Pyran isomer

Unfortunately, our carbon 13 and proton data (Table II) taken together with other data for the model compounds, do not allow an unambiguous assignment of the 3 stereoisomers which we have obtained. For the present, then, we must be content to leave the relative stereochemistry of the 5-membered ring model compounds in question. For now, we can only say that the fine structure in the copolymer spectra are certainly well within isomers shifts seen for the model compound. However, based on the fact that the relative stereochemistry of the tetrahydrofuran models are unknown and that we have only one pyran isomer, we are unable to make a definite assignment as to the relative stereochemistry of ring junctions in the original copolymer. 

Still another possibility of isomerization is illustrated by the easy interconversions between pentaphenylpentadienoie aeid chloride and 2-chloropentaphenyl-3-eyelopenten-l-one. Interestingly, 2,4,6-trimethylpjrrylium iodide maj be sublimed without decomposition in a vacuum, possibly as a covalent 6-iodo-4-methyl-3,5-heptadien-2-one or 2-iodo-2,4,6-trimethyl-2H-pyran valenee isomer. In a related case, chlorocyclopropenes are covalent and are converted into cyclo-propenium derivatives only by the action of Friedel-Crafts catalysts (electron-deficient metallic chlorides) (ef. also Section II,C, 2,c.)... 

No tautomeric interconversions have been observed between 4//-pyrans (1, X = O) and 2//-pyrans (2, X = O), 4//-thiopyrans (1, X = S) and 2H-thiopyrans (2, X = S), or their benzo-fused derivatives. Under normal conditions, all these compounds exist as sole isomers that are initially formed, both in solution and in the solid state. The only tautomeric equilibrium reported was that between the two possible 2H isomers of the unsymmetrical sulfone 3 (77TL1149). 

By contrast, heating benzo-2//-pyran 6 with acetic acid leads to the formation of the 4H isomer 7 (Scheme 2) [77ACS(B)496]. 

Transformations of 5,6-dihydro-2//-pyrans 8 into the corresponding 3,4-dihydro-2//derivatives 9 under basic conditions (88KGS291 98TL2025), as well as the reverse conversion under irradiation [97H(46)451], have been explained by the intermediate formation of electron-delocalized heterocyclic systems, giving rise to two possible isomers. 

Stereocontrol in the formation of spiroketals has been achieved in the alkylation of 2-(benzenesulfonyl)pyrans with allylsilanes <96JOC7860> and using a double carbonyl cycUsation strategy <96SL1065>. Spirocyclisation of protected dihydroxydiketones yields cis- and trans- l,7,9-trioxadispiro[5.1.5.3]hexadecanes the latter isomer is the thermodynamically more stable <96TL5461>. 

Scheme 43) [92]. Reaction of dienophiles such as 4-nitrobenzaldehyde with linker 80 at high temperature gave Diels-Alder products. Dihydro-pyrans were released from the support by Bronsted or Lewis acid-nucleo-phile combinations in moderate to good yield with stereoselectivity for the anti isomer. 

An interesting feature of the cyclization of y, -unsaturated alcohols is the marked effect on product isomer distribution by the nature of substituents remote from the double bond (cf. 42 and Scheme 59).98 Complete stereospecificity is observed for the phenyl derivative 42a in contrast to 42b and c, and the isomer ratio is reversed for 42d. The suggested mechanism98 is shown in Scheme 60 the trisubstituted alkene (45) is mainly converted into a pyran (46) rather than a tetrahydrofuran derivative (Scheme 61). 

A dichloromethane solution of 4 - m e th y I - 5,6 - d i hy dro - 2 // -pyran gives 4-me-thyltetrahydropyran in 35% yield when treated with a mixture of five equivalents of triethylsilane and 2.5 equivalents of aluminum chloride in the presence of excess hydrogen chloride at room temperature for one hour (Eq. 84).136 This behavior is essentially the same as that exhibited by the disubstituted 4-methylenetetrahydropyran isomer under similar conditions.136... 

