Chiral Synthesis via Organoboranes. 35. Simple Procedures for the Efficient Recycling of the Terpenyl Chiral Auxiliaries and Convenient Isolation of the Homoallylic Alcohols in Asymmetric Allyl- and Crotylboration of Aldehydes

Article abstract of DOI:10.1021/jo00050a044

Asymmetric allyl- and crotylboration of aldehydes, RCHO, with terpenyl-based allyl- and crotylborane reagents Ter2^*BAll (1), Ter2^*BCrt^Z (2), and Ter2^*BCrt^E (3, Ter^* = Ipc, 4-Icr and 2-Icr; All = allyl and Crt = crotyl), afford Ter2^*BOCH^*(R)C^*(¹R)(²R)CH=CH2 intermediates 4.In these reactions, the isolation of homoallylic alcohols, HOCH^*(R)C^*(¹R)(²R)CH=CH2 (5), can be accomplished via oxidation of 4 with alkaline hydrogen peroxide.Unfortunately, oxidative workup destroys the chiral auxiliary and produces a large amount of nonrecyclable byproduct, terpenol (Ter^*OH).Further, isolation of the pure homoallylic alcohol by distillation can be difficult if it boils in the range of the abundant byproduct.Therefore, in order to recycle the chiral auxiliaries and isolate the product homoallylic alcohols in an efficient manner, we have developed the following procedures: (1) elimination workup, in which enantiomerically pure α-pinene and Δ²- and Δ³-carenes are liberated from terpenylborinates 4 by treatment with isobutyraldehyde and 1 mol percent BF3*OEt2; (2) ethanolamine workup involving treatment of 4 with ethanolamine (EA) to achieve the precipitation of the ethanolamine adducts (EA-BTer2^*, Ter^* = Ipc and 2-Icr, 11 and 12) from which the Ter2^*BOMe can be easily regenerated; and (3) 8-hydroxyquinoline workup, involving treatment of 4 with 8-hydroxyquinoline (8-HQ) to precipitate the 8-HQ adducts (8-HQ-BTer2^*, Ter^* = Ipc, 4-Icr and 2-Icr, 13-15), from which the various Ter2^*BOMe intermediates can be conveniently liberated.It is hoped that these procedures will significantly enhance the scope of asymmetric allyl-/crotylboration of aldehydes with Ter2^*BAll (1), Ter2^*BCrt^Z (2), and Ter2^*BCrt^E (3) and serve as excellent alternatives for any catalytic versions yet to be discovered.