Enantioselective Reduction of Prochiral Ketones using Spiroborate Esters as Catalysts

Practical Enantioselective Reduction of Ketones Using Oxazaborolidine Catalysts Generated In Situ from Chiral Lactam Alcohols

Oxazaborolidine catalyst (CBS catalyst) has been extensively used for catalytic borane reduction with a predictable absolute stereochemistry and high enantioselectivity. However, the use of isolated CBS catalyst sometimes has the drawback of low reproducibility due to the aging of the CBS catalyst during storage. Therefore, we investigated a more reliable and practical method for the reduction of a variety of ketones including challenging substrates, primary aliphatic ketones, α,β-enones, and trifluoromethyl ketones. This review surveys the developments in borane reduction using oxazaborolidine catalysts generated in situ from chiral lactam alcohols and borane.

Chiral epoxides via borane reduction of 2-haloketones catalyzed by spiroborate ester: application to the synthesis of optically pure 1,2-hydroxy ethers and 1,2-azido alcohols

An enantioselective borane-mediated reduction of a variety of 2-haloketones with 10% spiroaminoborate ester 1 as catalyst is described. By a simple basic workup of 2-halohydrins, optically active epoxides are obtained in high yield and with excellent enantiopurity (up to 99% ee). Ring-opening of oxiranes with phenoxides or sodium azide is investigated under different reaction conditions affording nonracemic 1,2-hydroxy ethers and 1,2-azido alcohols with excellent enantioselectivity (99% ee) and in good to high chemical yield.

Chiral spiroaminoborate ester as a highly enantioselective and efficient catalyst for the borane reduction of furyl, thiophene, chroman and thiochroman containing ketones

Prochiral heteroaryl ketones containing furan, thiophene, chroman and thiochroman moieties were successfully reduced in the presence of 1 - 10 mol % of spiroaminoborate ester 1 with different borane sources to afford non-racemic alcohols in up to 99% ee. In addition, modest enantioselectivity, around 80% ee, was achieved in the reduction of linear α,β-unsaturated heteroaryl ketones.

A New and Efficient Approach to the Synthesis of Nicotine and Anabasine Analogues

A straightforward and practical approach was established for the synthesis of nicotine and anabasine analogues by the cyclization of mesylated 1-(3-pyridinyl)-1,4, and 1,5-diol derivatives to form the pyrrolidino or piperidino fragments. Nicotine analogue (S)-15 was prepared with good enantioselectivity using the developed azacyclization procedure of nonracemic (R)-1-pyridin-3-yl-butane-1,4-diol, which was obtained by the borane-mediated reduction of ketone 12 in the presence of the spiroborate ester derived from diphenyl prolinol and ethylene glycol.

Spiroborate ester-mediated asymmetric synthesis of beta-hydroxy ethers and its conversion to highly enantiopure beta-amino ethers

Borane-mediated reduction of aryl and alkyl ketones with alpha-aryl- and alpha-pyridyloxy groups affords beta-hydroxy ethers in high enantiomeric purity (up to 99% ee) and in good yield, using as catalyst 10 mol % of spiroborate ester 1 derived from (S)-diphenylprolinol. Representative beta-hydroxy ethers are successfully converted to beta-amino ethers, with minor epimerization, by phthalimide substitution under Mitsunobu's conditions followed by hydrazinolysis to obtain primary amino ethers or by imide reduction with borane to afford beta-2,3-dihydro-1H-isoindol ethers. Nonracemic Mexiletine and nAChR analogues with potential biological activity are also synthesized in excellent yield by mesylation of key beta-hydroxy pyridylethers and substitution with five-, six-, and seven-membered ring heterocyclic amines.

Novel dimethoxy(aminoalkoxy)borate derived from (S)-diphenylprolinol as highly efficient catalyst for the enantioselective boron-mediated reduction of prochiral ketones

The novel dimethoxyl(aminoalkoxy)borate 1 was isolated as a white crystalline dimer joined by H-bonding as evidenced by X-ray analysis, and demonstrated to be a highly effective catalyst for the asymmetric reduction of representative prochiral ketones with borane-DMS. Optically pure alcohols were obtained using only 1 mol% of catalyst 1 in up to 99% ee.

Recent development and improvement for boron hydride-based catalytic asymmetric reduction of unsymmetrical ketones

Catalytic asymmetric reduction of prochiral ketones with hydrides such as boranes and borohydrides is one of the simplest and the most convenient methods for obtaining chiral alcohols. This tutorial review describes the most significant advances recently achieved for enhancing the enantioselectivity and practicability of the reduction using this methodology. The review covers the development of new homogeneous and/or immobilized catalysts for asymmetric reduction using borane or borohydride reagents and practical improvement of the well-known oxazaborolidine (OAB)-catalysed reduction through developing more stable, cost effective and recoverable OABs, scalable and environmental friendly borane sources, and one-pot procedures for the reduction.