Isomerization carbocyclic

Introduction of nitrogen into the anulene ring (e.g. of 95) leads to a methano-azaanulene 107 121) with Q-symmetry which is therefore chiral (like its mono- or disubstituted derivatives)118). The low basicity of 107 (pKa 3.20) prevented its optical resolution by conventional methods (e.g. through salts with optically active acids). Excellent results were obtained, however, (as also in the case of the two isomeric carbocyclic methylesters 97 and 101 and of several derivatives of azaanulene) by chromatography on microcrystalline triacetyl cellulose in ethanol at 7 bar 1221 (see also Section 2.7.1). In many cases base line separations were accomplished to give both (optically pure) enantiomers. Enantiomeric relations were confirmed in all cases by recording the CD-spectra of both fractions. Some results of these separations are shown in Fig. 4 together with the optical rotations ([a]D in ethanol) of the enantiomers. 

Prompted by the success achieved with simple carbocyclic compounds (e.g., 35), which are potent inhibitors of influenza neuraminidase, several other Neu5Ac2en mimetics of this kind were studied. The isomeric carbocyclic derivatives 37 and 38 are selective for influenza A sialidase, but only 38 showed signihcant inhibition (IC50 = 2 X 10 M) [87]. Other carbocyclic neuraminidase inhibitors include the guanidino substituted cyclohexene 39 [88] and amine derivatives [89] such as 40, which was found to be an inhibitor of influenza B neuraminidase comparable with oseltamivir 35, but was not as potent against influenza A virus [89a]. 

Our conclusion regarding the importance of reaction via an adsorbed carbocyclic intermediate, together with the data in Fig. 12, lead to the conclusion that over ultrathin film catalysts, the specific rate of isomerization via the carbocyclic pathway decreases as the average platinum particle... 

There are many more syntheses of heterocyclic propellanes from 1,1,2,2-substituted carbocyclic starting materials. The tetrol discussed above, when treated with KHS04 at 170-190 °C affords the dioxa[3.3.2]propellane shown no isomeric spiran is mentioned. Although the yield is only 50% perhaps some dispiran is hiding in the brauner Ruckstand from which the propellane diether is either crystallized at low... 

Reaction of nitromethane and monosaccharide-derived dialdehydes is a useful tool that has been broadly used for the preparation of nitro and amino sugars, and carbocycles.30 Dialdehydes can easily be obtained by oxidative cleavage of conveniently protected monosaccharides with sodium periodate. Their subsequent Henry reaction with a nitroalkene, commonly nitromethane, usually gives isomeric mixtures that require the isolation of the major isomer.31 Thus, treatment of the D-ribose derivative 27 with sodium periodate gave dialdehyde 28, which was subjected to a Henry reaction with nitromethane, to afford nitrosugar 29 as an epimeric mixture (Scheme 11).32... 

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

A proposed mechanism for the SiCaT reaction using the 1,6,11-triyne system as an example is illustrated in Scheme 7.21. The reaction proceeds through insertion of one of the terminal alkynes into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, generating an ethenyl-[Rh] intermediate, which undergoes addition to the second and third alkyne moieties to form intermediate III.2a. Subsequent carbocyclization followed by /9-hydride elimination gives the tricyclic silylbenzene derivative 70. Alternatively, ethenyl-[Rh] intermediate can be isomerized to the thermodynamically more... 

Carbonylative [4+1] carbocyclization of 64 a was effected by simply stirring a solution of the complex in dichloromethane under an atmosphere of CO (1 atm.). The conjugated cyclopentenone 66 was formed in 96% yield, presumably by isomerization of the initially formed cyclopentenone 65 a. 

The asymmetric synthesis of (+)-Codeine 432 devised by White and colleagues included a Beckmann rearrangement to introduce the nitrogen atom in the carbocyclic structure (equation 182). Even though two isomeric lactams 430 and 431 were obtained as a result of the rearrangement, the preferential migration of the bridgehead carbon atom produced 430 as the predominant isomer. The synthesis of the non-natural enantiomer of Codeine was completed after oxidation, olefin formation and reduction. 

The photochromic compounds of potential interest, based on the 2//-chromene ring system, are the 2//-benzopyrans (1.18) or the three isomeric naphthopyrans (1.19-1.21). However, 2H-naphtho[2,3-( ]pyrans (1.21) show little or no useful photochromic behaviour and can be discounted from any further discussion. Although R and can be part of a carbocyclic spiro ring, they are more commonly unconnected substituents such as gem dialkyl or aryl groups. 

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). 

Homo-l//-azepines (29), unlike their carbocyclic counterparts, show little tendency to isomerize to the thermodynamically less favoured bicyclic aziridine tautomers (30). Presumably, the homoazepine gains its stability from delocalization of the nitrogen lone pair through the dienamine system <71AG(E)ll). However, the aziridine tautomers (30 R1 = C02Me, R2 = Ac, C02Me or p-Ts) have been trapped as their all-cTs bis-l,3-dipolar cycloadducts with diazomethane (76CB3505). 

As with the carbocyclic systems, the hydrogens on both N and C adopt axial and equatorial positions, and in the change from one conformer to the other the axial H inverts to the equatorial position and vice versa. There are many examples of ring systems in coordination chemistry which can adopt these conformational isomeric positions. Four systems currently being studied are the complexes of 1,2-propylenediamine, 1,2-f/ww-cyclo-hexanediamine, 1,2-fraw.y-cyclopentanediamine, and 2,3-butanediamine, and some reference will be made to each of these. 

No evidence was found of isomerization of the CH to the COMe adduct on addition of methanol to the DMSO solution,41 although a solvent effect of this kind was observed in carbocyclic series.48 However, methanol seems to have a depressing effect on the stability of the CH adduct and to promote its decomposition. Slow ring opening has been suggested to occur in MeOH-DMSO (1 9) starting from the 4-methoxy isomer.41... 

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

The success of the reaction, as well as its stereoselectivity, is dependent on both the disilane and the bis-diene employed. For example, the ethoxycarbonyl-substituted bis-diene reacts with either hexamethyldisilane or 1,2-diphenyltetramethyldisilane to give high yields of isomeric mixtures of the carbocycle products. In contrast, hexamethyldisilane does not have sufficient activity in reactions with the other bis-dienes studied. In addition, stereoselective formation of only one isomer is observed in some cases. 

In alloys with 0-23% Cu the activation energy of the total conversion of n-hexane is only marginally influenced and the observed effects are consequently connected with the preexponential factors. Since the selectivity of nickel diluted with copper is near the value found by Anderson et al. (113) for highly dispersed films, considering a common cause is suggested (60). Anderson assumes that with a large fraction of surface atoms in very small crystals the isolated corner atoms favor the formation of carbocyclic intermediates of isomerization, whereas hydrogenolysis requires two or more adjacent platinum atoms in a crystal plane. 

The formation of 5-hexenal (reaction 18) is believed to be an intramolecular rearrangement since the addition of oxygen does not cause its suppression. At least in a methyl substituted cyclohexanone the analogous process has been shown to occur by the transfer of a hydrogen atom from the beta position to the carbonyl group before the fission of the six-membered carbocyclic ring (29) as only one of the two possible isomeric heptenals is formed. 

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