64869-27-8Relevant academic research and scientific papers
Acyclic Stereoselection. 12. Double Stereodifferentiation with Mutual Kinetic Resolution. A Superior Class of Reagents for Control of Cram's Rule Stereoselection in Synthesis of erythro-α-Alkyl-β-hydroxy Carboxylic Acids from Chiral Aldehydes
Heathcock, Clayton H.,Pirrung, Michael C.,Lampe, John,Buse, Charles T.,Young, Steven D.
, p. 2290 - 2300 (2007/10/02)
Chiral α- ketones 7-9 have been prepared and their aldol condesations studied.Compound 8 shows from good to excellent inherent diastereoface selectivity in reactions with achiral aldehydes.Stereoselectivity is related to the size of the alkyl group attached to the aldehyde carbonyl; highest selectivity is observed with diphenylacetaldehyde (>10:1) and pivaldehyde (>19:1).Ketone 8 also shows high diastereoface selectivity in its reactions with chiral, racemic aldehydes 21, 25, 29, and 17, only one stereoisomeric aldol being obtained in each case.Furthermore, the four aforementioned aldehydes show much higher diastereoface selectivity with ketone 8 than they do with the related ketone 1.As a result, the reactions of racemic 8 with these chiral, racemic aldehydes show a high degree of "mutual kinetic resolution".In fact, the rate of the (R)-enolate plus (R)-aldehyde condensation is at least 35 times the rate of the (R)-enolate plus (S)-aldehyde condensation.It is shown by simple logical argument that such mutual kinetic resolution is expected in reactions between two chiral racemic compounds and that the magnitude of the effect should be proportional to the inherent diastereoselectivity shown by each compound in its reaction with achiral reaction partners.Thus, reagents such as 8 can be used to obtain the benefits of double stereodifferentiation even in the racemic form.As an application of the chemistry developed, (+/-)-blastmycinone (47) has been prepared in four steps from ketone 9 (20percent overall yield).
Acyclic Stereoselection. 7. Stereoselective Synthesis of 2-Alkyl-3-hydroxy Carbonyl Compounds by Aldol Condensation
Heathcock, Clayton H.,Buse, Charles T.,Kleschick, William A.,Pirrung, Michael C.,Sohn, John E.,Lampe, John
, p. 1066 - 1081 (2007/10/02)
The stereochemistry of the aldol condensation of preformed lithium enolates of a variety of ethyl ketones and propionic acid derivatives with aldehydes has been investigated.It is found that certain compounds give completely or nearly completely one diastereomeric enolate and that the stereostructure of the resulting aldol is correlated with the stereostructure of the enolate from which is formed.The observed stereochemistry may be understood in terms of an ordered transition state in which both oxygens are oriented in more or less the same direction.It is shown that the observed stereochemistry is kinetically controlled.In many cases, the initial aldol adduct equilibrates to furnish predominantly a threo isomer.The rate of equilibration varies widely, ranging from very fast at -60 deg C with the propiophenone-benzaldehyde adduct to slow at 25 deg C for the ethyl tert-butyl ketone-benzaldehyde adduct.The equilibration behavior of lithium ketolates is compared with that of zinc ketolates, and some differences are noted.A method for achieving erythro-threo equilibration via a chloral hemiacetal is presented.A new reagent is introduced (trimethylsilyloxy ketone 36) which may be used to stereoselectively homologate an aldehyde to an erythro α-methyl-β-hydroxy acid.As an application of the use of stereoselective aldol condensations in synthesis, (+/-)-ephedrine (48) has been synthesized from benzaldehyde in 71 percent overall yield.