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

Recently for the first time, a highly stereoselective surfactant-catalyzed INAC reaction in aqueous media, leading to the exclusive formation of a single isomer has been reported (726a). Either oxepane (mode A) or pyran (mode B) (Scheme 2.230) is formed from 3-O-allyl furanoside derivatives, which constitute... 

Compounds are generally classified according to their fully unsaturated parent compound (but see below). Thus substituted, partially saturated, and fully saturated derivatives of, for example, pyrrole are all indexed under pyrrole. Benzo and similar derivatives are included under the most unsaturated parent system (e.g., quinoline, thienofuran, etc.). For any given heterocyclic parent only one indicated hydrogen isomer appears in the text, typically the most stable or the lowest numbered form thus all instances of pyran, whether of the 2H- or 4H-form, are indexed under 2H-pyran. The charges and additional valences for any heterocyclic parent structure are not indicated. 

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

Utilization of bases with different cations made it possible to obtain pyrans with predominantly either the 4-(R) (K, Na, C5Hi2N ) or the 4-(S) (Li, Mg) isomer. 

Often it is difficult to predict the preferred product, basing on the literature. Since structures 282 and 283 are isomers, their identification is complicated. For example, in 86JPR35 the products of pyran acylations are... 

A problem arises with trivial names when a sp hybridized atom is present in an otherwise unsaturated ring. A good example is pyran, a heterocycle that is formally the product of the addition of a single hydride ion to the pyrylium cation. However, as this addition could occur either at C-2 or C-4, two isomers of pyran are possible so the question is, how can you distinguish between them The solution is to call one compound 2/f-pyran and the other 4/f-pyran, using the number of the ring atom and the letter H, in italics, to show the position of the hydrogen (see Box 1.2). This system of nomenclature works tolerably well in many related cases and is widely used other examples will be found in this book. 

A related 1,7-cyclization has been invoked to account for the formation of diazepine 292 from the electrophilic diazoalkane substitution reaction of ethyl diazoacetate and dimethyl diazomethylphosphonate, with the 2,4,6-trimethylpyr-ylium salt 289 (337) (Scheme 8.70). While the expected 4-(diazomethyl)-47/-pyran 290 could be isolated (20-22%), the 2-substituted isomer 291 was not. It was proposed that this latter species underwent 671-electrocyclic ring opening followed... 

Pyranopyrrole isomers, shown in Figure 4, consist of a six-membered pyran ring fused to a five-membered pyrrole ring. Pyranopyrrolium and furopyridinium ions are known, where the oxygen in the structure bears a positive charge however, such ionic compounds are not the focus of this chapter. 

A tungsten-promoted intramolecular [3-1-2] cycloaddition reaction of an alkyne and epoxide leads to the formation of air-isomers of reduced furo[3,4-c]pyrans, as shown in Equation (68) <2001JA7427>. The alkynyltungsten complexes were prepared in good yields from the corresponding alkynes upon treatment with CpWlCOlsCl and diethylamine <1997JA4404>. 

The 7-aryl-5-cyanomethyl-l,3-dimethyl-2,4-dioxopyrido[2,3- pyrimidines 458 have been regioselectively synthesized through the nucleophilic attack of the amino groups in 6-amino-l,3-dimethyluracil 452 on the highly electrophilic center at C-6 in 6-aryl-3-cyano-4-methylthio-2//-pyran-2-ones 457. Ring opening of 457 followed by decarboxylation and recyclization with the elimination of methylmercaptan gave 458 with no detection of the 5-aryl-7-cyanomethyl isomers 459 (Equation 38) <20008541 >. 

In the early 1990s, several nitro-polycyclic aromatic compounds that are powerful direct mutagens were identified in ambient particulate matter, including the nitrophenanthrene lactones 2- and 4-nitro-6//-dibenzo[6,r/]pyran-6-one, whose mutagenicities are given in Table 10.20. The 2-isomer (XI) is not only very... 

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