title: Stereoselective allyl transfer to chiral alpha-methoxycarbaldehydes: a model study related to the C-9/C-15 fragment of geldanamycin.
abstract: The enantiomerically pure alpha-methoxycarbaldehyde 3 was prepared from l-leucine in five steps and 31% overall yield. The aldehyde was subjected to a diastereoselective BF3-mediated crotylation with silane 4 and to various reagent-controlled addition reactions. The configuration of aldol addition products 19 and 20 was proven by single crystal X-ray crystallography. Based on these data spectral comparison allowed an unambiguous assignment of the desired anti,syn-crotylation product 2b and of the syn,syn-crotylation product 2a. Unexpectedly, product 2a of the BF3-mediated crotylation is formally the product of chelation-control. The anti,syn-crotylation product 2b, which is a model for the C-9/C-15 fragment of geldanamycin, was obtained by a reagent-controlled crotylation with the chiral (Z)-crotylborane 23.
authors.affiliation: Lehrstuhl für Organische Chemie I, Department of Chemistry, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany.
authors.forename: Tony
authors.initials: T
authors.lastname: Horneff
authors.forename: Eberhardt
authors.initials: E
authors.lastname: Herdtweck
authors.forename: Sören
authors.initials: S
authors.lastname: Randoll
authors.forename: Thorsten
authors.initials: T
authors.lastname: Bach
mesh_terms: D000447:Aldehydes / Q000138:chemical synthesis* / Q000737:chemistry; D000498:Allyl Compounds / Q000737:chemistry*; D016227:Benzoquinones / Q000737:chemistry*; D018360:Crystallography, X-Ray; D047029:Lactams, Macrocyclic / Q000737:chemistry*; D007930:Leucine / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D013237:Stereoisomerism
chemical_list: C514593:2-methoxy-4-methylpentanal; D000447:Aldehydes; D000498:Allyl Compounds; D016227:Benzoquinones; D047029:Lactams, Macrocyclic; D007930:Leucine; C001277:geldanamycin