60b-d, 61b-d) were shown to be competitive inhibitors of this enzyme [131], The nature of the p-substituents in the more potent frans-isomer (c/ s-relationship between fluorine and the amino group) of 2-fluoro-2-arylcyclopropylamine influenced the inhibitory potency in a consistent fashion. Thus, electron-withdrawing groups (F, Cl) slightly... 

We have examined the above-described series of trans- and c/s-2-fluoro-2-phenylcyclopropylamine analogues (60a-d, 61a-d) as inhibitors of recombinant human liver MAO A and B [134]. The presence of fluorine attached to a cyclopropane ring, especially for frans-isomer 8a, was found to result in an increase in inhibitory activity toward both MAO A and B (Table 4). In addition, p-substitution of electron-withdrawing groups, such as Cl and F, in the aromatic ring of the frans-isomers (60b-d) increased the inhibition of both enzymes. On the other hand, the introduction of fluorine at 2-position of c/s-isomer 8b resulted in loss of inhibitory activity for both MAO A and B, and no further p-aromatic substitution for c/s-isomer greatly affected on the inhibitory activity with either enzymes. In addition, both MAO A and B were selectively inhibited by the (1S,2S)-enantiomer of 60a, while no inhibition was observed with the (1f ,2f )-enantiomer [134]. As already described in the former section, several questions on the mechanistic pathway for MAO inhibition by cyclopropylamines still remain. However,... 

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. 

Richards and Pisker-Trifunac50 have irradiated cw-styrylferrocene (14) under several different sets of conditions and in all cases high conversion to the frans-isomer was the only reaction observed. It is noteworthy that no cyclobutane or oxetane derivatives are formed. The same seems to apply to ferrocenyl analogs of stilbene. That is, no ring formation is observed and the photostationary state consists almost exclusively of the Jrans-isomer. 

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 above considerations lead us naturally to the question of the nature of the one or more intermediate states involved directly in the photochemical reaction. We have displayed in Figure 27 the simplest scheme consistent with the above, admittedly preliminary, experimental results. The prime superscript refers to the frans-isomer, while the processes denoted 1, 2, 3,4, and 5 describe fluorescence, Sx S0 internal conversion, Si -> X intersystem crossing, isomerization, and deactivation to the ground state, respectively. Again, X and X may represent a common state, as, for example, is the case with the twisted triplet in ethylene (cf. Mulliken and Roothaan190 and Kaldor and Shavitt191). As late as 1962, investigators were still unable to determine the source of the temperature... 

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. 

Figure F2.4.1 Liquid chromatography/mass spectrometry (LC/MS) analysis of isomeric carotenes in a hexane extract from 0.5 ml human serum. Positive ion electrospray ionization MS was used on a quadrupole mass spectrometer with selected ion monitoring to record the molecular ions of lycopene, p-carotene, and a-carotene at m/z (mass-to-charge ratio) 536. A C30 HPLC column was used for separation with a gradient from methanol to methyl-ferf-butyl ether. The a -trans isomer of lycopene was detected at a retention time of 38.1 min and various c/ s isomers of lycopene eluted between 27 and 39 min. The all-frans isomers of a-carotene and P-carotene were detected at 17.3 and 19.3 min, respectively.

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