Through bond asymmetric induction

Use of imines as synthetic intermediates has been limited to mainly two processes reduction to amines, and as precursors to azaallyl anions for reaction with a variety of electrophiles (equation 36). The former transformation can often provide the best access to highly substituted amines and the latter represents one of the highest yield methods for carbon-carbon bond formation a to the carbonyl group of an aldehyde or ketone. Thus, the following sections will deal not only with imines but also with the properties and chemical reactions of the derived anions. Several reviews are available (in addition to those that cover both enamine and imine anion chemistry) as the result of recently uncovered methods for asymmetric induction through reactions of the anions. > ... 

The examples in Scheme 2-1 show that in intra-annular chirality transfer the resident asymmetric center connects to the enolate through annular covalent bonds. The geometric configuration of the enolate can be either immobile or irrelevant to the sense of asymmetric induction. 

An alternative approach in the asymmetric catalysis in 1,3-dipole cycloaddition has been developed by Suga and coworkers. The achiral 1,3-dipole 106 was generated by intramolecular reaction of an Rh(ii) carbene complex with an ester carbonyl oxygen in the Rh2(OAc)4-catalyzed diazo decomposition of <9-methoxycarbonyl-o -diazoacetophenone 105 (Scheme 12). The asymmetric induction in the subsequent cycloaddition to G=G and G=N bond was achieved by chiral Lewis acid Sc(iii)-Pybox-/-Pr or Yb(iii)-Pybox-Ph, which can activate the dipolarophile through complexation. With this approach, up to 95% ee for G=0 bond addition and 96% ee for G=G bond addition have been obtained, respectively. ... 

The rest of the catalyst cycle is identical to that illustrated for the rhodium complex-catalyzed reactions in Scheme 1. It has been proposed that the asymmetric induction occurs during the formation of alkyl-Pt(CO)L2 intermediate through olefin insertion into the Pt-H bond [13]. 

Desymmetrization of an achiral, symmetrical molecule through a catalytic process is a potentially powerful but relatively unexplored concept for asymmetric synthesis. Whereas the ability of enzymes to differentiate enantiotopic functional groups is well-known [27], little has been explored on a similar ability of non-enzymatic catalysts, particularly for C-C bond-forming processes. The asymmetric desymmetrization through the catalytic glyoxylate-ene reaction of prochiral ene substrates with planar symmetry provides an efficient access to remote [28] and internal [29] asymmetric induction (Scheme 8C.10) [30]. The (2/ ,5S)-s> i-product is obtained with >99% ee and >99% diastereoselectivity. The diene thus obtained can be transformed to a more functionalized compound in a regioselective and diastereoselective manner. 

PHTP is a chiral host which can be resolved into enantiomers DCA and ACA are (or derive from) naturally occurring optically active compounds. Using these hosts inclusion polymerization can be performed in a chiral environment and can be used for the synthesis of optically active polymers. This line of research has been very fruitful, both on the synthetic and on the theoretical plane. It has been ascertained that asymmetric inclusion polymerization occurs by a "through space" and not by a "through bond" induction only steric host-guest interactions (physical in nature) and not conventional chemical bonds are responsible for the transmission of chirality (W). 

An interesting use of removable chiral auxiliaries in photocycloaddition reactions concerns imminium salts. With cyclic enones, the observed asymmetric induction does not result from an approach of the double bonds in parallel planes because of the triplet nature of the reactive excited state. In contrast, the corresponding imminium salts react through their singlet excited state, and an approach of the reactants in parallel planes is now required during the cycloaddition process. For chiral imminium salts 130 derived from a cyclohexenone and a pyrrolidine having a C2 axis of symmetry, the intramolecular [2 -I- 2] photocycloaddition process occurs with a de up to 82%. As expected, the stereochemis-... 

We view acetylenic sulfoxide 1 as a two-carbon synthon in alkaloid synthesis. Our general approach, as depicted in Scheme 4, called for a Michael addition of Nu1 to the terminal acetylenic position followed by a cyclization by Nu2 (an intramolecular second Michael addition). This Michael addition cyclization step will build up the basic skeleton of the alkaloid system and at the same time control the absolute stereochemistry of the newly created chiral center through asymmetric induction of the chiral sulfoxide moiety. Finally, the sulfoxide can be transformed to another functional group (X) or used to promote the formation of another bond with Nu3 via trapping of the sulfenium ion intermediate under Pummerer rearrangement conditions (Scheme 4). 

One of the first examples of the asymmetric hydrocarboxylation of a-methylstyrene (2-phenyl-1-propene) in the presence of palladium(II) chloride and Diop was reported with a maximum enantiomeric excess of 14.2 % 6. In this case /7-induetion was achieved in the linear product via C-H bond formation. a-Induction through C.-C bond formation occurs in the branched product of styrene. Interestingly, the reaction of a-methylstyrene with different alcohols gives varying amounts of asymmetric induction. While 9.7% op is observed with ethanol, the more sterically hindered 2-propanol gives the linear product with 14.2% op6. 

The tandem Cope Claisen reaction of substrate 8 yields a 22 78 mixture of aldehydes 10 and 11. The initial Cope rearrangement proceeds exclusively through the chairlike transition state. The Claisen step results from the relative asymmetric induction exhibited by the ring substituent with preferred irans-C —C bond formation. The major diastereomer 11 has been transformed into the pseudoguaianolide aromatin399,400 1165. 

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