boron and propadiene

A practical catalytic method for enantioselective addition of an allene unit to aldimines is disclosed. Transformations are promoted by an in-situ-generated B-based catalyst that is derived from a simple, robust, and readily accessible (in multigram quantities) chiral aminoalcohol. A range of aryl-, heteroaryl-, and alkyl-substituted homoallenylamides can be obtained in 66-91% yield and 84:16 to >99:1 enantiomeric ratio through reactions performed at ambient temperature and in the presence of 0.1-3.0 mol% of the chiral catalyst and a commercially available allenylboron reagent. The catalytic protocol does not require strict anhydrous conditions, can be performed on gram scale, and promotes highly selective addition of an allenyl unit (vs a propargyl group). The utility of the approach is demonstrated through development of succinct approaches to syntheses of anisomycin and epi-cytoxazone.

Topics: Alkadienes; Amides; Amino Alcohols; Boron; Catalysis; Stereoisomerism

The kinetic hydroboration of allenylsilane 5 with ((d)Ipc)(2)BH at -40 °C provides allylborane 9Z with ≥12:1 selectivity. When the hydroboration is performed at temperatures above -40 °C, 9Z isomerizes to the thermodynamically more stable allylborane 9E with >20:1 selectivity. Subsequent treatment of 9Z or 9E with aldehydes at -78 °C provides syn- or anti-β-hydroxyallylsilanes, 7 or 8, respectively.

Topics: Aldehydes; Alkadienes; Alkenes; Boron; Hydroxylation; Kinetics; Molecular Structure; Silanes; Stereoisomerism

All-ene one! Three out of four stereoisomers of 2-vinyl-1,3-diols can be obtained from a single allene. A simple variation of the reaction conditions modifies the stereochemical outcome of the addition of an allene to an aldehyde via hydroboration. Stereocontrol is dependent upon the order in which the reagents are mixed (leading to E or Z boron species) and the type of aldehyde (aliphatic or aromatic) used.

Topics: Aldehydes; Alkadienes; Boron; Propanols; Stereoisomerism

The hydroboration of allene 7 with ((d)Ipc)(2)BH at 0 degrees C provides the kinetic allylborane 12Z with >20:1 selectivity. However, when the hydroboration is performed at 85 degrees C, the kinetically formed allylborane isomerizes to give the thermodynamic allylborane 12E with >or=12:1 selectivity. Subsequent treatment of 12Z or 12E with aldehydes at -78 degrees C, followed by oxidative workup, provides the 2-methyl-1,2-diols 8 and 9 in good yield and with 80-92% e.e.

Topics: Alcohols; Aldehydes; Alkadienes; Boron; Stereoisomerism; Substrate Specificity