- Regio- and enantioselective synthesis of α-silyl aldehydes and ketones via SAMP/RAMP hydrazones
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An efficient, highly regio- and enantioselective methodology for the synthesis of α-silyl aldehydes and ketones 2 and 6 using two different procedures was developed. Direct α-silylation of the azaenolates derived from SAMP/RAMP hydrazones 3 with various silyl trifluoromethanesulfonates resulted after oxidative removal of the chiral auxiliary in highly enantiomerically enriched α-silyl aldehydes and ketones (R)- or (S)-2. Alternatively, hydrazones 5 derived from acetaldehyde or methyl ketones were initially α-silylated followed by highly diastereoselective α-alkylation with suitable electrophiles. The latter variant allows the regiocontrolled synthesis of α-silyl ketones (S)-6 with high enantiomeric purity, not available by direct α-silylation of unsymmetrical ketone hydrazones. The absolute configuration of the resultant α-silyl carbonyl compounds 2 and 6 was derived from earlier mechanistic investigations of electrophilic substitutions via deprotonated SAMP/RAMP hydrazones and was unambiguously assigned by X-ray-crystallographical analysis of the α-silyl-SAMP hydrazone (S,R)-4c. VCH Verlagsgesellschaft mbH, 1996.
- Enders, Dieter,Lohray, Braj B.,Burkamp, Frank,Bhushan, Vidya,Hett, Robert
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p. 189 - 200
(2007/10/03)
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- ENANTIOSELECTIVE SYNTHESIS OF PROTECTED α-HYDROXY ALDEHYDES AND KETONES VIA HYDROXYLATION OF METALATED CHIRAL HYDRAZONES
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α-Benzyloxy aldehydes and α-acetoxy ketones 4 of high enantiomeric purity are prepared in good overall yields via oxaziridine mediated hydroxylation of chiral hydrazone azaenolates.As auxiliaries novel proline derived hydrazine reagents 5 are used.
- Enders, Dieter,Bhushan, Vidya
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p. 2437 - 2440
(2007/10/02)
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- ASYMMETRIC SYNTHESES VIA METALATED CHIRAL HYDRAZONESOVERALL ENANTIOSELECTIVE α-ALKYLATION OF ACYCLIC KETONES
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A general method is described, which allows the overall enantioselective α-alkylation of acyclic ketones in good overall yields (44-86percent, 4 steps) and enantioselectivities ranging routinely from >94percent ee up to virtually complete asymmetric induction (99.5percent ee).The acyclic ketones are transformed to their corresponding "SAMP-hydrazones" (S)-2 by reaction with the enantiomerically pure hydrazine (S)-1-amino-2-methoxymethyl-pyrrolidine , readily available from (S)-proline.Metalation to form chiral azaenolates (S)-3 of ECCZCN-configuration and then alkylation to product hydrazones 4, followed by hydrazone cleavage via acidic hydrolysis of methiodides 9 in a two phase system or ozonolysis, leads to α-substituted, enantiomerically enriched, acyclic ketones 5.In special cases, where a phenyl group is directly attached to the newly generated center of chirality (5n,o,p), only low enantiomeric excesses are observed. 17 Examples, including first applications in natural product synthesis (cf 5a,b,e, and h) are summarized.
- Enders, D.,Eichenauer, H.,Baus, U.,Schubert, H.,Kremer, K. A. M.
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p. 1345 - 1359
(2007/10/02)
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