In ring-opening reactions

Rearrangement of fluorine with concomitant ring opening takes place in fluorinated epoxides Hexafluoroacetone can be prepared easily from perfluo-ropropylene oxide by isomerization with a fluorinated catalyst like alumina pre treated with hydrogen fluoride [26, 27, 28] In ring-opening reactions of epoxides, the distribution of products, ketone versus acyl fluoride, depends on the catalyst [29] (equation 7) When cesium, potassium, or silver fluoride are used as catalysts, dimenc products also are formed [29]... 

Electron transfer to vinylaziridines results in ring-opening reactions, yielding allyl amines. Treatment of 268 with SmI2/DMEA (N,N-dimethylethanolamine) provided allyl amine 269 as a 2 1 mixture of olefmic isomers in 88% yield (Scheme 2.66) [97]. 

Typically in ring-opening reactions of aziridines, the amine functional group is retained in the product molecule. An example of such a reaction where the amine group has been lost has recently been reported <06TL977>. An intramolecular Friedel-Craft reaction of aziridine 91 leads to the expected product as an intermediate. Under the rather drastic reaction conditions, the sulfonamide is lost leading to formation of the naphthalene ring. 

Different immobilization methods were employed, and the new heterogenized catalysts were applied in asymmetric hydrogenation and epoxidation of olefins, as well as in ring-opening reactions of epoxides. In some cases, the typical catalytic properties of homogeneous catalysts could be transferred to the heterogeneous systems. The immobilization of complexes on solid supports facilitated recovery and recycling of the catalysts. 

Perhaps the most versatile cation for initiation of polymerisation is the tri-phenylmethyl species (Ph3C+) which is reactive towards both electron rich olefins and cyclic monomers. The more stable cycloheptatrienyl cation (C H,) has proved very useful in vinyl polymerisations but has been little used in ring opening reactions (/5). Not surprisingly, therefore, the dissociation of numerous salts of these two cations have been widely studied, and data for each is summarised in Tables 2 and 3 respectively. 

A new ring transformation of 1,2,3-thiadiazoles into furan-2-carbothioamides has been described.133 An unprecedented rearrangement of 5,5-diazidobarbituric acids to parabanic, tetrazoles and benzyl allophanate has been reported.134 Computational data for the energetics of internal nucleophilic reactions, both in ring-opening reactions... 

Nitrogen-based nucleophiles continue to remain popular in ring-opening reactions of aziridines. a-Substituted-a-methoxycarbonyl-V-nosylaziridines were opened with a variety of functionalized amines to provide access to enantiopure a,a -disubstituted (3-lactam scaffolds for ditopic peptidomimetics <07OL101>. A related intramolecular regioselective 3,Y-aziridine ring opening with an a-amino functionality was reported in the synthesis of... 

Sometimes, especially when fluoride sails are used as the reagents for the ring opening of aziridines, the substrate can be activated by quaternization to the corresponding A/.A-dialkyl-aziridinium compound, which reacts more readily with the fluoride anion than the aziridine itself. Thus, the tV.A -diallylaziridinium mesylate salts 23 have been employed in ring opening reactions of aziridine derivatives of pyranosides. " " ... 

Lanthanide catalysis was again effective in ring-opening reactions of cyclic epoxides (eq 8). Finally, MAO-activated l-RuCL provides block copolymers of ethylene and hex-l-ene. ... 

When bisphenol A is treated with excess epichlorohydrin, this stepwise process continues until all the phenolic OH groups have been used in ring-opening reactions, leaving epoxy groups on both ends of the polymer chains. This constitutes the fluid prepolymer, as shown in Figure 30.6. 

The strain in the three-membered ring is one of its most important properties. It is also the basis of the explanation of many of the features of the molecule, for example, the high reactivity in ring-opening reactions and the low electron-donor ability. These properties can be explained in terms of the hybridization of the bonding and nonbonding electron orbitals and the angular strain in the transition state. A short account of the MO model with reference to the oxiranes is found in the work of Rosowsky. ... 

In some cases, six-membered rings can be involved in ring-opening reactions. For example, the reaction of 4,6-O-benzylidene glucopyranoside 34 with Ai-bromosuccinimide (NBS) in the presence of barium carbonate leads to the corresponding 4-0-benzoyl-6-bromo-6-deoxy... 

It is easier to see the FMO interactions in electrocyclic ring closing reactions than in ring opening reactions. However, the TS s for ring closing and ring open-... 

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