Seven structurally distinct pentadienyl type lithium and potassium compounds were screened against a variety of electrophiles in order to assess the regioselectivity of the trapping reactions. Organoborates and analogs thereof (fluorodimethoxyborane) proved to be perfectly regioreliable attacking only unsubstituted terminal positions and thus providing, after oxidation, exclusively primary allylic alcohols. 2,4-Pentadienyllithiums or -potassiums, that carry a methyl group at the 1- or 3-position, exhibit the same extreme regioselectivity towards halotrialkylsilanes or carbon dioxide. Although the unsubstituted parent compounds combine with such electrophiles still preferentially at the terminal position, considerable proportions of branched products are concomitantly formed as well (1/3-attack ratios ranging from 2:1 to > 20:1). Hydroxyalkylating and alkylating reagents such as formaldehyde, oxirane or butyl iodide invariably afford regioisomeric mixtures generally varying in composition between 3:1 and 1:3. The condensation reaction with halotrialkylsilanes appears to follow a concerted (SN2-like) rather than an addition/elimination (ate complex-mediated) mechanism.
Enantioselective Syntheses of 1,4-Pentadien-3-yl Carbinols via Brønsted Acid Catalysis
作者:Shang Gao、Ming Chen
DOI:10.1021/acs.orglett.9b04089
日期:2020.1.17
asymmetric addition of substituted 1,3-pentadienylboronates to aldehydes via Brønstedacidcatalysis is reported. Under the developed conditions, a variety of synthetically useful 1,4-pentadien-3-yl carbinols were obtained in good yields with high enantioselectivities in the presence of a catalytic amount of a chiral phosphoric acid.