Oxidative cleavage steric hindrance

The next key step, the second dihydroxylation, was deferred until the lactone 82 had been formed from compound 80 (Scheme 20). This tactic would alleviate some of the steric hindrance around the C3-C4 double bond, and would create a cyclic molecule which was predicted to have a greater diastereofacial bias. The lactone can be made by first protecting the diol 80 as the acetonide 81 (88 % yield), followed by oxidative cleavage of the two PMB groups with DDQ (86% yield).43 Dihydroxylation of 82 with the standard Upjohn conditions17 furnishes, not unexpectedly, a quantitative yield of the triol 84 as a single diastereoisomer. The triol 84 is presumably fashioned from the initially formed triol 83 by a spontaneous translactonization (see Scheme 20), an event which proved to be a substantial piece of luck, as it simultaneously freed the C-8 hydroxyl from the lactone and protected the C-3 hydroxyl in the alcohol oxidation state. 

Evidently, the cleavage of the weak endocyclic N-0 bond is the driving force of the ring opening. The nucleophilicity of the /V-oxide oxygen atom in nitronates facilitates the backward cyclization reaction (in the case of minimization of steric hindrance). With regard to the above mentioned one cannot exclude the tautomerism between cyclic nitronates 100 and open (or chain) isomers 101. 

Ozonolysis of cyclic olefins in the presence of carbonyl compounds gives the corresponding cross-ozonides.1329 In the ozonation of 1,2,4,5-tetramethyl-1,4-cyclohexadiene, oxidative dehydrogenation (formation of 1,2,4,5-tetramethylben-zene) was found to compete with oxidative cleavage because of steric hindrance.1330 Secondary ozonides (the 76 1,2,4-trioxolanes) are formed in high yields in the gas-phase, low-temperature ozonation of terminal and disubstituted alkenes.1331... 

Whether the diol moiety is up or down will have an influence on its ability to bind the oxidizing agent used to effect cleavage, due to increased or decreased steric hindrance. 

U(Tp )2CH2Ph] and the high thermodynamic stabihty of the +4 oxidation state.The trivalent uranium complex [U(Tp )2(CH2Ph)] has been investigated by DFT as catalyst to promote the fimctionahzation ofC02 and CS2-It has been found that the interaction between CO2 and CS2 is not with central uranium atom, but due to the steric hindrance the first step is the cleavage of the U—C benzyl bond to form a C—C covalent bond. " ... 

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