Abstract
A library of pinane-based chiral aminodiols, derived from natural (−)-β-pinene, were prepared and applied as chiral catalysts in the addition of diethylzinc to aldehydes. (−)-β-Pinene was reacted to provide 3-methylenenopinone, followed by a reduction of the carbonyl function to give a key allylic alcohol intermediate. Stereoselective epoxidation of the latter and subsequent ring opening of the resulting oxirane with primary and secondary amines afforded aminodiols. The regioselectivity of the ring closure of the N-substituted secondary aminodiols with formaldehyde was examined and exclusive formation of oxazolidines was observed. Treatment of the allylic alcohol with benzyl bromide provided the corresponding O-benzyl derivative, which was transformed into O-benzyl aminodiols by aminolysis. Ring closure of the N-isopropyl aminodiol derivative with formaldehyde resulted in spirooxazolidine. The obtained potential catalysts were applied in the reaction of both aromatic and aliphatic aldehydes to diethylzinc providing moderate to good enantioselectivities (up to 87% ee). Through the use of molecular modeling at an ab initio level, this phenomenon was interpreted in terms of competing reaction pathways. Molecular modeling at the RHF/LANL2DZ level of theory was successfully applied for interpretation of the stereochemical outcome of the reactions leading to display excellent (R) enantioselectivity in the examined transformation.
Highlights
Chiral synthons, applied successfully in asymmetric homogenous and heterogeneous catalysis, have achieved increasing importance in organic chemistry in recent years [1,2,3]
The enantioselective addition of dialkylzinc to aldehydes catalyzed by different types of chiral ligands has been investigated intensively [4,5,6], because the preparation of enantiomerically pure or enriched alcohols is of considerable interest for the synthesis of bioactive compounds [7,8,9] and natural products [10,11]
Starting from natural ( )-β-pinene, a monoterpene-based 3-amino-1,2-diol library has been created via the epoxide ring opening of epoxyalcohol as key intermediate, whereas the reactions of
Summary
Chiral synthons, applied successfully in asymmetric homogenous and heterogeneous catalysis, have achieved increasing importance in organic chemistry in recent years [1,2,3]. Aromatic and aliphatic aminodiols bearing a 1,2- or a 1,3-aminoalcohol moiety have proven to be highly efficient building blocks [12,13,14,15] They have been applied as starting materials in the stereoselective synthesis of compounds of pharmacological interest, including 1,3-oxazines [16], 1,3-thiazines [17,18,19] and 2-iminothiazolidines [20]. In addition to their synthetic importance, aminodiols can be applied as chiral ligands and auxiliaries in enantioselective transformations [21,22,23,24] including intramolecular radical cyclizations [25], intramolecular [2+2] photocycloaddition [26] and Grignard addition [27,28]. Abbott-aminodiol bears the core the display remarkable pharmacological activities.
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