Allylboronates, Roush allylboronation

The minimization of gauche pentane interactions in the transition stales is also an important consideration in the reactions of substituted allylboronates and a-heteroatom-substituted aldehydes4,52d 54,56. Transition states 8 and 11 have been identified by Roush and Hoffmann as the least sterically hindered ones accessible in reactions with (E)- and (Z)-allylboronates. 

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... 

Hunt and Roush <1995TL501> achieved a synthesis of swansonine 230 via a stereoselective allylation of a silyl allylboronate derived from tartaric acid followed by a Tamao-Fleming oxidation to introduce the C-2 hydroxy functionality (Scheme 54). 

Roush et al. discovered that the tartrate ester-modified allylboronates, such as diisopropyl tartrate allylboronate (.S, .S )-41, react with achiral aldehydes to give the homoallylic alcohols 42 in good yields and high levels of enantioselectivity of up to 87% ee when the reaction is carried out in toluene in the presence of 4-A molecular sieves20 (Scheme 3.1q). To rationalize the asymmetric induction realized by 41, two six-membered transition states were compared (Scheme 3.1r). It was reasoned that transition state A was favored over transition state B due mainly to the nonbonded electronic repulsive interactions of the lone-pair electrons of the aldehyde oxygen and the carbonyl oxygen of the tartrate ester. 

The biologically important 2-deoxypentoses can be prepared readily by the two-carbon chain elongation of 2,3- -isopropylidene-D-glyceraldehyde following Roush s allylation method (Scheme 13.54), relying on the highly diastereoselective additions of enantiomerically pure allylboronates derived from (R,R) and (5, 5 )-tartaric acid [100]. Similarly, the 2,6-dideoxyhexose derivative 155 was obtained by Roush and Straub (Scheme 13.54) [101]. 

Roush, W R, Adam, M A, Walts, A E, Harris, D J, Stereochemistry of the reactions of substituted allylboronates with chiral aldehydes. Factors influencing aldehyde diastereofacial selectivity, J. Am. Chem. Soc., 108, 3422-3434, 1986. 

The total synthesis of the 20-membered macrolide (+)-lasonolide-A was undertaken by S.H. Kang and co-workers. During the construction of the C15-C25 subunit, they utilized the Roush asymmetric allylation reaction to introduce the C21 and C23 stereocenters. First, (R,R)-diisopropyltartrate derived allylboronate was used to provide the (S)-homoallylic alcohol with 78% ee. A second asymmetric allylation was achieved utilizing the (S,S)-diisopropyltartrate-derived allylboronate to form the (R)-homoallylic alcohol with a 91% ee. 

Roush asymmetric allylation Reaction of allylboronates with aldehydes to give homoallylic alcohols. 386... 

Roush, W. R., Walts, A. E., Hoong, L. K. Diastereo- and enantioselective aldehyde addition reactions of 2-allyl-1,3,2-dioxaborolane-4,5-dicarboxylic esters, a useful class of tartrate ester modified allylboronates. J. Am. Chem. Soc. 1985,107, 8186-8190. 

Roush, W. R., Grover, P. T. Diisopropyl tartrate (E)-Y-(dimethylphenylsilyl)allylboronate, a chiral allylic alcohol 3-carbanion equivalent for the enantioselective synthesis of 2-butene-1,4-diols from aldehydes. Tetrahedron Lett. 1990, 31,7567-7570. 

For a study of the dependence of reactivity of allylboronates on the diol unit, see W. R. Roush, L. Banfi, J. C. Park and L. K. Hoong, Tetrahedron Lett., 1989, 30, 7305. While pinacol esters are commonly employed owing to their stability, these derivatives are considerably less reactive than other esters. 

Evans aldol reaction Hajos-Wiechert reaction Keck stereoselective allylation Roush allylboronation... 

The synthesis and use of tartrate-modified allylboronate 1 was first reported by Roush and co-workers in 1985. The synthesis and use of the corresponding (E)- and (. -crotylboronate reagents 2 and 3 was published by Roush and co-workers shortly thereafter. The ease of synthesis, stability and efficient reactivity of these reagents offers advantages over many other allyl- and crotylmetal reagents. Roush and co-workers have extensively explored the enantioselective allylations with achiral aldehydes as well as the... 

The allylboronate reagent 1 was used twice in the synthesis of (-)-lasubine I by Wang and Liao. (-)-Lasubine I (67) is a bioactive alkaloid isolated from the Lythraceae plant.(-)-Lasubine I contains three stereocenters in a quinolizidine core. Roush allylation of aldehyde 63 with provided the 1,3-aminoalcohol 67 in > 90 < 10 diastereoselectivity. Alcohol protection and alkene oxidation provided an intermediate aldehyde which underwent Roush allylation with to provide homoallylic... 

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