Biomedical subjects
Publications and source records attributed to Reinhard W Hoffmann.
The addition of alpha-haloalkyl Grignard reagents to benzaldehyde occurs with simple diastereoselectivity substantially higher than that of the corresponding lithium reagents. Reaction in the presence of dimethyl-aluminum chloride suppresses subsequent Oppenauer oxidation of the resulting Mg-alkoxides by excess benzaldehyde.
The Grignard reagent 2, in which the magnesium-bearing carbon atom is the sole stereogenic centre has been coupled with vinyl bromide under Pd(0) or Ni(0)-catalysis to give compound 3 with full retention of configuration. Coupling using Fe(acac)3 or Co(acac)2 as catalyst was accompanied by considerable racemisation. These findings are discussed with respect to a dichotomy between concerted polar and stepwise SET transmetallation pathways.
After more than a quarter of a century of development, the methodology of stereoselective synthesis appears to be fully matured. In line with this, the potential that meso compounds offer in stereoselective synthesis is clearly recognized. The use of meso compounds in synthesis is, however, in no way commensurate with this potential, because, ironically, the synthesis of meso compounds in the first place is a problem of stereoselective synthesis. Present-day methodology does not provide many useful solutions to this problem. This Review therefore addresses the strategies available for the synthesis of more elaborate meso compounds whose stereogenic centers have a distance >1,4 between them. meso Compounds with more than four stereogenic centers are also considered. The criteria used in choosing from several strategies in the synthesis of such compounds are discussed.
[reaction: see text] The conversion of the iodine ate-complex 5 to the Grignard reagent 6 follows a rate law first order in 5 and in magnesium ions. In addition to this normal pathway of the halogen-metal exchange process, the conversion of 5 to 6 is catalyzed by a number of diverse "electrophiles". This points to a SET-initiated radical chain process as a second pathway for the conversion of the iodine ate-complex 5 to the Grignard reagent 6.
The involvement of single electron transfer, i.e. of free radicals in the reactions of organomagnesium reagents could be detected with the aid of a chiral secondary Grignard reagent, in which the magnesium-bearing carbon atom is the sole stereogenic centre. So far, however, such reagents have not been accessible, because the standard preparation of Grignard reagents proceeds via free radicals. We review and summarize here our efforts to generate such a Grignard reagent 36 by asymmetric synthesis starting from an enantiomerically pure alpha-chloroalkyl-sulfoxide 30b using a sulfoxide/magnesium exchange reaction to give 33 followed by a carbenoid homologation reaction. Grignard reagent 36 turned out to be an ideal probe to learn about the extent to which SET is involved in reactions of organomagnesium reagents.
A set of three-armed urea-containing anion receptors was prepared. The receptors all have the same binding topology but differ in the level of conformational preorganization with respect to the arrangement of the side-arms relative to the platform and within the side arms themselves. This is mirrored in a specific increase (x 2.5) in the binding constant for chloride and in a 12-fold increase in the chloride/nitrate-selectivity.
Transmetalation of a chiral secondary Grignard reagent to Cu(I) or Mn(II) occurs with partial racemization in a manner that suggests the sequence of two transmetalation cycles, one with racemization (caused by ET ?) and the other one with (predominant) retention of configuration.
The anion binding of tripodal hosts 2-4 has been studied. Increasing levels of conformational preorganisation of the side arms of the hosts led to increased (Cl-) unaltered (Br-) or decreased (NO3-) binding; it was thus possible to change guest selectivities by about an order of magnitude through conformational preorganisation of the flexible host.
Efforts are described to design simple, fully flexible but conformationally preorganised omega-hydroxy-nonanoic acids that could serve as the conformation controlling unit in analogues of the potent protein-kinase C activator aplysiatoxin. Such analogues are macrodilactones incorporating the designed omega-hydroxy-nonanoic acid and 3,4-dihydroxy-pentanoic acid, which contains the pharmacophoric groups. The design process (replacement of CH(2) groups by an oxygen atom, annelation of a six-membered ring and placement of alkyl substituents) of the omega-hydroxy-nonanoic acids was monitored by force-field calculations. In the end of this process simple analogues of aplysiatoxin are proposed in which the proper disposition of the pharmacophoric groups is secured by a conformationally flexible but preorganised template structure as part of the macrodilactone ring.