Bond cleavage or formation

Scheme 37 C - C bond cleavage or formation - Gault mechanism...

Except for some vitamin B12-dependent reactions, the cleavage or formation of carbon-carbon bonds usually depends upon the participation of carbonyl groups. For this reason, carbonyl groups have a central mechanistic role in biosynthesis. The activation of hydrogen atoms (3 to carbonyl groups permits (3 condensations to occur during biosynthesis. Aldol or Claisen condensations require the participation of two carbonyl compounds. Carbonyl compounds are also essential to thiamin diphosphate-dependent condensations and the aldehyde pyridoxal phosphate is needed for most C-C bond cleavage or formation within amino acids. 

Position of Bond Cleavage or Formation in Some Other Phosphate Transfers... 

Usually, the proton acceptor ability of amines (reaction A of Scheme 3) lead to ammonium ion formation, while the N—H bond cleavage (or formation) (reaction B of Scheme 3) is a less common process which may be obtained under drastic experimental conditions or via metal aniline complexes29. 

Mechanistically, enzymatic activation of the aldol donor substrates is achieved by stereospecific deprotonation along tw o different pathw ays (figures 5.5 and 5.6) [29]. Class I aldolases bind their substrates covalently via imine-enamine formation w ith an active site lysine residue to initiate bond cleavage or formation (Eigure 5.5) this can be demonstrated by reductive interception of the intermediate w ith borohydride w hich causes irreversible inactivation of the enzyme as a result of alkylation of the amine [30, 31]. 

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