Exocyclic cleavage

In these studies cleavage of the endocyclic alkoxy group produces ethylene glycol and since only about 5% was found, it was concluded that hydrolysis proceeds 95% via exocyclic cleavage, in excellent agreement with results found for poly(ortho esters). 

Scheme 14-1. General in-line monoanionic mechanism of phosphodiester cleavage transesterification catalyzed by hairpin ribozyme the first proton transfer (PT1), the nucleophilic attack (Nu), and the exocyclic cleavage (Cl) steps are shown, and the Oip and O2p pathways are indicated by blue and red colored hydrogens, respectively. For the uncatalyzed model reaction in solution, the Ojp and O2p pathways are energetically equivalent...

Figure 14-7. Snapshots of the active site structures near the transition state of (top) the nucleophilic attack and (bottom) the exocyclic cleavage for the in-line monoanionic O2p mechanism of cleavage transesterification in the hairpin ribozyme. The yellow and red colored cartoon is for the substrate and ribozyme strands, respectively, and water molecules interacting with non-bridging oxygens and O5/ are shown...

The A -acyl derivatives of 4-substituted-3,4,5,6-tetrahydro-27/-l,3-oxazin-2-ones proved to behave as effective chiral auxiliaries in asymmetric enolate alkylations and aldol reactions, the stereoselectivities of which were found to be higher for 4-isopropyl than for 4-phenyl derivatives <2006OBC2753>. The transformations of 4-isopropyl-6,6-dimethyl-3-propa-noyl-3,4,5,6-tetrahydro-2/7-l,3-oxazin-2-one 251 to 252 or 253 proceeded with excellent diastereoselectivities (Scheme 47). 6,6-Dimethyl substitution within the oxazine ring facilitated exclusive exocyclic cleavage upon hydrolysis of the C-alkylated and the aldol products 252 and 253, to furnish a-substituted carboxylic acids 254 or a-methyl-/ -hydroxycarboxylic acids 256. 

Species (23) and (24) function as the intermediates for the water-catalyzed hydrolysis of methyl ethylene phosphate which gives 25% exocyclic cleavage. Ionization to generate (26) followed by pseudorotation to (27), viz. 

In strong alkali (10 M NaOH) up to 15% exocyclic cleavage is again found. The anticipated intermediates in alkali are the dianions (28) and (29), viz-... 

Since (28) possesses a negative charge at an apical position, it should be unstable relative to (29). Rapid pseudorotation to (29) should then compete with decomposition of (28) to yield products arising from exocyclic cleavage. This explanation requires, however, a pentacovalent intermediate and that the rate of pseudorotation of (28) be increased by a minimal 103 relative to (26) to accommodate the experimental findings. By implication then the above base-catalyzed hydrolysis also is thought to involve the pentacovalent species. 

The roles of nucleophilic assistance and stereoelectronic control in determining endo-versus exo-cyclic cleavage of pyranoside acetals have been investigated for a series of a- and j8-anomers.15 Exocyclic cleavage of a-anomers, via a cyclic oxocarbenium ion, is predicted by the theory of stereoelectronic control, and was found exclusively for the cases studied. The endocyclic route, with an acyclic ion, is predicted for the /1-structures, and a measurable amount was found in all cases, but its extent was dependent on temperature, solvent, and the nature of the aglycone group. 

According to these rules a pseudorotation of the pentacoordinate intermediate in the hydrolysis of [120] is required for exocyclic cleavage to [121],... 

Reactions of Phosphoric Acids and Their Derivatives. - Numerous investigations of phosphate ester hydrolysis continue to be reported. The hydrolysis between 1.5 < pH < 4 of five- and six-membered cyclic phosphoramides (71) has been followed by UV and NMR spectroscopy. Small differences in hydrolysis reactivity for n = 5 and n = 6 constitutes evidence for syn lone pair catalysis. The product ratios from the hydrolysis shows that in the five-membered rings the main product is the one produced by endocyclic cleavage meanwhile, in the six- membered cyclic phosphoramide the kinetic product is the one produced by exocyclic cleavage. The syn orientation of two electron pairs on nitrogen stabilizes the transition state of water approach to the phosphoramides by ca. 3 kcal/moN when compared to the orthogonal attack. (Scheme 12). ... 

Efforts to trap an open-ring intermediate in the enzyme have not been successful (J. Knowles, private communication). However, evidence is accumulating that endocyclic, as well as exocyclic, cleavage contributes to non-enzymatic hydrolysis (Bunton et al. 1955 Shafizadeh 1958 Miller 1967 Franck 1992 Liras and Anslyn 1994). 

Lim, C. and Tole, P. (1992) Endocyclic and Exocyclic Cleavage of Phosphorane Monoanion A Detailed Mechanism of the RNase A Transphosphorylation Step, J. Am. Chem. Soc. 114,7245-7254. 

The exocyclic cleavage pathway is taken by all common pyranosides evidence includes ... 

When this 6-isomer was added to the reaction product of 1,2,6-hexanetriol and acetic acid, the 11.62 peak was enhanced. Thus, the 10.98 and 11.24 peaks in chromatogram 31a are the 1 and 2-isomers. Because the sensitivity of the gas chromatographic assay is such that as little as 0.01% of the 6-isomer can be detected, hydrolysis of this particular polymer proceeds exclusively by the exocyclic cleavage path shown in Scheme 20. This is in very good agreement with a reported hydrolysis study of the closely related model compounds 2-methoxy-l,3-dioxalane (1) and 2-phenyl, 2-methoxy-l,3-dioxalane (2) which have been reported to undergo respectively about 2.5% and less than 0.1% endocyclic cleavage [52]. 

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