Ring Junction Research Articles

6β,8β-Dihydroxyeremophil-7(11)-en-8α,12-olide

The title compound, C15H22O4, an eremophilane sesquiternoid, was isolated from the roots of Ligularia virgaurea. Both six-membered rings (A and B) adopt chair conformations and the five-membered ring is almost planar (r.m.s. deviation = 0.016 Å). The two methyl and two hy­droxy groups adopt a syn conformation and the A/B ring junction is cis-fused. An intra­molecular O—H⋯O hydrogen bond generates an S(6) ring. In the crystal, O—H⋯O hydrogen bonds link the mol­ecules into [100] chains.

Deacetylnomilin monohydrate

In the title compound (systematic name 1-hy­droxy-1,2-dihydro­obacunoic acid 3,4-lactone monohydrate), C26H32O8·H2O, the dihedral angles between the planes of the ester groups and the furan plane are 43.06 (12) and 56.06 (7)°, while that between the furan plane and the keto group is 58.50 (9)°. The A/B, B/C and C/D ring junctions are all trans-fused. Inter­molecular O—H⋯O hydrogen bonds between the hy­droxy and carbonyl groups and the water mol­ecule give rise to a three-dimensional structure.

The Chemistry of B-Norsteroidal 6-Ketones and their Relatives

The preparation and reactions of B-norsteroidal 6-ketones and their relatives are reviewed in the context of the stability of the A/B and B/C ring junctions and the stereochemistry of the addition reactions to the carbonyl group. This review is a sequel to a previous article on the reactions of B-norsteroidal 4- and 5-enes.

Enantioselective synthesis of chelidonine, a B/C- cis-11-hydroxyhexahydrobenzo[ c]phenanthridine alkaloid

Both enantiomers of chelidonine, a B/C- cis-11-hydroxyhexahydrobenzo[ c]phenanthridine alkaloid, were synthesized by manipulation of the B/C-dehydro ring juncture of benzo[ c]phenanthridine skeleton using Sharpless asymmetric dihydroxylation and stereospecific catalytic hydrogenation after introduction of oxygen functions on the C ring as key reaction steps for the construction of stereogenic centers.

Concise Synthesis of 1,4a-Bifunctionalised Decalin Building Blocks by C-H Activation of Decalin

Friedel-Crafts acylation of decalin introduces functionality at C1 and C4a, which may be elaborated to novel building blocks possessing a methyl ketone, methyl carbinol, or vinyl group at the ring junction.

Magnetic Field Control of the Current through Molecular Ring Junctions

Whereas conducting loops are, in principle, sensitive to external magnetic field, as is pronouncedly exemplified by the Aharonov–Bohm (AB) effect, the small radius of molecular rings makes the observation of such effects challenging. Indeed, the unrealistically large magnetic field needed to realize the AB effect in molecular rings has led to a widespread belief that molecular conduction is insensitive to laboratory realizable fields. Here we revisit this issue, presenting conditions under which magnetic field control of molecular ring conduction is realizable with pronounced effects on the I–V characteristics. We find these conditions to be (a) weak molecule-lead coupling, implying relatively distinct conduction resonances, (b) asymmetric junction structure (e.g., meta- or ortho-connected benzene rather than a para structure), and (c) minimal dephasing (implying low temperature) so as to maintain coherence between multiple pathways of conduction.

New analogs of steroid estrogens

Aiming at the investigation of the mechanism of functioning of steroid estrogens a series of compounds with unnatural rings junction was synthesized. All investigated compounds exhibit a reduced uterotropic activity. It was established applying the NMR spectroscopy that 7α-methyl-3-methoxy-D-homo-6-oxa-8α,14β-estra-1,3,5(10)-trien-17a-one existed in solution in two conformations distinguished by the structure of the rings B, C, and D simultaneously. The reaction of 17-methylidene-3-methoxy-6-oxa-8α-estra-1,3,5(10)-triene with hydrobromic acid in acetic acid promotes a rearrangement with the migration of a methyl group into the position 17 resulting in the formation of 17,17-dimethyl-6-oxa-8α-gona-1,3,5(10),13(14)-tetraene derivatives.

