Fragmentation stereospecific

The ability of silicon to stabilize positive charge to itself has been exploited in silicon-directed fragmentations of p-silyl oximes. Oxime 0-acetate (61) fragmented stereospecifically to the ( )-alkenic product in the presence of catalytic TMSOTf (equation 41). ... 

Whilst spirophosphetanes (35) have been generated for reactivity studies via the Wittig reaction, they have also been generated as oxyanions (35 R = 0 , R = R = Ph) from )3-hydroxyalkylphos-phine oxides and found to fragment stereospecifically to cis stilbene <85JCS(P1)1953>. Another exam-... 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

Similarly, the stereospecific formation of cis-2-butene from cis-2,3-dimethylthiirane dioxide19 may be rationalized in terms of a stereospecific ring opening to give the threo-sulfinate 120 which, in turn, decomposes stereospecifically to yield the cis-alkene, hydroxide ion and sulfur dioxide73. The parent thiirane dioxide fragments analogously to ethylene, hydroxide ion and sulfur dioxide (equation 49). 

It was further confirmed that although the fragmentation pattern is dependent on the substitution pattern, most thiirane dioxides formed in situ decompose rapidly and stereospecifically under alkaline conditions to yield the corresponding alkenes with retention of configuration156. 

Anti stereospecificity is associated with a concerted reductive elimination, whereas single-electron transfer fragmentation leads to loss of stereospecificity and formation of the more stable A-stereoisomer. 

The most synthetically useful cheletropic elimination involves 2,5-dihydrothiophene-1,1-dioxides (sulfolene dioxides). At moderate temperatures they fragment to give dienes and sulfur dioxide.301 The reaction is stereospecific. For example, the dimethyl derivatives 22 and 23 give the E,E- and Z,E-isomers of 2,4-hexadiene, respectively, at temperatures of 100°-150°C.302 This stereospecificity corresponds to disrotatory elimination. 

Entry 4 involves nitrogen participation and formation of an iminium ion that is reduced by NaBH4. The reaction in Entry 5 creates an 11-methylenebicyclo[4.3.1]undecen-3-one structure found in a biologically active natural product. Note that this fragmentation creates a bridgehead double bond. Entry 6 involves construction of a portion of the taxol structure. The reaction in Entry 7 is stereospecific, leading to the E-double bond. 

It was shown that the fragmentation of 143 occurs stereospecifically in the sense of a trans-elimination erythro-145 gives the olefin (E)-144 while threo-745 furnishes (Z)-14495K... 

Despite the aromatic character, the arylphospholes could also participate in Diels-Alder reactions to give new type of 7-phosphanorbomenes. These products together with phosphanorbomenes obtained by the regio- and stereospecific dimerization of arylphosphole oxides were useful model compounds in the UV light mediated fragmentation-related phosphorylation of alcohols. A novel mechanism was substantiated. 

Mass spectra of the cis- and /ram-isomers of the pyrimido[2,TA [l,3]thiazin-6-ones 294 and 295 were studied. Retro-Diels-Alder fragmentation of the hydrocarbon ring was of medium to low stereospecificity. A number of highly selective processes were discovered allowing differentiation between stereoisomers <1996RCM721>. The mass spectral fragmentation pattern of 296 was studied in detail <1996PS(113)67>. 

The fragmentation is stereospecifically anti as shown by complementary geometry obtained in the cleavage of the epimeric pair of epoxycyclobutanones 91 and 92 (Eq. 110). The fragmentation product 93 of cyclobutanone 91 is transformable into the dimethyl ester of the pheromone of the Monarch butterfly. Considering the availability of the starting epoxy ketones from enones, the oxasecoalkylation serves to reorient the oxidation pattern with chain extension as summarized in Eq. 111. 

Nozaki reported the reaction of trialkylboranes with styryl sulfoxides and sulfones. Alkyl radicals generated from trialkylboranes add at the -position of /3-styryl sulfoxides and sulfones (a- to the sulfur atom). The resulting radicals fragment and deliver the -styryl adducts [108]. Interestingly, the sulfoxides eliminate very rapidly leading to partially stereospecific substitution (Scheme 44). The radical nature of the process is demonstrated by the presence of a side product derived from the solvent (THF) by hydrogen atom abstraction. 

Karpati, A. Rave, A. Deutsch, J. Man-delbaum, A. Stereospecificity of Retro-Diels-Alder Fragmentation Under Electron Impact. J. Am. Chem. Soc. 1973,90,4244. 

On the other hand, a double asymmetric version where both radical fragments are chiral has been investigated (equation 12) . The stereochemistry of the rearrangement products 20a and 20b is not the result of chelation control, but is determined by the stereochemistry of the hthium-bearing terminus. The significant difference in the stereospecificity of 19a and 19b reveals that a substantial level of mutal recognition of the radical enantiomers takes place during the recombination process. 

The stereospecific construction of acycHc anti-1,3-carbon stereogenic centers and C2-symmetrical fragments was also accompHshed via the sequential allylic alkylation... 

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