Ring openings

The cleavage of the cyclopropyl ring with its relase of the total strain is a powerful driving force. Electrophilic attack on the electron rich ring does provide one approach as shown in Eq. 1 3). The lack of selectivity in the cleavage of one of the 

A cyclopropylcarbinyl cation can be trapped to form either a cyclopropane product (Path a, Eq. 3) or a homoallyl product (Path b, Eq. 3). The latter has proven useful to create acyclic units containing olefins of defined geometry as in the synthesis 

This section on ring closure is linked to the following section on ring opening by the kinetic study of the reaction shown in equation (4). Values of the observed rates for the approach of this system to equilibrium were analysed in terms of an appropriate rate law. The ring-closure reaction in fact involves the rapidly established equilibrium between the chloro-complex shown on the right-hand side of equation (4) and its aquo-analogue.  

Bis-acetylacetonatoplatinum(ii) reacts with triphenylphosphine or with pyridine to give [Pt(acac)2(PPh3)2] or [Pt(acac)2(py2)l respectively, in which the acetylaceto-nate is carbon-3-(y-)bonded to the platinum. The mechanism of the reaction with pyridine is said to involve a unidentate O-bonded acetylacetonate derivative on the way to the y-(C-)bonded form.  

Krylova, I. G. Luk yanova, L. D. Dikanskaya, and L. M. Volshtein, Tezisy Doklady Vses. Chugaevskoe Soveshch. Khim. Kompleksn. Soedinenii, 12th, 1975, 2, 150 Chem. Abs., 1976, 

Pyrido[1,2-c][1,3]oxazines and Their Benzo Derivatives 1. Ring Opening 

4-tetrahydroquinoline-8-carboxylic acid (64M59), respectively. The rate of the solvolysis of lran5-3,4a-H-3-substituted perhydropyrido[l,2-c][l,3]ox-azines was higher than that of cw-3,4a-H epimers (57CLY927). The (rans-3,4fl-H-3-substituted perhydropyrido[l,2-c][l,3]oxazines, containing the R substituent in an axial position, were hydrolyzed faster then the 3-epimeric derivatives in 2 A acetic acid at 25°C in the presence of dimedone [70LA(737)24]. 

Treatment of r-4 , c-5a, t-9o-H-1 -ethylideneperhydro[l,3]oxazino[3,4-hjisoquinoline (42) with water in methylene chloride gave 2-(rra s-perhy-droisoquinolin-3-yl)ethyl propionate (80HCA1158). 

3-dione with amines in dioxane led to TV-substituted 1,2,3,4-tetrahy-droquinoline-8-carboxamides (78JHC645 79JHC897). 

Depending on the reaction conditions, treatment of 4-hydroxymethyl-3,4-dihydro-l-oxo-l//-pyrido[2,l-c][l,3]oxazinium chloride with NH3 afforded either 4-hydroxymethyl-l,3-dihydropyrido[2,l-c][l,3]oxazin-l-one or different ring-opened products (92JOC5764). 

The reactions of 3,5,6,7-tetrahydro-l//-pyrido[3,2,l-y][3,l]benzoxazine-1,3-dione with amines in dioxane led to iV-substituted 1,2,3,4-tetrahy-droquinoline-8-carboxamides (78JHC645 79JHC897). 

The boiling of optically active 3-deuterio-3-p-methoxyphenylperhydro-pyrido[l,2-c][l,3]oxazine in ethanol in the presence of 20% w/w of 10% Pd/ C catalyst yielded A-formyl-2-(2-deuterio-2-p-methoxyphenylethyl)piperi-dine with no deuterium loss [72JCS(CC)1152], 

The reaction of oxiran with HF to yield CH2FCH2OH was chosen as the model for the ring-opening of epoxides that is caused by halogen acids, and theoretical ab initio calculations for the reaction were performed. For the gas-phase reaction, the preferred mechanism leads to retention of configuration at the carbon atoms of the ring. An alternative pathway via preliminary formation of the conjugate acid of the oxiran was found to fit a borderline A2 mechanism. 

A detailed study of eleven possible isomers of C2H4OI has been carried out with the aid of ab initio M.O. theory.Results indicate that (90) is relatively stable, with a calculated energy that is in reasonable agreement with thermochemical data. 

Alagona, E. Scrocco, and J. Tomasi, Theor. Chim. Acta, 1979, 51, 11. 

Politzer and V. M. Estes, Jerusalem Symp. Quantum Chem. Biochem., 1979, 12, (Catal. Chem. Biochem. Theory Exp.) 305 Chem. Ahstr. 1980, 92, 163 440). 

Morelli, S. Catalano, V. Scartoni, M. Ferretti, and A. Marsili, J. Chem. Soc., Perkin Trans. 1,1979, 1665. 

For the sake of similarity this section has been kept 84CHEC-I(5)167 as it corresponds to the ring opening of bicyclic systems already containing the pyrazole nucleus. 

Takahashi et al. (1996) described another case of cation-radical cycloreversion. Benzocyclobu-tenols undergo ring opening induced by electron transfer to generate quinodimethide intermediates, which then tautomerize to benzophenones. The reaction proceeds on photoirradiation in the presence of tetracyanoanthracene (X 350 nm). Yields (based on NMR (proton) analyses) are quantitative 

Irradiation of pyran-2-one gives the ketene (46) reversibly. Similar reactions are known for aza and diaza analogues. Thus, l,3-oxazin-6-ones isomerize photochemically to ketene imines (47), and flash vacuum pyrolysis converts the oxadiazinone (48) reversibly into (49). 