Synthesis of (+)-Lysergic Acid, (+)-Lysergol, and (+)-Isolysergol

Lysergic acid, lysergol, and isolysergol are indole-based alkaloids that are secondary metabolites produced by filamentous fungi. These ergot alkaloids (and synthetic analogues) have been reported to exhibit broad biological ­activity. Despite considerable interest from the synthetic community only three asymmetric syntheses have been reported, with many racemic syntheses due to the challenge of setting the ­stereocenter at the ring junction.

Rapid Assembly of Quinolizidines via Consecutive Nucleophilic Cyclizations onto Activated Amides

A new approach to the synthesis of quinolizidines involving a cascade of nucleophilic cyclizations triggered by chemoselective amide activation is reported. Particular attention was given to the effect of the nature of the tethered nucleophiles on the cascade of cyclizations. As a result, simple acyclic amides gave rapid access to functionalized quinolizidines bearing either a tertiary or quaternary center at the ring junction. Such a fused bicyclic motif is found in several alkaloids.

ChemInform Abstract: Tandem Phospha‐Friedel—Crafts Reaction Toward Curved π‐Conjugated Frameworks with a Phosphorus Ring Junction.

Abstractm‐Teraryl iodides are subjected to lithium—halogen exchange with subsequent trapping with PCl3 and tandem phospha‐Friedel—Crafts reaction to afford the heptacyclic phosphine sulfudes (II).

3,6-Didehydro-5-hydroxy-1,2-O-isopropylidene-5-C-nitromethyl-α-D-glucofuranose

The title compound, C10H15NO7, consists of one methyl­enedi­oxy ring and two fused tetra­hydro­furan rings. The three fused rings exhibit cis arrangements at the ring junctions. One O atom of a tetra­hydro­furan ring and the H atoms bound to the neighboring C atoms are disordered over two orientations with site-occupancy factors of 0.69 (1) and 0.31 (1). intra­molecular O—H⋯O and C—H⋯O inter­actions stabilize the mol­ecular conformation. In the crystal structure, inter­molecular O—H⋯O and C—H⋯O inter­actions link the mol­ecules into a three-dimensional network.

Gold Catalysis: Domino Reaction of En‐Diynes to Highly Substituted Phenols

By Sonogashira coupling of 1,7-heptadiynes and 1,8-octadiynes with 2-iodoallyl alcohols, various substrates that bear a 2-alkynylallyl alcohol moiety tethered to an additional alkyne were prepared in one step. Subjection to nitrogen acyclic carbene (NAC)/gold(I) catalysts delivered highly substituted phenols in an efficient domino reaction. Furan derivatives were formed as intermediates; this was proven by in situ NMR spectroscopy. The uncommon substitution pattern of these furans opens the way for a selective formation of phenols that contain the hydroxyl group in the meta position to the ring junction, which previously was not possible by gold-catalyzed furan-yne cyclization. Furthermore, interesting mechanistic insights were obtained by products derived from secondary allyl alcohols. In this case, in addition to the phenolic compounds, a ketone is formed by 1,2-alkyl shift.

Further expansion of the trans-Diels–Alder paradigm: reductive alkylation of α-cyanoketones

Reductive alkylation of Diels–Alder-derived ring junction α-cyanoketones provides a route to trans-fused bicyclic systems.

N-Benzyl-6-deoxy-3,6-iminomethylene-1,2,3,5-O-tetraacetyl-α-D-1(S)-epiallofuranose

The mol­ecule of the title compound, C22H27NO9, an azasugar derivative, consists of one benzene ring and two fused rings, which have the cis arrangement at the ring junctions, and gives a V-shaped geometry. The inter­planar angle between the five- and six-membered rings is 65.69 (11)°. The crystal structure is stablized by weak inter­molecular C—H⋯O hydrogen bonds.

