Crotylboronation, Roush asymmetric

Roush WR, Palkowitz AD, Ando K. Acyclic diastereoselec-tive s3mthesis using tartrate ester modified crotylboronates-double asymmetric reactions with alpha-methyl chiral aldehydes and synthesis of the C(19)-C(29) segment of rifamycin S. J. Am. Chem. Soc. 1990 112(17) 6348-6359. 

Liu and Zhou applied Roush s crotylboration to the stereoselective synthesis of the orostanal 70, a novel sterol that induces apoptosis in human acute promyelotic leukemia cells28 (Scheme 3.ly). The aldehyde 72, prepared from hyodeoxycholic acid methyl ester, underwent asymmetric reaction with crotylboronate (R,R)-43E to furnish 73. Hydrogenation of the terminal alkene followed by Swem oxidation gave the ketone 74. Methylenation of the ketone and removal of the protective groups afforded orostanal in 50% yield. 

A final example of the use of tartrate-derived crotylboronates in natural product synthesis is illustrated in the formal total synthesis of ikarugamicin (Scheme II-11) [179]. Here, Roush and Wada used the asymmetric crotylboration of meso-(t/" -2,4-hexadien-1,6-dial)iron tricarbonyl 266 with (S,S)-(E)-219 to set three stereocenters in their synthesis of the a,s-indacene unit of ikarugamycin. This key reaction provided 267 in 90% yield and >98% ee. Homoallylic alcohol 267 was converted to the allylic acetate 268, which underwent stereoselective ethylation with EtsAl with retention of stereochemistry. The resulting adduct 269 was subsequently elaborated to as -indacene unit 271 through a 15-step synthetic sequence, including the intramolecular Diels-Alder reaction of 270. 

Roush, W. R. Halterman, R. L. Diisopropyl Tartrate Modified E) Crotylboronates Highly Enantioselective Propionate (li)-Enolate Equivalents J. Am. Chem. Soc. 1986, 108, 294-296. Roush, W. R Ando, K. Powers, D. B. Palkowitz, A. D. Halterman, R L. Asymmetric Synthesis Using Diisopropyl Tartrate Modified ( )- and (Z)-Crotylboronates Preparation of the Chiral Crotylboronates and Reactions with Achiral Aldehydes J. Am. Chem. Soc. 1990, 112, 6339-6348. 

Roush WR. Ando K, Powers DB, Palkowitz AD, Halterman RL. Asymmetric synthesis using diisopropyl tartrate modi-Hed (E)- and (Z)-crotylboronates preparation of the chiral crotylboronates and reactions with achiral aldehydes. J. Am. Chem.Soc. 1990 112 6339-6348. 

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