Ring opening processes

Other classes of yellow couplers reported ia the Hterature include the iadazolones (71) and the benzisoxazolones (72). Neither of these stmctures contains an active methylene group dye formation is beUeved to occur through a ring-opening process. 

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

A major limitation of this method is the low pH at which the reactions are performed, which resulted in substantially lower yields in reactions with substrate progenitors of acid-sensitive epoxides, in which competing ring-opening processes effectively reduced the usefulness of the method. As an example, the oxidation of styrene had proceeded with 70% conversion after 3 h at 70 °C, but the observed yield of styrene oxide was only 2% (Table 6.5, Entry 5). 

The ring-opening process leading to 164 (route a) is analogous to that which has been demonstrated to follow the cycloadditions of tosyl azide to certain enamines176. Similar results have been reported for the reaction of 2,3-diphenylcyclopropenone with 2-diazopropane177. Other 1,3-dipolar cycloadditions with thiirene dioxides could also be affected (see below). 

Fig. 2. Potential energy profile of 1,2-insertion and ring opening processes of Cp2LnH-MCP systems obtained at RPBE level...

SCHEME 2.18 Generation of tropone from o-QM via a ring-opening process. Enthalpies of formation for the intermediates and activation enthalpies (data above the arrows) are reported in kcal/mol. Enthalpies of formation and activation enthalpy for the rate-determining step are in bold (data have been taken from Ref. [23]). 

The second reaction pathway investigated was a o-QM decomposition initiated by a ring-opening process, generating a conjugated ketenes as intermediate, as shown in Scheme 2.18. 

The formation of the linear polymer from the cyclic monomer requires a decrease of the free energy. Because usually entropy is lost during polymerization, the main driving force for the ring-opening process is the release of the angular strain upon conversion of the cycles to linear macromolecules. Thus, a majority of three- and four-membered rings can be readily and quantitatively converted into polymers. 

Cyclosilazanes are found to be reluctant to polymerize by the ring-opening process, probably for thermodynamic reasons. On the other hand, six- and eight-membered silazoxane rings are able to undergo anionic polymerization under similar conditions to those which have been widely used for cyclosiloxane polymerization provided there is no more than two silazane units in the cyclic monomer. They can also copolymerize with cyclosiloxanes however, the chain length of the linear polymer formed is substantially decreased with increasing proportion of silazane units. 

Figure 1.81 Succinic anhydride reacts with primary amine groups in a ring-opening process, creating an amide bond and forming a terminal carboxylate.

Figure 1.84 Glutaric anhydride reacts with amines in a ring-opening process to create an amide bond linkage and a terminal carboxylate group.

An epoxide or oxirane group can react with nucleophiles in a ring-opening process. The reaction can take place with primary amines, sulfhydryls, or hydroxyl groups to create secondary... 

Figure 3.9 An azlactone reacts with amine groups through a ring-opening process, creating amide bond linkages with the attacking nucleophile.

Porath, 1974). B/s-oxirane compounds also can be used to introduce epoxide groups into soluble dextran polymers in much the same manner (Boldicke et al., 1988 Bocher et al., 1992). The epoxide group reacts with nucleophiles in a ring-opening process to form a stable covalent linkage. The reaction can take place with primary amines, sulfhydryls, or hydroxyl groups to create secondary amine, thioether, or ether bonds, respectively (Chapter 2, Section 1.7). 

The tautomerism of furoxan (l,2,5-oxadiazole-2-oxide) has been investigated by different computational methods comprising modern density functions as well as single-reference and multi-reference ab initio methods. The ring-opening process to 1,2-dinitrosoethylene is the most critical step of the reaction and cannot be treated reliably by low-level computations (Scheme 2). The existence of cis-cis-trans- 1,2-dinitrosoethylene as a stable intermediate is advocated by perturbational methods, but high-level coupled-cluster calculations identify this as an artifact <2001JA7326>. 

Most of the reactions of interest involve the opening of the isoxazole ring. This reductive opening has been known for years and was reported in CHEC(1984) and CHEC-II(1996). Over the years, many different reagents have been used and/or developed for this ring-opening process examples of the most useful ones are collected in Table 5 and give an overview of the protocols available for N-O bond reduction. 

Cyclopropyl ions and radicals (23) can undergo conversion to allyl (24) by typical electrocyclic ring opening processes we have carried out calculations for ring opening by both conrotatory and disrotatory paths. Table 5 shows calculated activation energies for the various processes. 

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. 

In practice, the equivalent synthon of 2 was l-cyano-4,5-dimethoxybenzocyclobutene 22 (Scheme 3.7) which on heating generates a reactive o-quinodimethane by a conrotatory electrocyclic ring opening process (Cf. Scheme 3.7) and reacts, at 150-160 °C, with the 3,4-dihydroisoquinoleine 23 to give 80-88%yield of 13-cyanoprotoberberine 24. A simple reductive decyanation with lithium in liquid ammonia in the presence of isopropyl alcohol afforded xylopinine (19) in 84.6% yield [19]. 

There appears to be a common intermediate whose formation from Zr4(02)2(OH)4 is rate-determining in the oxidation and reduction reactions. A slow ring-opening process may expose an — OjFI arm, which would be expected to react rapidly with Ce(IV) or S(IV). This same intermediate may be involved in the acid hydrolysis. 

These results clearly show that an unsubstituted C-2 position in 6-halogenopyrimidines is an important prerequisite for ring-opening processes that can lead to nucleophilic substitutions involving a degenerate ring transformation. 

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