1,4-Di-O-tert-alkyl-l-threitols as Chiral Auxiliaries in the Asymmetric Nucleophilic Addition of Alkyllithiums to Hydrazones
摘要:
The applications of 1,4-di-O-tert-alkyl-L-threitols as chiral auxiliaries in the asymmetric nucleophilic addition of alkyllithiums to hydrazones are investigated. Chiral acetal-hydrazones 9, obtained from the chiral acetals 8 by ozonolysis followed by treatment with dimethylhydrazine, are allowed to react with organolithium reagents in toluene at -78 degrees C to give 15 with excellent diastereoselectivity. The stereochemical assignments were based an the X-ray crystal structure of 17a. The absolute configuration at C-2 Of the major isomer of the adducts 15 was thereby determined to be S. The nucleophile thus attacked from the si face of the C=N moiety. The effect of solvent on the diastereoselectivity of the reactions of 9 with organolithium reagents is reported. Polar aprotic solvent shows poor diastereoselectivity, and the diastereoselectivity; is reversed when the reaction is carried out in THF. Reaction of dl-14 with methyllithium has been studied for comparison purposes and the reaction shows the opposite selectivity. Chelation intermediates 18 and 26 are proposed for these reactions to account for the observed stereoselectivities.
Highly Diastereoselective Ring-Opening Reactions of Chiral Acetals with Secondary or Sterically Hindered Grignard Reagents
摘要:
There has been an increasing interest in the stereoselective ring opening of acetals by means of nucleophiles.(1-4) Most of these approaches involve the preferential complex formation of acetals with a Lewis acid and subsequent invertive substitution by nucleophiles (Scheme 1).(2) The retentive ring cleavage of acetals has been investigated only briefly, mostly related to the reductive ring opening with metal hydrides.(3,4) Recently, Yamamoto and his co-workers reported the highly diastereoselective retentive alkylation of chiral acetals using novel organoaluminum reagents.(4) However, only methyl- or primary alkylaluminum reagents can be used. We now wish to describe our investigations on the highly diastereoselective reactions of secondary or sterically bulky Grignard reagents with chiral acetals prepared from aldehydes and tunable chiral diols.
The reactions of C-60 with C-2 chiral 1,4-tert-alkoxy-2,3-bisazidobutanes give the corresponding chiral bisazafulleroids 2. The enantiomeric pairs of chiral bisazafulleroids 2 exhibit mirror image CD curves.
1,4-Di-<i>O-tert-</i>alkyl-<scp>l</scp>-threitols as Chiral Auxiliaries in the Asymmetric Nucleophilic Addition of Alkyllithiums to Hydrazones
The applications of 1,4-di-O-tert-alkyl-L-threitols as chiral auxiliaries in the asymmetric nucleophilic addition of alkyllithiums to hydrazones are investigated. Chiral acetal-hydrazones 9, obtained from the chiral acetals 8 by ozonolysis followed by treatment with dimethylhydrazine, are allowed to react with organolithium reagents in toluene at -78 degrees C to give 15 with excellent diastereoselectivity. The stereochemical assignments were based an the X-ray crystal structure of 17a. The absolute configuration at C-2 Of the major isomer of the adducts 15 was thereby determined to be S. The nucleophile thus attacked from the si face of the C=N moiety. The effect of solvent on the diastereoselectivity of the reactions of 9 with organolithium reagents is reported. Polar aprotic solvent shows poor diastereoselectivity, and the diastereoselectivity; is reversed when the reaction is carried out in THF. Reaction of dl-14 with methyllithium has been studied for comparison purposes and the reaction shows the opposite selectivity. Chelation intermediates 18 and 26 are proposed for these reactions to account for the observed stereoselectivities.
Highly Diastereoselective Ring-Opening Reactions of Chiral Acetals with Secondary or Sterically Hindered Grignard Reagents
There has been an increasing interest in the stereoselective ring opening of acetals by means of nucleophiles.(1-4) Most of these approaches involve the preferential complex formation of acetals with a Lewis acid and subsequent invertive substitution by nucleophiles (Scheme 1).(2) The retentive ring cleavage of acetals has been investigated only briefly, mostly related to the reductive ring opening with metal hydrides.(3,4) Recently, Yamamoto and his co-workers reported the highly diastereoselective retentive alkylation of chiral acetals using novel organoaluminum reagents.(4) However, only methyl- or primary alkylaluminum reagents can be used. We now wish to describe our investigations on the highly diastereoselective reactions of secondary or sterically bulky Grignard reagents with chiral acetals prepared from aldehydes and tunable chiral diols.