Ketone biological halogenation

Remarkably, ketone halogenation also occurs in biological systems, particularly in marine alga, where dibromoacetaldehyde, bromoacetone, 1,1,1-tri-bromoacetone, and other related compounds have been found. 

As already described, with the exception of iodo, all halogens have proved useful DMGs in the pyridine series for the preparation of potential intermediates and final products related to biologically interesting molecules (Schemes 36, 41-47, 57). As specific applications, the reaction of chloro ketones 524 with ethylene diamine and 1,2-phenylenediamine give, respectively, pyrido[4,3-c]-l, 4-diazepines 525 and pyrido[4,3-6]-1,5-benzodiazepines 523 in good yields (Scheme 160) (87AP704). 

Pushing simple halogenation of ketones into more complex molecules of biological importance. 

Diastereoselective halogenations using removable chiral auxiliaries have been the purpose of recent significant developments. a-Halo aldehydes, a-halo ketones and a-halo carboxylic acid derivatives are very useful precursors involved for total syntheses of pharmaceutical drugs and phytochemicals. In most cases, the biological activity is associated with one of the two enantiomers. So, diastereoselective halogenation of carbonyl compounds and carboxylic acid derivatives has attracted considerable attention in recent years, as a tool for the production of enantiomerically pure substances. The numerous examples of diastereoselective halogenation of compounds with non removable chiral adjuvants are outside the scope of this review. 

Apart from syntheses aimed at producing substances for biological evaluation, which will be discussed later, most work in this area has employed aldehydes rather than ketones. Acidic conditions must be avoided during the isolation of the alcohols, since ethers are readily formed.47,76 In the great majority of cases the alcohol has been successfully reduced by the lithium aluminum hydride-aluminum chloride method to which we have already referred. Halogen substituents on the thiophene ring (even iodine) survive this procedure. The methods of synthesis discussed in the preceding sections are not suitable for the preparation of unsymmetric dithienylmeth-anes with free thiophene a-positions, and it is for these substances that the... 

Depressants may be hydrocarbons, halogenated hydrocarbons, alcohols, ethers, ketones, weak acids (like the barbiturates), weak bases, or sulphones. They are the selectively toxic agents used in medicine as hypnotics and general anaesthetics. This is the only kind of biological activity in which structure simply does not matter (there is much more about this in Chapter 15). See Section 3.3 for the general function of partition effects in securing selective distribution of drugs. 

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