Ring opening Subject

The ring-opening polymerization of is controUed by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cycHzation equihbrium constants of dimethyl siloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equihbrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equihbrium constant of the formation of each cycHc is approximately equal to the equihbrium concentration of this cycHc, [(SiR O) Thus the total... 

In this section three main aspects will be considered. Firstly, the basic strengths of the principal heterocyclic systems under review and the effects of structural modification on this parameter will be discussed. For reference some pK values are collected in Table 3. Secondly, the position of protonation in these carbon-protonating systems will be considered. Thirdly, the reactivity aspects of protonation are mentioned. Protonation yields in most cases highly reactive electrophilic species. Under conditions in which both protonated and non-protonated base co-exist, polymerization frequently occurs. Further ipso protonation of substituted derivatives may induce rearrangement, and also the protonated heterocycles are found to be subject to ring-opening attack by nucleophilic reagents. 

The bis(silyloxy)cyclobutenes are also subject to a variety of special reactions. Probably the most interesting is the observation that they readily undergo a ring-opening reaction leading to a butadiene derivative. This reaction has already been used to prepare large-ring diketones from cyclic 1,2-diesters. 

A decisive solvent effect is also observed with other a,/ -epoxy ketones. Specifically, 3jS-hydroxy-16a,17a-epoxypregn-5-en-20-one and its acetate do not react with thiocyanic acid in ether or chloroform. However, the corresponding thiocyanatohydrins are formed by heating an acetic acid solution of the epoxide and potassium thiocyanate. As expected, the ring opening reaction is subject to steric hindrance. For example, 3j6-acetoxy-14f ,15f5-epoxy-5) -card-20(22)-enoIide is inert to thiocyanic acid in chloroform, whereas the 14a,15a-epoxide reacts readily under these conditions.Reactions of 14a,15a-epoxides in the cardenolide series yields isothiocyanatohydrins, e.g., (135), in addition to the normal thiocyanatohydrin, e.g., (134). 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

Cations are more subject to ring-opening than are neutral species or anions. Thus ring-opening (slow) has been observed in the cations but not in the neutral species of 1,3,5-, 1,3,7-, and 1,3,8-triazanaph-thalene at 20° it is followed by further degradation. 1,3,6-Triaza-naphthalene decomposes much faster than its isomers in acidic solution, but follows the usual sequence, 47 49,... 

One of the first indications that the antiinflammatory potency of the corticoids could be increased was the observation that incorporation of a 9a-fluoro group in hydrocortisone resulted in a tenfold increase in activity. Treatment of hydrocortisone acetate (170a) with phosphorus oxychloride in pyridine yields the corresponding olefin, 172. This, on being subjected to the reaction sequence depicted in the transformation of 104 to 108 (addition of HOBr, closure to the epoxide and ring opening with HF),... 

Various cyclic esters have been subjected to hpase-catalyzed ring-opening polymerization. Lipase catalyzed the ring-opening polymerization of 4- to 17-membered non-substituted lactones.In 1993, it was first demonstrated that medium-size lactones, 8-valerolactone (8-VL, six-membered) and e-caprolactone (e-CL, seven-membered), were polymerized by lipases derived from Candida cylindracea, Burkholderia cepacia (lipase BC), Pseudomonas fluorescens (lipase PF), and porcine pancreas (PPL). °... 

The susceptibility of the sydnone ring to acid-catalyzed hydrolysis at elevated temperature is exploited for the preparation of hydrazines <2000MI227>. Primary amines are first converted to sydnones, which are then hydrolyzed to give hydrazines that may be isolated or reacted further without isolation <1996CHEC-II(4)165>. As masked hydrazines, 3-arylsydnones are firstly subjected to acid-catalyzed ring opening and then allowed to react further in situ with 2-(4-chlorophenyl)hydrazono-3-oxo-butyric acid to form substituted pyrazolidinones (Equation 2) <2005SC2169>. 

More traditional carbon nucleophiles can also be used for an alkylative ring-opening strategy, as exemplified by the titanium tetrachloride promoted reaction of trimethylsilyl enol ethers (82) with ethylene oxide, a protocol which provides aldol products (84) in moderate to good yields <00TL763>. While typical lithium enolates of esters and ketones do not react directly with epoxides, aluminum ester enolates (e.g., 86) can be used quite effectively. This methodology is the subject of a recent review <00T1149>. 

Equilibrium studies. Acidities of thiazolium cations, such as 213, cannot be measured in basic aqueous solution because the hydroxyl adduct ( pseudobase ) 216 of the thiazolium cation is formed rapidly and is subject to base-catalyzed ring opening (Scheme 36).151 In DMSO, formation of the carbene dimer 217 from 213 and 214 is a complicating factor.8 If indicator anion (In") was added to a solution of 213, a very rapid drop in absorbance was followed by a somewhat... 

The fact that tetrazolo[l,5- ]pyridine reacts with phosphines - via ring opening to the valence bond isomer azide -to give a phosphorane has been long recognized. Some novel applications of this transformation have been published during the recent period. The fused tetrazoles subjected to this reaction, the resulting phosphoranes, and the literature sources are summarized in Table 4. 

Basic hydrolysis of the [l,2,4]triazolo[5,l-3][l,3]thiazine derivative 77 was described to yield triazole-thione 78, the reaction proceeding in 59% yield <2005ZOR1092>. Related partially saturated triazolothiazines 79 were also subjected to ring-opening reaction aqueous hydrolysis afforded the acid 80 <2004KGS1256>, whereas reaction of 79 with hydrazine hydrate yielded the acid hydrazine 81 <2004ZOR260>. Both transformations took place in high yields. 

Both sets of observations confirm that the ring-opening process is subject to both steric and electronic influences, the combination of which produces some subtle effects in product distribution. Ultimately, the regiochemistry of the product must be decided after a cyclopropene-Fe(CO)4 complex is formed, but probably during the cleavage of one of the C-C bonds in the cyclopropene ring. 

Elaboration of 248 to 257 is depicted in Scheme 32. Acidic deketalization, basic epoxide formation, and silylation of the remaining hydroxyl group led to 249, which was subjected to ring opening with lithium dimethylcuprate in a regioselective (78 15) fashion to provide the desired alcohol 250 as the major... 

To avoid the inherent limitations of a kinetic resolution process, the reaction was extended to desymmetrization of prochiral meso epoxides. A number of cyclic di-methylidene epoxides were synthesized and subjected to treatment with Et2Zn in the presence of Cu(OTf)2 and ligands 42 or 43. As in the case mentioned above, ligand 42 was superior in terms of selectivity. Cydohexane derivative 46 gave the ring-opened product with a 97% ee and in a 90% isolated yield, with a y/a ratio of 98 2 (Scheme 8.28). The other substrates investigated produced sigmficantly lower ees of between 66% and 85%. 

The rare 4- and 5-alkylindan-2-ols (33) and (34) have been prepared in 62-72% yields by formal 1,6- and 1,4-nucleophilic ring opening of the 2-hydroxyindan-3a, 7a-oxide (32), respectively. In a comparison of gas-phase and condensed-phase 5 Ni reactions, the competitive four- and five-centre cyclizations [yielding (36) and (37), respectively] of the 3,4-epoxybutoxide anion (35) have been subjected to both experimental and theoretical study. In the gas phase, the barriers to the transition... 

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