By Rainer Mahrwald
Aldol Reactions presents a entire up to date evaluate of aldol reactions together with software of alternative steel enolates; catalytic aldol additions catalyzed through assorted Lewis acids and Lewis bases; enantioselective direct aldol additions; antibodies and enzyme catalyzed aldol additions and the new competitive improvement of organocatalyzed aldol additions. the ability of every approach is verified via numerous purposes in overall synthesis of traditional items. the professionals and cons of those methodologies with reference to stereoselectivity, regioselectivity and alertness in overall synthesis of common items are mentioned. nice significance is determined to the various chances of the guide of aldol response to put in required configurations in advanced normal product synthesis.
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Quantity four makes a speciality of additions and the ensuing substitutions at carbon-carbon &pgr;-bonds. half 1 comprises procedures usually regarded as uncomplicated polar reactions, reactive electrophiles and nucleophiles including to alkenes and alkynes. a big subject is Michael-type addition to electron poor &pgr;-bonds, featured within the first six chapters.
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7,8,9 After these initial reports the attention was drawn to the stereoselective execution of this method. The aldol addition proceeds via a chair-like, six-membered transition state, which is more rigid than those of alkali metal enolates. This is due to the shorter boron oxygen bond length, which guarantees a maximum of 1,3-diaxial interactions (R3 ↔ L) and thus the formation of the more stable transition states A and B. For that reason higher stereoselectivities were observed when used with boron enolates, compared to aldol additions of corresponding lithium enolates.
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24 The same is true for the generation of defined boron enolates of carboxylic esters. (E)-boron enolates of carboxylic esters were obtained when used with tert-butyl esters and dicyclohexylboron triflates in the presence of triethylamine selectively. 25 As pointed out in the introduction there exist in general three different ways for an asymmetric execution of aldol reactions. Conventional chiral enolates have been used for this purpose extensively. Several chiral auxiliaries have been developed to achieve this aim.
Aldol Reactions by Rainer Mahrwald