CH(C0CH3)C02R NaCH(C0CH3)CO2R J Heterocyclic Chem 28 1757 (1991) 

Me3SiCN, Ce(CN)3 RMgX, CuBr SMej RMgX, cat CuBrSMcj R2CuLi 

J Heterocyclic Chem 6 273 (1969) I Heterocyclic Chem 6 273 (1969) J Heterocyclic Chem 6 273 (1969) 

ClCOjCI Ph, NaHCOj aCOjPh, KHCOj ClC02Ph, (LiT) / hydrolysis 

NH4CI NaN3, UCIO4 NaN3, BF3 OEt2 

CF3CO2H or BF3 OEt2 R2NH, CF3C02H or BF3OEt2 

Kaverin and six others, Izv. Akad. Nauk SSSR, Ser. Khim., 1980, 2657. 

Most of the work done in the pteridine series has been concerned with the equilibria between the neutral species and the anions. This work was more fruitful than that involving the cations because all three of the values, piT/, pA /, and pK/ (for definitions, see Section II, A), could be determined, and, from these, ratios of the hydrated to the anhydrous forms were calculated. Furthermore, the kinetics in the 

The ratio, at equilibrium, of the hydrated to anhydrous forms (for both neutral species and anions) has been measured for the following  

2-hydroxypteridine and its 4-, 6-, and 7-methyl and 6,7-dimethyl derivatives 6-hydroxypteridine and its 2-, 4-, and 7-methyl derivatives 2,6-dihydroxypteridine and 2-amino-4,6-dihydroxypteridine. The following showed no evidence of hydration 4- and 7-hydroxy-pteridine 2,4-, 2,7-, 4,7-, and 6,7-dihydroxypteridine and 2-amino- 

2-hydroxypteridine and its C-methyl derivatives (also 2-mercapto-pteridine) have been measured in the pH region 4-12, and all these reactions were found to be acid-base catalyzed. The amount of the hydrated form in the anions is always smaller than in the neutral species, but it is not always negligible. Thus, the percentages in 

2-hydroxy-, 2-hydroxy-6-methyl-, 2-mercapto-, and 2,6-dihydroxypteridine are 12, 9, 19, and 36%, respectively (see also Table VI in ref. 10). 

ClC02CH2Ph, NaHC03 ClC02Ph, KHC03 ClC02Ph, (Lil) / hydrolysis 

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For Woodward-Hoffm an allowed thermal reactions (such as the con rotatory ring opening of cyclobulan e), orbital symmetry is conserved and there is no change in orbital occupancy. Hven though bonds are made and broken, you can use the RHFwave fun etion. 

Cyclic compounds capable of undergoing ring opening (alkylene oxides, lactones, lactams, anhydrides, etc.). 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Epoxides provide another useful a -synthon. Nucleophilic ring opening with dianions of carboxylic acids (P.L. Creger, 1972) leads to y-hydroxy carboxylic acids or y-lactones. Addition of imidoester anions to epoxides yields y-hydroxyaldehyde derivatives after reduction (H.W. Adickes, 1969). 

The last group of reactions uses ring opening of carbonyl or 1-hydroxyalkyl substituted cyclopropanes, which operate as a -synthons. d -Synthons, e.g. hydroxide or halides, yield 1,4-disubstituted products (E. Wenkert, 1970 A). (1-Hydroxyalkyl)- and (1-haloalkyl)-cyclopropanes are rearranged to homoallylic halides, e.g. in Julia s method of terpene synthesis (M. Julia, 1961, 1974 S.F. Brady, I968 J.P. McCormick, 1975). 

The thermal ring opening of l,2-bis(trimethylsiIoxy) cyclobutenes (from acyloin condensation of 1,2-dicarboxylic esters) was used in ring expansion prodecures (see p. 53f.). 

Another useful route to cyciopentanes is the ring contraction of 2-bromo-cydohexanones by a Favorskii rearrangement to give csrdopcntanecarboxylic acids. If a 0 dibromoketones are used, ring opening of the intermediate cydopropanone leads selectively to, y-unsaturated carboxylic acids (S.A, Achmad, 1963, 1965 J. Wolinsky, 1965). 

Tertiary alcohols are usually degraded unselectively by strong oxidants. Anhydrous chromium trioxide leads to oxidative ring opening of tertiary cycloalkanols (L.F. Fieser, 1948). 

Irradiation of steroidal 3,7-dienes with ultraviolet light may result in ring opening and formation of various trienes. The most important reaction of this type is the conversion of ergosterol to previtamin Dj. 

The alkenyloxirane 126 in excess reacts with aryl and alkenyl halides or triflates in the presence of sodium formate to afford the allylic alcohol 127[104], Similarly, the reaction of the alkenyloxetane 128 gives the homo-allylic alcohol 130[105]. These reactions can be explained by insertion of the double bond in the Ar—Pd bond, followed by ring opening (or /3-eliraination) to form the allylic or homoallylic alkoxypalladium 129, which is converted into the allylic 127 or homoallylic alcohol 130 by the reaction of formate. The 3-alkenamide 132 was obtained by the reaction of the 4-alkenyl-2-azetizinone 131 with aryl iodide and sodium formate [106]. 

An efficient carboannulation proceeds by the reaction of vinylcyclopropane (135) or vinylcyclobutane with aryl halides. The multi-step reaction is explained by insertion of alkene, ring opening, diene formation, formation of the TT-allylpalladium 136 by the readdition of H—Pd—I, and its intramolecular reaction with the nucleophile to give the cyclized product 137[I08]. 

The aryl- and heteroarylfluorosilanes 541 can be used for the preparation of the unsymmetrical ketones 542[400], Carbonylation of aryl triflate with the siloxycyclopropane 543 affords the 7-keto ester 545. In this reaction, transme-tallation of the siloxycyclopropane 543 with acylpalladium and ring opening generate Pd homoenolate as an intermediate 544 without undergoing elimination of/3-hydrogen[401],... 

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