Asymmetric Dihydroxylations of 1-Substituted (E)- and (Z)-3-Methylpent-2-en-4-ynes: Full Compliance with the Sharpless Mnemonic Re-established and Embellished
摘要:
Asymmetric dihydroxylations ("ADs") of the pentenynyl chlorides (E)- and (Z)-1 or the pentenyne-based ester (Z)-3 in the presence of (DHQ)(2)-containing ligands delivered diol stereoisomers (2R,3S)-2, (2R,3R)-2, and (3S,4R)-4, respectively. The ADs of pentenynyl ethers (E)-10 and (Z)-12, respectively, have the same stereochemical preference under analogous conditions; these reattributions correct previous reports of the contrary. The Sharpless mnemonic rationalizes all these results implying that each substrate prefers a Sharpless/Norrby instead of a Chapleur orientation in the transition state.
para-Methoxyphenyl (PMP) ether is a very convenient protectinggroup for the alcohol function; however, its cleavage requires strong oxidative conditions. In this field, the use of powerful ceric ammonium nitrate has been widely described, despite the fact that its strong nature results in the formation of degradation products in acid- or oxidation-sensitive substrates. In this paper, a new anodic