72507-44-9Relevant articles and documents
Protocols for the Preparation of Each of the Four Possible Stereoisomeric α-Alkyl-β-hydroxy Carboxylic Acids from a Single Chiral Aldol Reagent
Draanen, Nanine A. Van,Arseniyadis, Simeon,Crimmins, Michael T.,Heathcock, Clayton H.
, p. 2499 - 2506 (2007/10/02)
Protocols have been devised whereby all four possible stereoisomeric α-alkyl-β-hydroxy carboxylic acids can be derived from a single aldol reagent, hydroxy ketone 3.Compound 3, obtained in enantiomerically homogeneous form in 50percent overall yield from tert-butylglycine (1), is used for aldol reactions in the form of its trimethylsilyl and tert-butyldimethylsilyl derivatives, 4 and 5.The Z lithium and Z boron enolates of 4 react with various aldehydes to give aldols 8 and 9, respectively.Deprotonation of 4 by bromomagnesium 2,2,6,6-tetramethylpiperidide (MTMP) gives the E enolate, which may be trapped by trimethylsilyl chloride to obtain the E silyl enol ether 11.The E bromomagnesium enolate of 4 reacts with aldehydes to give aldols of structure 15.Transmetalation of the bromomagnesium enolate of keto ether 5 is accomplished by reaction with (triisopropoxy)titanium chloride.The resulting E (triisopropoxy)titanium enolate reacts with aldehydes to provide aldols of structure 17.The aldols resulting from the foregoing reactions are hydrolyzed to keto diols 19-22, which are oxidized to the stereoisomeric α-methyl-β-hydroxy carboxylic acids 23-26.
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).