Norrish–Prins reaction as a key step in the synthesis of 14β-hydroxy-5α (or 5β or Δ 5,6)-pregnane derivatives

Numerous bioactive glycosteroids are characterized by aglycones bearing a 14β-hydroxy pregnane skeleton like boucerin and isoramanone. In general, the syntheses of the latter are achieved by acidic hydrolysis of the corresponding glycosteroids. These aglycones were also obtained by a combined Norrish type I–Prins reaction starting from the corresponding 12-keto-pregnane derivatives. However, for the Norrish–Prins reaction, no reports describe the influence of the A/B ring junction (cis or trans or Δ 5,6 double bond) or the influence of the substitution pattern at position 20. Herein, we describe the use of Norrish type I–Prins reactions to synthesize isoramanone and boucerin derivatives and their A/B cis and trans analogs. The influence of the parameters mentioned above is also presented. These studies showed that the A/B ring junction has little influence on the Norrish type I–Prins reaction but that the substitution pattern at position 20 is important. The presence of a dioxolane group induced not only the formation of the desired 14β-hydroxy pregnane derivatives in the highest yields but also the formation of new spiro derivatives.

Separation and Characterization of C70(C14H10) and C70(C5H6) from an Acetylene–Benzene–Oxygen Flame

Although derivatives of fullerenes are prevalent in the fullerene-producing flame, the chemistry of these derivatives has rarely been discussed in the previous literature. In this paper, two D5h-C70 derivatives, C70(C14H10) and C70(C5H6), were isolated from the soot of an acetylene–benzene combustion. On the basis of detailed MS, NMR, IR, and UV/vis analyses in combination with DFT calculations, the cycloadduct structures of C70(C14H10) and C70(C5H6) were identified. Both the anthracene (C14H10) and the cyclopentadiene (C5H6) adducts, supposed as the intermediate species produced during the combustion process, were characterized to bond at a [6,6] ring junction at the end of the olivary C70 cage. The present work exemplifies the capture of possible intermediates by the C70 fullerene from the flame and thus provides insight into the mechanism responsible for the formation of fullerene-containing soot.

Stereodivergent Synthesis of Two Novel α‐Aminophosphonic Acids Characterised by a cis‐Fused Octahydroindole System

AbstractThe synthesis of two new α‐aminophosphonic acids, namely (2S*,3aS*,7aS*)‐ and (2R*,3aS*,7aS*)‐octahydroindole‐2‐phosphonic acids, is described. They are analogues of phosphoproline with a cyclohexane ring fused to the pyrrolidine moiety. The ring junction has cis stereochemistry in both cases, but the two compounds differ in the relative orientation of the cyclohexane and phosphonate moieties. The two amino acids were prepared from a common starting material following stereodivergent routes that provide the desired stereoisomer with complete stereocontrol. The relative configurations of the compounds synthesised were confirmed by X‐ray diffraction analysis.

Rh(I)-catalyzed intramolecular [2 + 2 + 1] cycloaddition of allenenes: Construction of bicyclo[4.3.0]nonenones with an angular methyl group and tricyclo[6.4.0.01,5]dodecenone

The [RhCl(CO)dppp]2-catalyzed intramolecular carbonylative [2 + 2 + 1] cycloaddition of allenenes was developed to prepare bicyclo[4.3.0]nonenones possessing a methyl group at the ring junction, which is difficult to achieve by the Pauson–Khand reaction of the corresponding enynes. This method also provided a new procedure for the construction of the tricyclo[6.4.0.01,5]dodecenone framework in a satisfactory yield.

Tandem Phospha-Friedel−Crafts Reaction toward Curved π-Conjugated Frameworks with a Phosphorus Ring Junction

The tandem phospha-Friedel-Crafts reaction transforms dichloro(m-teraryl)phosphine to the corresponding triarylphosphine derivatives containing curved π-conjugated frameworks with a phosphorus ring junction. The rigid molecular frameworks enable these unprecedented phosphine compounds to hold an extended π-conjugation spread over the whole molecule.

ChemInform Abstract: A Novel Oxidative Cyclization onto Vinyl Silanes.

AbstractA variety of tetrahydrofurans bearing a silyl group at the ring junction is prepared in good yields stereoselectively via a osmium‐catalyzed oxidative cyclization of appropriate terminal vinyl silanes.