Introduction Of Ring Research Articles

Stereoselective Synthesis of the I-L Fragment of the Pacific Ciguatoxins.

The I–L ring system found in all the Pacific ciguatoxins has been prepared from a tricyclic precursor in a highly stereoselective manner. Subtle differences in the reactivity of the enones present in the seven- and eight-membered rings of the tricyclic ether starting material have been exploited to allow selective protection of the enone in the eight-membered ring. Subsequent distereoselective allylation of the seven-membered ring has been accomplished by a palladium-mediated Tsuji-Trost reaction. The K-ring methyl and hydroxyl groups have been installed in a highly stereoselective manner by sequential conjugate reduction and enolate oxidation reactions. Ring L has been constructed by a use of a novel relay ring-closing metathesis reaction to complete the tetracyclic framework, which possesses the functionality necessary for elaboration of rings I and L and the introduction of ring M.

A novel approach to ring A analogues of the marine pyridoacridine alkaloid ascididemin

A novel approach to ring A analogues of the marine pyridoacridine alkaloid ascididemin starting from readily available 4-bromobenzo[c][2,7]naphthyridine (10) comprises a high-yield Minisci-type homolytic methoxycarbonylation at C-5, followed by introduction of the ring A scaffold via Suzuki cross-coupling reaction, and a trifluoromethanesulfonic acid-aided Friedel–Crafts-type intramolecular acylation. This protocol allows for the introduction of various electron-rich carbocyclic and heterocyclic ring A substitutes.

Structural studies of sparsely substituted synthetic chlorins and phorbines establish benchmarks for changes in the ligand core and framework of chlorophyll macrocycles

Understanding the changes in molecular structure of tetrapyrrole macrocycles upon derivatization of the organic framework is essential for diverse studies ranging from metal complexation to formation of supramolecular assemblies. New, sparsely substituted free base chlorin, 17-oxochlorin, phorbine and 13 1-oxophorbine macrocycles provide benchmarks for naturally occurring hydroporphyrins and have been examined here by X-ray crystallography, resonance Raman spectroscopy, and density functional theoretical (DFT) calculations. The macrocycles contain no substituents other than a geminal-dimethyl group attached to the reduced, pyrroline ring. The X-ray studies indicate that the benchmark compounds exhibit only slight distortion from planarity, which increases along the series porphine < chlorin < oxochlorin < phorbine < oxophorbine. The elongated C β C β bond distance due to sp 3 versus sp 2 hybridization in the pyrroline ring (ring D) of the (oxo)chlorins and (oxo)phorbines (1.52–1.54 Å) versus that of porphine (1.35 Å) is accompanied by altered bond angles in ring D. Introduction of ring E (exocyclic ring) in a chlorin to give the phorbine or oxophorbine causes alteration of the bond angles at many sites in the framework of the macrocycle; for example, the bond angles of N3 C14 C15 in the (oxo)phorbine are widened by ∼11° compared to those of porphine or the analogous sparsely substituted chlorin. As a result, the shape of the macrocycle core changes along the series of porphine (nearly square), (oxo)chlorin (kite-shaped), and (oxo)phorbine (right-angled trapezoid), and the core size increases in the order porphine < phorbine ∼ oxophorbine < oxochlorin ∼ chlorin. Comparison of the bond distances and angles in ring E of the phorbine versus oxophorbine indicates that the shortening of the C13 C13 1 bond owing to the presence of the oxo-group is quite small, only 0.024 Å; thus, the unsymmetrical structure of ring E does not appear to be due to conjugation with the C13 1 O group but may be a characteristic feature of the (oxo)phorbine framework. The X-ray data further indicate that the lengths of the oxochlorin C17 O and oxophorbine C13 1 O groups are essentially identical, a result also predicted by DFT calculations. Regardless, the observed frequencies for the stretching vibrations of the C17 O (1721 cm −1) and C13 1 O (1701 cm −1) groups are different and suggest that conjugation of the latter group with the π-system of the macrocycle is greater than that of the former group. Collectively, the studies provide new insights into the individual factors that give rise to the overall structural characteristics of various macrocycles.

ChemInform Abstract: INVESTIGATIONS ON STEROLS. 40. THE SYNTHESIS OF RING D SUBSTITUTED 9β,10α‐STEROIDS

AbstractDas Pregnen (I) wird zum 5,7‐Dien (II) umgesetzt, das beim Bestrahlen das 9ß,l0oz‐konfigurierte Dien (III) liefert.

Investigations on sterols. 40. The synthesis of ring D substituted 9β,10α‐steroids

AbstractThe retro configuration (9β,10α) was introduced into pregnane derivatives containing substituents at C(16) and/or C(17) by irradiation of a suitable 5,7‐diene. In this way the title compounds became much more accessible than by the introduction of ring D substituents into 20‐ketoretro‐steroids.

Stable Curved Theta-Pinch Plasma in a Series of Caulked Cusp Fields

Curved theta-pinch experiments show that a toroidal drift of plasma is suppressed by a series of caulked cusp fields, produced with the aid of metal rings located inside the discharge tube. The introduction of rings is effective in the suppression of toroidal drift of the initial high-beta plasma, as well as in the stabilization of low-beta plasma in the later stage.