Bond cleavage acetals

The transition state involves the carbonyl oxygen of one carboxyl group—the one that stays behind—acting as a proton acceptor toward the hydroxyl group of the carboxyl that IS lost Carbon-carbon bond cleavage leads to the enol form of acetic acid along with a molecule of carbon dioxide... 

Both types of processes, 7r -assisted y, -bond cleavage and P -bonding, have been invoked to operate in the phototransformations of the aldehyde-ketone (153) to products (155), (156) and (158). The conversions have been observed at room temperature in dioxane, t-butanol, ethanol and benzene using light of wavelengths 2537 A or above 3100 A or sensitization by acetophenone. The phosphorescing excited triple state of (153) is very similar to that of testosterone acetate (114), but its reactions are too rapid... 

Deoxy-2-fluoroglucosides ( ) are mechanism-based glucosidase inhibitors Fluorine at C-2 slows the rate of the acetal C-OR (R = 2,4-dinitrophenyl) bond cleavage in S by destabilizing the proposed oxocarbonium lon-like transition state for glucosidase-catalyzed hydrolyses [28]... 

A similar approach with [l- C]tributyltin acetate showed that carbon hydroxylation occurred at the a-, as well as at the /8-, carbon atoms, followed by Sn-C bond-cleavage, to afford dibutyltin derivatives. 

Because of certain misconceptions with regard to the choice of solvent and the occurrence of sulfur-oxygen bond fission in hydroxylic solvents - , it is important to emphasize that one can greatly reduce the rate of this competing process by the use of weak bases. In systems which can undergo facile C—O as well as S—O bond fission, it is possible to control the type of bond cleavage by choosing the appropriate base . A remarkable illustration of this behavior is found in the ethanolysis of sulfinate 6a. In anhydrous ethanol at 90.0° with acetate ion as the added base, 6a yielded ethyl 2, 6-dimethylbenzenesulfinate plus a trace of sulfone 7a. Under the same conditions but with 2,6-lutidine the reaction was slower and sulfone 7a was the only detectable reaction product . ... 

Using ethyl chloroformate as a reagent for C-8—N bond cleavage (Section II,A,4), synthesis of ( )-canadaline (272) from tetrahydroberberine (26) was independently achieved by Ronsch (40) and Hanaoka et al. (41,45). Bond-cleaved iodide 40a or chloride 40b was converted to the acetate 280 or the alcohol 281, both of which were easily derivatized to 272 through reduction of the urethane and oxidation of the alcohol 274 (Scheme 48). 

The indenobenzazepines 314, obtainable from the corresponding protoberberines (Sections V,F,2 and V,G,2), were converted to the spirobenzyliso-quinolinediones 315 in 76% yield through hydrolytic bond cleavage and recyclization by sequential treatment with 4 N hydrochloric acid, bromine in acetic acid, and triethylamine, via the indanediones (Scheme 58) (166). A one-step stereoselective rearrangement of an indenobenzazepine to a spirobenzylisoquinoline was developed by Blasko et ah (167). O-Methylfumarofine (316)... 

O-acetylophiocarpine (381) with ethyl chloroformate afforded the C-8—N cleaved urethane 382 in quantitative yield. Sequential treatment of 382 with silver nitrate, PCC, sodium hydroxide, and p-toluenesulfonic acid in ethanol furnished acetal 384, which was reduced with lithium aluminum hydride followed by hydrolysis to afford the hemiacetal 385. Oxidation of 385 with PCC provided (+ )-a-hydrastine (369). Similar treatment of O-acetylepi-ophiocarpine (386) afforded ( )-/J-hydrastine (368) however, in this case, C—N bond cleavage of 386 with ethyl chloroformate proceeded without regioselectivity. 

Selective removal of the iodine from fluorinated compounds was performed by 5% Pd/C catalyzed hydrogenolysis in the presence of triethylamine or sodium acetate.467 Ra-Ni and 1% NaOH were used for the cleavage of the C-I bond.468 The adsorption of chloroiodomethane was studied on a Pt(lll) surface. Dissociation began with C—I bond cleavage at about 150 K. Co-adsorbed deuterium atoms weaken the bonding between the starting compound and the surface and decrease the amount of dissociated molecules.469... 

In the author s opinion, the main reaction pathway for nitroso acetals (243) is determined by the ease of N-O bond cleavage in the O-N-O fragment. 

The product ratio of 8K/9K is similar to that of the Lewis acid-mediated reaction of 4a-c. These products of the C2-C3 bond cleavage (8K and 9K) may be formed via alkylideneallyl cation intermediate, which is formed by the oxygen protonation of 4. Thus, the product ratio of 10/(8K + 9K) is controlled by the protonations at the olefinic carbon and at the acetal oxygen of 4. 

Reduction of 2-bromo-3-pentanone at mercury affords a mixture of 3-pentanone and l-hydroxy-3-pentanone, whereas electrolysis of Q ,Q -dibromoacetone in the presence of benzoate gives a mixture of products arising from both a carbon-bromine bond cleavage and an Sn2 displacement of bromide by benzoate [94]. In an acetic acid-acetate buffer, branched dibromo ketones, such as 2,4-dibromo-2,4-dimethyl-3-pentanone, are reduced to a-acetoxy ketones however, less highly substituted compounds, such as 4,6-dibromo-5-nonanone, undergo simple cleavage of both carbon-bromine bonds [95]. Other work dealing with the reduction of Q, Q -dibromoketones has been described [96]. 

Ketones containing sulfur or nitrogen atoms bound to a-carbons suffer carbon-sulfur or carbon-nitrogen bond cleavage under the conditions of the Clemmensen reduction [159, 864 (p. 118). A ketosulfone was reduced to a sulfone-alcohol with zinc in refluxing 80% acetic acid in 70% yield [920]. 

In the presence of acetic acid, the radical-anion of a nitrosamine is protonated before bond cleavage can occur and further reactions give the dialkylhydrazine ... 

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