A new access to spiro-isozazolines derivatives

Total Synthesis of Psammaplysins A, M, O, and Q and Ceratinamide A

The psammaplysins are a unique class of brominated marine alkaloids bearing a signature 5/7-spiroisoxazoline-oxepine core linked to a variable tyramine-derived unit. Here, we report the total synthesis of several members of this family via a dipolar cycloaddition between an in situ generated nitrile oxide and an unusual seven-membered enediol diether dipolarophile. Carefully orchestrated oxidative transformation toward the fully functionalized spirocycle and direct coupling with tyramine-derived amines provides access to five representative family members, psammaplysins A, M, O, and Q and ceratinamide A, the latter four for the first time. Additionally, kinetic resolution of a late-stage intermediate enables the first asymmetric synthesis of (-)-psammaplysin A, thereby confirming its absolute configuration.

Rapid Assembly of Spironaphthalenones by Dearomative Spiroannulation of Naphthols and Dielectrophiles

A novel [4 + 1] and [5 + 1] dearomative spiroannulation has been developed by the use of commercial naphthols and phenols with dielectrophiles. Various spirocycles, including spiro[4.5] and spiro[5.5] have been constructed successfully by employing four-atom or five-atom dielectrophilic synthons. This transformation was realized through a sequence of site-selective C-alkylation/dearomative spiroannulation. Moreover, the potential application of this method was exemplified by several further transformation.

2-Methyl-3′-(4-methylphenyl)-4′-(2-nitrophenyl)-4′H-spiro[chroman-3,5′-isoxazol]-4-one

The title compound, C25H20N2O5, crystallizes with two molecules (A and B) in the asymmetric unit with similar conformations. The five-membered rings are both in envelope conformations with the spiro C atom as the flap. The six-membered heterocycles display half-chair conformations. The mean plane through the isoxazole ring is nearly perpendicular to those through the spiro-chroman system and the 4-nitrophenyl moieties, as indicated by the dihedral angles of 81.42 (9) and 87.58 (8)°, respectively, between them in molecule A. Equivalent data for molecule B are 75.58 (9) and 84.15 (8)°, respectively. The p-tolyl plane makes a dihedral angles of 24.10 (9) and 28.78 (8)° with the isoxazole ring in molecules A and B, respectively. In the crystal, molecules are linked by C—H...O and C—H...N hydrogen bonds and C—H...π interaction, forming a three-dimensional network.

Synthesis and Investigation of Novel Spiro-isoxazolines as Anti-Cancer Agents

A series of structurally diverse 4-bromo spiro-isoxazolines possessing a variety of aromatic and aliphatic substituents at the 3 position, were synthesized through a 1,3-dipolar cycloaddition followed by intramolecular cyclization of a pendant hydroxyl or carboxylic acid group. The biochemical antiproliferative activity was evaluated in vitro by using two breast cancer cell lines (MCF-7 and MDA-MB-231) and two prostate cancer cell lines (PC-3 and DU-145) using the MTT viability assay, and the IC50 values were obtained. Spiro-isoxazoline derivatives bearing a p-chloro or an o-dichloro aromatic substituent at the 3-position of the isoxazoline showed considerable antitumor activities in all four cell lines with IC50 value ranging from 43μM to 56μM.

Synthesis and Tautomerism of Spiro-Pyrazolines

An experimental study on the synthesis, tautomerism and acid promoted structural changes of spiro-pyrazolines is described. The target was achieved through a [3+2]-dipolar cycloaddition of an alkene with nitrile imines generated in situ and was isolated in high yield. The synthesized cycloadduct displayed a tendency to exhibit an imine-enamine type of tautomerism as evidenced by X-ray crystal and NMR studies. Furthermore, addition of an acid resulted in the transformation of an imine tautomer to an enamine. The current report constitutes a first formal observation of this kind of tautomerism observed in spiro-indoline pyrazolines.

Oxonium Ion Mediated Synthesis of 4-Substituted Spiro-Isoxazolines

The stereoselective synthesis of 4-bromo-spiro-isoxazolines was achieved in one step through the bromination of various isoxazoles that contain a pendant alcohol or carboxylic acid functional group. Isoxazole bromination leads to a bromonium ion intermediate which opens either by neighboring oxygen lone pair electrons or by intramolecular nucleophilic attack. Single X-ray crystal data provides evidence that the two contiguous stereocenters of the spiro-isoxazoline are formed by the anti intramolecular attack of the nucleophile relative to bromine since there is an anti stereochemical relationship between the spirocyclic ring oxygen and the bromine atom.

4'-Methyl-3-(4-nitro-phen-yl)-4-phenyl-4,5,1',2',3',4'-hexa-hydro-spiro-[isoxazole-5,2'-naphthalen]-1'-one

The title compound, C₂₅H₂₀N₂O₄, is a new spiro-isoxazoline derivative. It contains a five-membered isoxazoline ring (A), a tetra­lone unit (E and D), a 4-nitro­phenyl substituent (B), and a phenyl ring (C). The isoxazoline ring (A) has an envelope conformation, while the cyclo­hexenone ring (D) has an inter­mediate sofa/half-chair conformation. The aromatic ring of the 4-nitro­phenyl substituent (B) is inclined at an angle of 78.97 (10)° to the phenyl ring (C). The rigid pharmacophore site, Osp²—C—C—Osp³, is characterized by an O⋯O distance of 3.113 (2) Å and an O—C—C—O torsion angle of 97.8 (2)°. In the crystal structure, mol­ecules are linked by C—H⋯O contacts.

3,4-Bis(4-nitro-phen-yl)-1,2,5-oxadiazole 2-oxide

The title compound, C₁₄H₈N₄O₆, a new 1,2,5-oxadiazole N-oxide derivative, was formed by dimerization of 4-nitro­benz­al­de­hyde oxime. The compound crystallizes with two independent mol­ecules per asymmetric unit. The N-oxide O atom is disordered over two sites in each mol­ecule; site occupancy factors are 0.57/0.43 and 0.5/0.5. The mean planes through the two benzene rings are inclined to the planar 1,2,3-oxadiazole ring by 25.03 (11) and 41.64 (11)° in one mol­ecule, and 22.58 (11) and 42.66 (11)° in the other mol­ecule, the smaller angle being for the ring on the oxide side of the oxadiazole ring in each case. In the crystal structure, the individual mol­ecules form centrosymmetric dimers linked via C—H⋯O hydrogen bonds. The dimers of one mol­ecule are then linked to those of the other mol­ecule via C—H⋯O hydrogen bonds, forming a three-dimensional network.

Stereoselective synthesis of unnatural spiroisoxazolinoproline-based amino acids and derivatives

A route to spiroisoxazolinoproline-based amino acid derivatives is reported in which exo-methyleneprolinate 4 (tert-butyl ester) reacts as a dipolarophile with nitrile oxides to generate spiroisoxazolinoprolinates 7/10/11 in good yields (70-75%) and with ca. 1:4 cis:trans diastereoselectivity. tert-Butyl spiroisoxazolinoprolinates were separable by column chromatography and amenable to scale-up leading to single diastereoisomers of N-Boc and N-Fmoc protected spiroisoxazolinoproline amino acids.