Catalytic Asymmetric Hetero Diels–Alder Reactions of N‐Sulfinyl Dienophiles with Chiral Bis(oxazoline)copper(II) and ‐zinc(II) Triflates

Abstract

Asymmetric hetero Diels–Alder (HDA) reactions of N‐sulfinyldienophiles with bis(oxazoline)copper(II) and ‐zinc(II) triflatesare described. The cycloaddition with cyclic and acyclic 1,3‐dienes has been studied. The copper catalyst was found to be more efficient. With 10 mol‐% of catalyst loading, a pronounced enhancement in turnover, diastereoselectivity (54–>90 % de), and enantioselectivity (30–98 % ee) was obtained by the addition of 100 mol‐% of TMSOTf. The role of the additive is unclear, but we hypothesize that it is involved in the breakdown of catalyst‐sulfine aggregates assists in the release of catalyst from the HDA adducts. Mechanistic studies revealed a positive nonlinear effect with the zinc catalyst, assumed to arise from the formation of less reactive heterochiral complexes. The copper catalyst showed a nearly linear relationship between the enantioselectivity of the ligand tested and the HDA product formed. The relative configurations and, in one case, the absolute configuration of the HDA products were established by X‐ray analysis. Finally, an enantioselective route to (3aR,6aS)‐3,3a,4,6a‐tetrahydrocyclopenta[d][1,3]oxazol‐2‐one, a precursor of the C‐ring in agelastatin A, applying the asymmetric HDA reaction is described. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis of enantiopure 1,2-azido and 1,2-amino alcohols via regio- and stereoselective ring-opening of enantiopure epoxides by sodium azide in hot water

“…[α] D 22 = − 50 ( c 0.09, CHCl 3 ); Lit. 40 [α] D 20 = + 26 ( c 1.0, EtOH) for the ( S )-configuration. 1 H NMR (400 MHz, CDCl 3 ): δ 1.99 (s, 3H), 2.34 (s, 3H), 3.54 (dd, J = 8.4, 10.8 Hz, 1H), 3.73 (dd, J = 4.0, 10.8 Hz, 1H), 4.01 (dd, J = 4.4, 8.0 Hz, 1H), 7.16 (d, J = 8.0 Hz, 2H), 7.21 (d, J = 8.4 Hz, 2H); 13 C NMR (100 MHz, CDCl 3 ): δ 21.1, 57.0, 68.0, 126.3, 129.3, 137.2, 139.8.…”

Practical Synthetic Procedures for the Iron-Catalyzed Intermolecular Olefin Aminohydroxylation Using Functionalized Hydroxylamines

“…By following the general procedure under the conditions described in Scheme 7, 9 was isolated through a silica gel flash column (hexanes/EtOAc: from 4:1 to 1:1) as a white solid, which is a known compound; 14 yield: 30.5 mg (61%); mp 118–121 °C.…”

Update 1 of: C2-Symmetric Chiral Bis(oxazoline) Ligands in Asymmetric Catalysis

“…If the catalyzed reactions are performed under stoichiometric conditions, then excellent results in terms of yields and selectivity are obtained. However, when performed under catalytic conditions using 10 mol % of catalyst, then it is less selective, but with 1 equiv of trimethylsilyl trifluoromethanesulfonate (TMSOTf), it becomes strongly endoselective affording 347 with good to excellent enantiomeric excess (see Scheme and Table ). − The reaction is applicable to unusual N-substituents, and excellent enantioselectivities are obtained for both zinc(II) and copper(II) triflates …”

“…374 When the same N-sulfinyl amides 346 and acyclic dienes 348 are reacted with the Cu(OTf) 2 complex of box 1 as the catalyst in the presence of 1 equiv of TMSOTf, highly diastereoselective hetero DielsÀAlder reactions with cis-349 as the major isomer (Scheme 132) are the result. 373 To give an idea of the stereoselectivity of this reaction and the absolute configuration of the products, some selected examples are reported in Table 81.…”