Alkenes ozonolysis steps

An efficient and straightforward synthesis of isomers of DMDP 36 (2,5-dihy-droxymethyl-3,4-dihydroxypyrrolidine), a known naturally occurring glycosidase inhibitor, has been recently reported. o-Xylose is converted into nitrone 37 in a few steps. Vinylation of 37 gives hydroxyamine 38 which is reduced into the corresponding pyrrolidine with zinc. N-Protection, alkene ozonolysis, and reduction with NaBH4 lead to compound 39. Final deprotection gives 2,5-dideoxy-2,5-imino-L-mannitol (e f-36) (Scheme 14) [92]. 

Ozonolysis occurs in several steps. First, an unstable intermediate, called a molozonide, forms by a cyclic concerted addition of the terminal oxygen atoms of ozone to the 7i bond of the alkene. This step requires a total of three electron pair shifts, as shown below. 

The final step can involve introduction of the amino group or of the carbonyl group. o-Nitrobenzyl aldehydes and ketones are useful intermediates which undergo cyclization and aromatization upon reduction. The carbonyl group can also be introduced by oxidation of alcohols or alkenes or by ozonolysis. There are also examples of preparing indoles from o-aminophcnyl-acetonitriles by partial reduction of the cyano group. 

The reaction of alkenes with ozone constitutes an important method of cleaving carbon-carbon double bonds.138 Application of low-temperature spectroscopic techniques has provided information about the rather unstable species that are intermediates in the ozonolysis process. These studies, along with isotope labeling results, have provided an understanding of the reaction mechanism.139 The two key intermediates in ozonolysis are the 1,2,3-trioxolane, or initial ozonide, and the 1,2,4-trioxolane, or ozonide. The first step of the reaction is a cycloaddition to give the 1,2,3-trioxolane. This is followed by a fragmentation and recombination to give the isomeric 1,2,4-trioxolane. The first step is a... 

The formation of 1,2,3-trioxolanes from an alkene and ozone is the first step in the ozonolysis reaction, which is widely used in synthesis to convert alkenes to aldehydes or carboxylic acids. No instances of double bond migration during ozonolysis are known (since the first step is a cyclo-... 

The last step is an ozonolysis, which generates the aldehyde function from an alkene. 

There are six possible five-membered monocycles 1-6 containing three oxygen or sulfur atoms in the 1,2,3-positions <1996CHEC-II(4)545>. 1,2,3-Trioxolane 1 is the parent compound of the so-called primary ozonides, the primary reaction products in the reaction of alkenes with ozone. They are extremely unstable and rearrange to the more stable ozonides (1,2,4-trioxolanes). This rearrangement represents a key step in the reaction of ozonolysis. However, the parent compound 1 and a few derivatives have been characterized at low temperatures (see Section 6.05.10.1). 1,2,3-Trithiolanes have been synthesized (Section 6.05.10.3) some of them undergo slow decomposition at room temperature. Derivatives of 1,2,3-dioxathiolane 3 are unknown, and the other heterocycles of the mixed types 4-6 are known only in the oxidized forms, mostly as -oxides and J -dioxides, and also A-imino and A-thiono derivatives <1996CHEC-II(4)545>. The A-oxides and AA -dioxides of... 

The mechanism proposed by Criegee for the ozonolysis of alkenes <1975AGE745> considers an initial it-complex between the alkene and ozone which decays via a 1,3-dipolar cycloaddition into a 1,2,3-trioxolane or primary ozonide, known also as the molozonide . These compounds are unstable, even at low temperatures, and due to cycloreversion decompose into a carbonyl fragment and a CO, which may recombine by another 1,3-dipolar cycloaddition step to form the more stable 1,2,4-trioxolane ( secondary ozonide or final ozonide (see also Section 6.06.2). 

One of the key steps in the synthesis of the depsipeptide jasplakinolide involved the preparation of an aldehyde 273 by ozonolysis of an alkene 271 with the correct stereochemistry of the substituents (Scheme 83). The hydroxyl group was protected with R = fer7-butyl(dimethyl)silyl <2004ASC855>. 

Dipolar Cycloadditions and 1,3-Dipolar Cycloreversions as Steps in the Ozonolysis of Alkenes... 

The following reactions show the products obtained from ozonolysis of some representative alkenes. Note how (1) and (2) are used with a single reaction arrow to denote the steps in a two-step sequence. 

For the ozonolysis of linear alkenes only alk-l-encs should be used to avoid product mixtures as the resulting formaldehyde or formic acid are readily separated. This type of reaction is also a successful method for the preparation of perfluorinated carboxylic acids.The advantage over the oxidation with potassium permanganate is that the process does not form solid byproducts which are diflicult to separate. A disadvantage of this procedure is the fact that two reaction steps arc needed to obtain the required product. 

---