Intramolecular Diels-Alder reactions of optically active allenic ketones: chirality transfer in the preparation of substituted oxa-bridged octalones

The intramolecular Diels-Alder reaction of allenic ketones containing a furyl unit (IMDAF) to generate oxatricyclic systems in good yields is described. The alkene dienophiles 1ab give poor yields of the cycloadducts 2ab, presumably due to the facile retro Diels-Alder reaction. However, the analogous allenic dienophile 7 afforded the desired cycloadduct 8 in 91% yield on treatment with dimethylaluminum chloride. When the allene bears an alkyl substituent on the terminal carbon, complete diastereoselectivity is seen in the IMDAF, e.g. cyclization of 14 gave only the cycloadduct 15 in 80% yield presumably due to greater steric hindrance in the transition state II as compared to that in I. Finally we report complete chirality transfer of the stereochemistry of an allene to the carbon framework of the oxatricyclic system. Thus, the optically active allenic ketone 20 afforded only the desired cycloadduct 21 with the correct absolute stereochemistry needed for the synthesis of the arisugacin class of natural products.     

The Oxidation of Heterogeneous Tl/Ni/P‐Alloys — Preparation and Crystal Structures of the Thallium Nickel Phosphates TlNi4(PO4)3, Tl4Ni7(PO4)6, and Tl2Ni4(P2O7)(PO4)2

Single crystals of the first anhydrous thallium nickel phosphates were prepared by reaction of heterogeneous Tl/Ni/P alloys with oxygen. TlNi₄(PO₄)₃ (pale‐yellow, orthorhombic, space group Cmc2₁, a = 6.441(2)Å, b = 16.410(4)Å, c = 9.624(2)Å, Z = 4) crystallizes with a structure closely related to that of NaNi₄(PO₄)₃. Tl₄Ni₇(PO₄)₆ (yellow‐brown, monoclinic, space group Cm, a = 10.711(1)Å, b = 14.275(2)Å, c = 6.688(2)Å, β = 103.50(2)°, Z = 8) is isotypic with Na₄Ni₇(PO₄)₆, and Tl₂Ni₄(P₂O₇)(PO₄)₂ (brown, monoclinic, space group C2/c, a = 10.389(2)Å, b = 13.888(16)Å, c = 18.198(3)Å, β = 103.1(2)°, Z = 8) adopts the K₂Ni₄(P₂O₇)(PO₄)₂ structure. Tl₂Ni₄(P₂O₇)(PO₄)₂ could also be prepared in nearly single phase form by reaction of Tl₂CO₃, NiO, and (NH₄)₂HPO₄.     

Synthesis and Characterization of Push-Pull Porphyrins

A series of “push-pull” porphyrins with 4-nitrophenyl and 4-aminophenyl substituents were synthesized and separated by flash column chromatographic techniques. They were fully characterized by elemental analysis, FAB-MS, FTIR, UV-visible, and ¹H NMR spectroscopies. The unsymmetrical π-electron distribution of the porphyrins caused by the donor (amino) and acceptor (nitro) substituents were investigated by ¹H NMR technique. The pyrrole-H resonance positions can be correlated to the Hammett σ constants of the substituents. Although with strong donor and acceptor substituents, UV-visible spectra show the push-pull porphyrins have rather weak solvatochromism and hence limited intramolecular charge-transfer character.     

A facile synthesis of (6S,1′S)-(+)-hernandulcin and (6S,1′R)-(+)-epihernandulcin

A facile total synthesis of (+)-hernandulcin (1) was accomplished from (−)-isopulegol in 6 steps with 15% overall yield. Epoxidation of (−)-isopulegol with m-chloroperbenzoic acid followed by opening of the epoxide 3a with prenyl Grignard afforded the tertiary alcohol 4a with correct C-6 and C-1′ stereochemistry as a major product. Oxidation of the secondary alcohol in compound 4a to the ketone 5a was accomplished in high yield by using TPAP and N-methylmorpholine N-oxide. Conversion of the ketone 5a to α,β-unsaturated ketone via organoselenium intermediate gave (+)-hernandulcin (1). This method was also successfully applied to the synthesis of (+)-epihernandulcin (2).