Roush reagent

The reactions of Type I and Type 111 2-butenyl-mctal reagents with chiral aldehydes have been reviewed from the perspective of this model W. R. Roush in Comprehensive Organic Synthesis, C. H. Heathcock, Ed., Vol. 2, p 1, Pergamon, Oxford 1990. 

Roush reported another tartrate boronate, (E )-y-[(menthofuryI)-dimethyl silyl]-allylboronate 130, for untz -a-hydroxyallyation of aldehydes. Reagent 130 can be obtained from commercially available menthofuran, which was selected... 

The presence of a stereogenic center on the aldehyde can strongly inlinence the diastereoselectivity in allylboration reactions, especially if this center is in the a-position. Predictive rules for nucleophilic addition on snch a-snbstitnted carbonyl substrates such as the Felkin model are not always snitable for closed transition structures.For a-substituted aldehydes devoid of a polar substituent, Roush has established that the minimization of ganche-ganche ( syn-pentane ) interactions can overrule the influence of stereoelectronic effects. This model is valid for any 3-monosubstituted allylic boron reagent. For example, althongh crotylboronate (E)-7 adds to aldehyde 39 to afford as the major prodnct the diastereomer predicted by the Felkin model (Scheme 2), " it is proposed that the dominant factor is rather the minimization of syn-pentane interactions between the Y-snbstitnents of the allyl unit and the a-carbon of the aldehyde. With this... 

The control of the three consecutive asymmetric centers in a-alkylated y-amino-p-hydroxy acids is achieved by aldol condensation of a chiral aldehyde and a chiral reagent (Scheme 17 and Table 6), e.g. boron enolate 43,150 79 oxazolidinones of Evans type lit80 or 45 [68>69>801 or Brown s or Roush s crotylorganoboron reagents 46[81 and 47J81,82 respectively. 

For a review see A. J. Pearson, W. J. Roush (ed.) Handbook of Reagents for Organic Synthesis - Activating Agents and Protecting Groups, John Wiley Sons, Chichester 1999. 

In order to utilize such a chiral substructure as a source of absolute stereoehemical information in a synthesis, it is necessary to employ the complexes A and ent-A in non-racemie form. While the enan-tioselective preparation of such chiral complexes was achieved in the past more or less exclusively via resolution of raeemic mixtures, the diastereo-selective complexation of chirally modified ligands was shown more recently to be a practical alternative [2]. Another possibility, the enantiose-lective conversion of prochiral metal complexes by means of chiral reagents, has been achieved by W. R. Roush 3]. In a remarkable (formal) total synthesis of the antibiotic (-i-)-ikarugamycin (1), Roush et al. apply their method and demonstrate in a highly convincing fashion the synthetic usefulness of acyclic butadiene-FefCOfj complexes [4]. 

Pearson, AJ, Roush, WR, eds. Handbook of Reagents for Organic Synthesis Activating Agents and Protecting Groups. 1999. John 31. Wiley Sons, New York. pp. 370-373. 

Quartromicins are complex C2 symmetric macrocyclic natural products that have significant activity against a number of human viral targets.The diastereoselective synthesis of the endo- and exo-spirotetronate subunits of the quartromicins was accomplished by W.R. Roush and co-workers. The preparation of the exo-a-acetoxy aldehyde involved the Pummerer rearrangement oi a sulfoxide using acetic anhydride as the activating reagent and NaOAc as the co-catalyst. The yield of this transformation was modest and all attempts to improve its efficiency failed. 

Roush, W. R., Grover, P. T., Lin, X. Diisopropyl tartrate modified (E)-y-[(cyclohexyloxy)dimethylsilyl]allylboronate, a chiral reagent for the stereoselective synthesis of anti 1,2-diols via the formal a-hydroxyallylation of aldehydes. Tetrahedron Lett. 1990, 31,7563-7566. 

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