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Tetrahedron: Asymmetry

Volume 17, Issue 3, 6 February 2006, Pages 434-448
Tetrahedron: Asymmetry

Enantiomerically pure α-pinene derivatives from material of 65% enantiomeric purity. Part 2: C2-symmetric N,N′-3-(2α-hydroxy)pinane diimines and diamines☆

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Abstract

The conversion of enantiomerically enriched (ee >99%) 2α-hydroxypinan-3-one and its oxime (obtained in previously described procedures∗ from α-pinene of 65% ee) into a range of derivatives with potential application in asymmetric synthesis was attempted. C2-Symmetrical compounds, ligands for potential catalysts, were synthesized from 2α-hydroxypinan-3-one and either aliphatic or aromatic diamines. Reduction or etherification/reduction of selected diiminodiols afforded respective diaminodiols and diaminoethers, which were further transformed into azolium salts. Reactions of dipinanediaminoethers with dichlorophenylphosphine and subsequent in situ oxidation of the products afford the respective stable phosphinediamide oxides. Four selected compounds were crystallographically studied.

Introduction

The synthesis of enantiomerically pure ligands for potential catalysts of numerous reactions aimed at the generation of new chiral centers is one of the key challenges of today’s organic chemistry. Extensive literature is currently available with regards to chiral ligands and catalysts, including numerous reviews (e.g., an issue of Chemical Reviews was devoted exclusively to this topic).1

Pinenes are a handy subset of the chiral pool and pinene derivatives are common building blocks for a broad range of asymmetric syntheses. In recent years, pinene derivatives have been used for the synthesis of enantiomeric amino acids containing cyclobutane rings,2 pheromones3 and catalysts ligands (Fig. 1).4

α-Pinenes belong to a very small group of compounds naturally occurring in both enantiomeric forms. However, the research applicability of these compounds is limited by the high cost of the material with sufficient enantiomeric excess. Therefore, of special importance are those derivatives of α-pinene, which can be enantiomerically enriched to obtain pure chemical entities.

In 2002, we published a convenient method for enantiomeric enrichment of 2α-hydroxypinan-3-one and its oxime obtained from α-pinene of 65% ee. We have also documented significant differences of crystalline networks of enantiomerically pure oxime and the racemic compound.5 The synthesis of a C2-symmetric ligand, di[3α-(2α-hydroxy)pinane]amine (Fig. 2), is described in the same paper.

In addition to the number of straightforward transformations of the oxime and the ketol, we wanted to present a series of hydroxypinanone-derived diiminodiols (Fig. 3a) and their derivatives (Fig. 3b), comprising a group of C2-symmetric ligands for potential catalysts.

The growing interest in pinenes as a source of chirality can be illustrated by the following examples: in 2001, Hiroi and Watanabe6 obtained terpenic imine-phosphine ligands. One of them, based on 2α-hydroxypinan-3-one (Fig. 4a), allowed us to obtain a 44% enantiomeric excess of the product from palladium-catalyzed cycloaddition of acryloyl amide and cyclopentadiene. Recently, Basavaiah et al. have obtained a pinane-based compound with an N–Pdouble bondO structural framework (Fig. 4b) and successfully used this compound in borane-mediated asymmetric reduction of prochiral ketones.7

Section snippets

Results and discussion

The fact that 2α-hydroxypinan-3-one, some of its imines and particularly its oxime, can be enantiomerically enriched to obtain practically pure enantiomers has influenced our attempt to use this compound in the synthesis of building blocks and ligands for potential catalysts. We observed that the oxime of 2α-hydroxypinan-3one 2, when reacted with tosyl chloride in pyridine, forms ketonitrile 3 in close-to-quantitative yield (96.5% as calculated for the distilled, solidifying product) and with

Conclusion

The possibility of facile enantiomeric enrichment of 2α-hydroxypinan-3-one and its oxime, thus allowing us to obtain enantiomerically pure materials from inexpensive α-pinenes, makes it possible to use these materials for the synthesis of useful building blocks, such as cis-1-acetyl-3-cyanomethyl-2-2-dimethylcyclobutane or pinocarvone.

The formation of imines in the reactions of two molecules of ketol with aliphatic or aromatic amines led to a series of C2-symmetric products, characterized by an

General

Boiling points are uncorrected. Melting points were determined with a Büchi apparatus in open capillaries and are uncorrected. Elemental analyses were performed at CBMiM PAN (Łódź). Optical rotation was measured with a Horiba SEPA-200 apparatus in a 10 cm cell. 1H, 13C, and 31P NMR spectra were recorded at 250 or 300, 62.6 MHz, respectively, on Bruker Avance DPX-250 or Varian-300 apparatus in CDCl3 (TMS). The values of the coupling constants are reported in hertz. Mass spectra were determined

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Part 1: Markowicz, S. W.; Pokrzeptowicz, K.; Karolak-Wojciechowska, J.; Czylkowski, R.; Omelańczuk, J.; Sobczak, A. Tetrahedron: Asymmetry 2002, 13, 1981–1991. Preliminary communications; Figlus, M.; Markowicz, S. W. PTChem, Kraków, 2002, S16, p 1096 and Markowicz, S. W.; Figlus, M.; Lejkowski, M.; Karolak-Wojciechowska, J.; Verpoort, F. PTChem, Lublin, 2003, S1, p 91, Annals of the PTChem, Vol. 2, PI, pp 52–56.

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