Cleavage acetals, mixed

The mixed aliphatic - aromatic ethers are somewhat more reactive in addition to cleavage by strong hydriodio acid and also by constant b.p. hydrobromio acid in acetic acid solution into phenols and alkyl halides, they may be bromi-nated, nitrated and converted into sulphonamides (Section IV,106,2). 

Contained within intermediate 25 is an acid-labile mixed acetal group and it was found that treatment of 25 with camphorsulfonic acid (CSA) results in the formation of dioxabicyclo[3.3.0]octane 26 in 77 % yield. Acid-induced cleavage of the mixed cyclic acetal function in 25, with loss of acetone, followed by intramolecular interception of the resultant oxonium ion by the secondary hydroxyl group appended to C leads to the observed product. Intermediate 26 clearly has much in common with the ultimate target molecule. Indeed, the constitution and relative stereochemistry of the dioxabicyclo[3.3.0]octane framework in 26 are identical to the corresponding portion of asteltoxin. 

Mixed acid fermentations are not limited to bacteria. For example, trichomonads, parasitic flagellated protozoa, have no mitochondria. They export pyruvate into the bloodstreams of their hosts and also contain particles called hydrogenosomes which can convert pyruvate to acetate, succinate, C02, and H2.144 Hydrogenosomes are bounded by double membranes and have a common evolutionary relationship with both mitochondria and bacteria. The enzyme that catalyzes pyruvate cleavage in hydrogenosomes apparently does not contain lipoate and may be related to the pyruvate-ferredoxin oxidoreductase of clostridia (Eq. 15-35). The hydrogenosomes also contain an active hydrogenase. 

Triethylamine must be added immediately after mixing the alcohol and activated DMSO, in order to avoid the aeid-catalyzed cleavage of a very sensitive acetal. 

The DNA sequencing chemistry begins with a base-modification reaction, the extent of which determines the frequency of DNA cleavage in the subsequent phosphate-elimination reaction. The number of bases modified in each molecule depends on the concentration of dimethylsulphate (G and G A reactions) and hydrazine (C T reactions) as well as the temperature and duration of the reaction. For speed and convenience the Maxam-Gilbert procedure makes use of temperature shifts and dilution to control the rate and extent of these reactions. The reagents are mixed at 0°C, incubated at 20°C for the required time and the DNA precipitated with cold sodium acetate and ethanol to slow down or halt the reaction. A fixed concentration of the different reagents is usually used so the main factor determining the extent of reaction is the time of incubation at 20°C. 

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