Diastereoselective NaBH₄ Reductions of (dl) α‐Keto Esters

Abstract

The (dl) a-keto esters 16, were prepared from the (dl) carbinols 1 to and from (dl) -2-hydroxyheptaElicene 5. These (dl) a-keto esters underwent NaBHq reductions to give the corresponding diastereomeric (dl) a-hydroxy esters 11, with 54 to -100% diastereaneric excesses.Highly enantioselective and/or diastereoselective syntheses are of great importance in organic chemistry. As an ultimate goal we aimed at the preparation and use o f chiral secondary alcohols and phenolewhich could lead to highly effective asymmetric inductions.process to evaluate the steric approach control as it is easy to modulate the bulkyness of the alkoxy moety of the ester '.available2'3 and proved to be efficient inducers in the reduction of the corresponding a-keto esters with NaBH4.The NaBH' reduction of a-keto esters was considered to be an efficient We have studied four secondary (dl) alcohols (1 to i) which are readily

Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

“…390 It appears that helicene derivatives have been underexplored in asymmetric catalysis, although, the few works that have been published show promising results in some precise reactions in which the chiral helically moiety was decorated with oxygen, nitrogen, and phosphorus functionalities. After the earlier examples employing a substituted [7]helicene moiety as chiral auxiliary and as chiral reagent for some diastereoselective reactions, 391 a variety of helicenic molecules have been employed in asymmetric catalysis reaction as organocatalysts, organometallic catalyst and chiral inducers. Helicenes with alcohol functions, phosphines, phospholes, bipyridines, aminopyridines, NHCs, revealed efficient in enantioselective organocatalysis and organometallic catalysis, where the rigidity, the bulkiness, and the dissymmetric environment, match-mismatch effects, appeared as important features for the stereocontrol of the reaction.…”

Helicene-Based Chiral Auxiliaries and Chirogenesis

“…However, one of the first results on the use of helicenes as chiral auxiliaries in few diastereoselective reactions was reported in 1985-1987 by Martin et al with racemic 2-substituted- [7]helicene [46][47][48][49][50]. Hence, racemic 29 was employed as an inbuilt chiral auxiliary for the diastereoselective reaction of the carbonyl group of α-keto esters upon reduction with NaBH 4 , yielding 30 (99% yield; 100% de) [46], and on the nucleophilic addition of Grignard reagent to give 31 (95% yield; 100% de) [47] (Scheme 1). However, since the majority of recently published reviews [2][3][4][5][6] have focused on helicene synthetic chemistry, there is no need to describe this topic here in detail.…”

“…These examples (Schemes 1-4) [46][47][48][49][50] are the first cases of helicenes being used as chiral auxiliaries resulting in a high asymmetric conversion (de or ee), yet noncatalytic in nature. These excellent results were due to the judicious selection of [7]helicene with corresponding functionalization at the 2-position (monodentate) (refer, Figure 8), which locates inside the helical chiral cavity with sufficient accessibility to the reactants.…”

One hundred years of helicene chemistry. Part 3: applications and properties of carbohelicenes

“…It contrasts the early toxicity studies on carbohelicenes that were done almost 80 years ago. For instance, an old report on the carcinogenicity of [5]helicene on mice dated back to 1933, 2 with regard to polyaromatic hydrocarbons toxicity (PAH). As an anecdote, the authors made use of a carcinogenic solvent such as benzene for skin tests with PAH, but they concluded: ''If 3,4,5,6-dibenzphenanthrene has any carcinogenic activity, it is of a very low order, for the hydrocarbon (0.3% solution in benzene), it has now been applied to a series of ten mice for 18 months and two of the mice are still alive (6 were alive after 1 year) but so far only one transient papilloma has appeared''.…”