Isomers cyclisation

Ring-closing metathesis, which has proved to be a popular route to the marine toxins, has found a further application as the key step in the synthesis of the pheromone (-)- and ( )-frontalin <99TL1425>. The precursor in this reaction is a mixture of the syn- and anri-isomers 39. Ring closure in the presence of a ruthenium benzylidene catalyst occurs within minutes at room temperature when only the syn-isomer cyclises to 40. The unreacted anri-isomer can be re-equilibrated for a further cyclisation. 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

Chemoselectivity plays an important role in the benzannulation reaction as five-membered rings such as indene or furan derivatives are potential side products. The branching point is again the rf-vinylcarbene complex D intermediate which maybe formed either as a (Z)- or an ( )-metallatriene the (E)-configuration is required for the cyclisation with the terminal double bond. (Z)-Metallatriene D, however, leads to the formation of furan derivatives H (Scheme 8). Studies on the formation of (E)- and (Z)-isomers discussing stereoelectronic effects have been undertaken by Wulff [17]. 

Similar results are obtained from incineration of polymeric materials with octabromo- and pentabromodiphenyl ether (refs. 11,12). The temperature with the maximum PBDF-yield depends on the kind of polymeric matrix. All three bromo ethers 1-2 give the same isomer distribution pattern with preference for tetrabrominated dibenzofiirans. The overall yield of PBDF is lower for incineration of pentabromobiphenyl ether 2, 4 % at 700°C compared to 29 % for ether 1 at 500 °C (ref. 12). The preferred formation of tetrabrominated fiirans observed at all temperatures cannot be a result of thermodynamic control of the cyclisation reaction it is likely due to the special geometry of the furnaces. One explanation is that a spontaneous reaction occurs at approximately 400°C while the pyrolysis products are transferred to the cooler zones of the reactor details can be found elsewhere (ref. 12). 

Cyclisation of l-aryl-6-hydroxyhex-2-ynyl carbonates to the 2-arylidene-5,6-dihydropyran catalysed by Pd(0) is steieospeciBc, giving only the (Z)-isomer <96SL553>, whilst alk-l-yn-5-ols form dihydtopyranylidene carbenes under the influence of the tungsten pentacarbonyl-THF complex. The carbenes are a source of 2-stannyldihydropyrans <96TL4675>. 

In other cyclisations to functionalised oxepanes, Rychnovsky and Dahanukar have shown that the epoxide 36 cyclises with BF3 etherate and TMSCN to form the oxepane 37 as single product <96TL339>, and Evans and Roseman have prepared a series of cyclic ethers by radical cyclisation of the acylselenides 38 (Scheme 8) <96JOC2252>. The major product was always the cw-isomer and the best yields were obtained with (TMSjaSiH. 

Temperature control during pressure hydrogenation of cis- or tram-isomers is essential, since at 155°C violent decomposition to carbon, hydrogen and carbon monoxide with development of over 1 kbar pressure will occur. The material should not be heated above 100°C, particularly if acid or base is present, to avoid exothermic polymerisation [1], The m-isomer is readily cyclised to 2,3-dimethylfuran, which promotes lire and explosion hazards. These were measured for the cis- and tram-isomers, and for fram-3-methyl-l-penten-4-yn-3-ol [2],... 

Caryophyllenes, as an example of two naturally occurring isomeric sesquiterpenes containing a medium-sized ring, in which the success of the total syntheses lies in the stereoselective control of a chiral centre, in a common synthetic key intermediate, which governs the configuration (JE or Z) of the double bonds present in each one of the two isomers. In this context, a brief reference to Cecropia Juvenile Hormone synthesis by the Syntex group, as well as to Johnson s cationic cyclisation of unsaturated polyolefins to fused polycyclic compounds, is made. 

However, as already stated [12] this is probably unnecessary, since in the basic medium in which the cyclisation takes place the rrans-decalone with no angular methyl group would be in equilibrium with the corresponding cii-decalone. Even in the less favoured case, the equilibrium would shift to the cis-isomer since this is the only isomer which can cyclise to twistanone. Should this be the case, then the stereochemical control in preparing the starting decalone would be unnecessary and the synthesis would be greatly simplified. 

Fulgides can exist as or Z isomers by rotation around the double bonds in (1.33). This is illustrated for the furano derivative in Figure 1.10. Isomerisation of the yellow Z-fulgide (1.34) to the fi-fulgide (1.35) and cyclisation of this to the red coloured photochrome (1.36), designated as C here but often called the P state, occurs on irra-... 

The precursor was prepared via a lengthy sequence involving the elaboration of a Reformatsky adduct. The cyclic product was obtained as a 1.2 1 mixture of cis and trans isomers.The presence of the fluorine atoms had no effect on the efficiency of the cyclisation reaction. A subsequent study [384] extended the range of cyclisations to include trifluoromethyl alkyl and alkenyl radicals (Eq. 157). Free radical cyclisation reactions therefore show considerable promise for the rational synthesis of fluorinated carbocycles. 

