Applications of 1-Alkenyl-1,1-Heterobimetallics in the Stereoselective Synthesis of Cyclopropylboronate Esters, Trisubstituted Cyclopropanols and 2,3-Disubstituted Cyclobutanones

1-Alkenyl-1,1-heterobimetallics are potentially very useful in stereoselective organic synthesis but are relatively unexplored. Introduced herein is a practical application of 1-alkenyl-1,1-heterobimetallic intermediates in the synthesis of versatile cyclopropyl alcohol boronate esters, which are valuable building blocks. Thus, hydroboration of 1-alkynyl-1-boronate esters with dicyclohexylborane generates 1-alkenyl-1,1-diboro species. In situ transmetalation with dialkylzinc reagents furnishes 1-alkenyl-1,1-borozinc heterobimetallic intermediates. Addition of the more reactive Zn-C bond to aldehydes generates the key B(pin) substituted allylic alkoxide intermediates. An in situ alkoxide directed cyclopropanation proceeds with the formation of two more C-C bonds, affording cyclopropyl alcohol boronate esters with three new stereocenters in 58-89% isolated yields and excellent diastereoselectivities (> 15:1 dr). Oxidation of the B-C bond provides trisubstituted alpha-hydroxycyclopropyl carbinols as single diastereomers in good to excellent yields (75-93%). Facile pinacol-type rearrangement of the alpha-hydroxycyclopropyl carbinols provides access to both cis- and trans-2,3-disubstituted cyclobutanones with high stereoselectivity, (> 17:1 dr in most cases) from a common starting material. This methodology has been applied in the synthesis of quercus lactones A and B.