Oxidation-ring-opening process

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 whole process of chlorophyll disappearance in vascular plants is a complex multistep pathway, much as chlorophyll biosynthesis is, but for didactic reasons it can be abbreviated into two main stages. The first group of reactions produces greenish derivatives while the more advanced steps produce colorless compounds by an oxidative ring opening, analog to the oxygenolytic rupmre of the porphynoid macrocycle of haem. It is a very rapid process and despite considerable efforts, the detection of intermediates is difficult. ... 

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>. 

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. 

On the other hand, when this anodic oxidation was carried out in the presence of CIO4- or AcO salts instead of BF4, it led to both oxygen insertion and ring-opening processes, to form mainly cyclic and linear siloxanes (equation 46). The ratio of these products greatly depends both on the amount of electricity passed and on the presence of oxygen55. 

This paper is a review of studies on the ring-opening polymerization of cyclic ethers, e.g., styrene oxide, phenyl glycldyl ethers, epoxy aldehydes and derived oxacyclic monomers. Model reactions involving ring-opening processes occurring in those compounds have alsobeen discussed. 

Cyclic compounds constitute a potentially polymerizable class of monomers. The difunctionality criterion for polymerizability is achieved by a ring-opening process as shown below for ethylene oxide ... 

The isolation of chlorin Pe and mesochlorin Pe and possibly the purpurin 18 intermediates suggests that allomerization is an important process for chlorin formation by oxidative ring opening (Figure 1.7). It is also possible that petroporphyrins of the etio and phyllo type can be derived from these chlorins. 

Pyrimidine 1-oxides 72, which are less electrophilic than triazines, do not react with cyanamide in the presence of a base. However, treatment of 72 with cyanamide under acidic conditions (dry HCl) results in the formation of 4-aryl-2-ureidopyrimidines 75 (Scheme 49) [107]. It is evident that in situ protonation of these N-oxides significantly enhances their ability to give ring opening process (73 74 75), thus resulting in the formation of 75. Hydrolysis of the latter on reflux in formic acid provides the corresponding 2-aminoperimidines. 

There are some investigations on the influence of a proton and water molecules on the ring-opening process of propylene oxide. It was found that the ring-opening... 

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