In the same year the biotransformation of these monoterpenes by B. cinerea in model solutions was described by another group [41]. Although the major metabolites found were co-hydroxylation compounds, it is important to note that these authors only identified the -isomers in the extracts and that some new compounds were detected that were not described by the previous group, Fig. (9). Geraniol (20) was mainly transformed to (2 ,5 )-3,7-dimethyl-2,5-octadiene-l,7-diol (53), ( )-3,7-dimethyl-2,7-octadiene-l,6-diol (54) and (2 ,6 )-2,6-dimethyl-2,6-octadiene-1,8-diol (43), nerol (14) to (2Z,5 )-3,7-dimethyl-2,5-octadiene-1,7-diol (55), (Z)-3,7-dimethyl-2,7-octadiene-l,6-diol (56), and (2E,6Z) 2,6-dimethyl-2,6-octadiene-1,8-diol (47). Furthermore a cyclisation product (57) was formed which was not previously described. Finally citronellol (4) was converted to trans- (60) and cw-rose oxide (61) (a cyclisation product not identified by the other group), ( )-3,7-dimethyl-5-octene-l,7-diol (58), 3,7-dimethyl-7-octene-l,6-diol (59) and ( )-2,6-dimethyl-2-octene-1,8-diol (34). 

In the course of the catalytic hydrogenation of a,us dinitriles over Raney nickel, by-products are obtained from C-N and C-C bond formation. The mechanism of the formation of these compounds was investigated. Cyclic and linear secondary amines can result from the same secondary imine through a transimination process involving a ring-chain tautomerism. Stereochemical results for 2-aminomethyl-cyclopentylamine (AMCPA) are in accordance with a specific cyclisation pathway favored by an intramolecular hydrogen bond giving rise to the cis isomer from aminocapro-nitrile, unfavored in the case of adiponitrile which leads to the trans AMCPA as the major isomer. 

Intramolecular cyclisation of a 4-arylbutanoic acid system is also an important step in a convenient synthesis of the polycyclic system, chrysene, which is formulated and described in Expt 6.12. Here, methyl cinnamate is first subjected to reductive dimerisation to give methyl meso-ft,y-diphenyladipate, which is accompanied by some of the ( + )-form. The meso isomer (16) is the most easily isolable and cyclisation occurs smoothly in sulphuric acid to yield the diketone 2,1 l-dioxo-l,2,9,10,ll,18-hexahydrochrysene, which is obtained as the trans form (17) as shown in the following formulation. Clemmensen reduction of this ketone followed by dehydrogenation (in this case using selenium) completes the synthesis of chrysene. 

A novel approach to glycosidation with jV-acetylneuraminic acid was adopted by Sinay [268], Reaction of (385) with diazotrimethylphosphonoacetate in the presence of rhodium acetate gave the phosphonate (386) as a mixture of diastereoisomers which were allowed to react with the aldehyde (387) in the presence of sodium hydride in tetrahdrofuran to give a mixture of E- and Z-isomers (388) which were separated. The isopropylidene groups were hydrolysed and the products cyclised with mercury(II)... 

So, in this instance, cyclopropane formation is the first rather than the last step in the synthesis. Addition of ethyl diazoacetate 42 R = Et to styrene 41 gave a mixture7 of cis and trans isomers of 40 R = Et. Only the cis isomer will be able to cyclise so separation8 of the free acid gave 34% yield of cis-43. Chain extension by the Amdt-Eistert procedure9 (chapter 31) gave cis-35 and the acid chloride duly cyclised to 36. 

The condensation of 3-(substituted)-l,2,4-triazoles with an A-cyanocarbonimidate at high temperatures gives (1,2,4]triazolo 1,5-a [ 1,2,3]triazines (34) which are converted to other such products by standard metatheses. At lower temperatures l,2,4Jtriazolo[4,3- ][l,3,5]-triazines (35) can also be obtained which rearrange to the more stable (1,5-a] isomers on heating. Cyclisation of the hydrazides (36) also leads to [1,5-a] fiised triazines (Scheme 20) [95JCS(P1)801],... 

The radical cyclisation of o-bromophenyl sulfones affords the benzothiopyran 1,1-dioxide in which the trans isomer is either exclusive or predominant (95SL943). 

A traditional acid-catalyzed cyclisation to piperidines can control the stereo-outcome by the judicious choice of acid (Scheme 43) <2002OL3727>. Stereoselectivities of up to 98 2 were achieved with a Lewis acid giving the trans-isomer and Bransted acids yielding the air-isomers. 

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