Chiral Synthesis via Organoboranes. 11. Hydroboration. 82. Asymmetric Hydroboration of 1-Heteroarylcycloalkenes with Monoisopinocampheylborane. Synthesis of trans-2-Heteroarylcycloalkyl Boronates and Derived Alcohols of Very High Enantiomeric Purity

Article abstract of DOI:10.1246/bcsj.61.93

The hydroboration of represantative 1-heteroarylcycloalkenes with monoisopinocampheylborane (IpcBH2) was investigated systematically to establish the degree of asymmetric induction during hydroboration.The hydroboration of 1-heteroarylcyclopentene with IpcBH2 at -25 deg C proceeded cleanly to afford the corresponding dialkylboranes.These dialkylboranes, upon treatment with acetaldehyde and then oxidation, afforded the corresponding trans-2-heteroarylcycloalkanols of 85-86percent ee.Similarly, 1-heteroarylcyclohexenes, upon hydroboration with IpcBH2, followed by acetaldehyde treatment and oxidation, gave the corresponding alcohols in ca. 90percent ee.The dialylboranes or alkylboronic acids derived from 1-heteroarylcyclopentenes and IpcBH2 can be recrystallized to furnish materials approaching 100percent ee.From such dialkylboranes or alkylboronic acids, the corresponding boronates of ca. 100percent ee were prepared and isolated.These proving to be highly versatile synthetic intermediates for organic synthesis.