12-(4-Chloro­phen­yl)-9,9-dimethyl-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one

The title compound, C25H21ClO2, was synthesized via the three-component coupling of 4-chloro­benzaldehyde, 2-naphthol and 5,5-dimethyl­cyclo­hexane-1,3-dione. The pyran ring adopts a boat conformation, while the cyclo­hexenone ring is in an envelope conformation. The 4-chloro­phenyl ring is almost perpendicular to the pyran ring [dihedral angle = 87.39 (1)°]. In the crystal, mol­ecules are connected by inter­molecular C—H⋯O hydrogen bonds

3,3,6,6-Tetra­methyl-9-(2-nitro­phen­yl)-3,4,6,7-tetra­hydro-2H-xanthene-1,8(5H,9H)-dione

In the title compound, C23H25NO5, the pyran ring adopts a flattened boat conformation, while the two cyclo­hexenone rings are in envelope conformations. The 3-nitro­phenyl ring is almost perpendicular to the pyran ring, making a dihedral angle of 87.1 (3)°

9-(2,4-Dichloro­phen­yl)-3,3,6,6-tetra­methyl-3,4,5,6-tetra­hydro-9H-xanthene-1,8(2H,7H)-dione

The title compound, C23H24Cl2O3, was synthesized by reaction of 2,4-dichloro­benzaldehyde and 5,5-dimethyl­cyclo­hexane-1,3-dione in ethyl­ene glycol. The central ring of the xanthene moiety is almost planar (with an r.m.s. deviation of 0.0268 Å from the least-squares plane) while the two outer rings, in a cis arrangement, display envelope conformations. The ring of the 2,4-dichloro­phenyl substituent is nearly perpendicular [85.89 (4)°] to the xanthene ring system

3,3,6,6-Tetra­methyl-9-phenyl-3,4,5,6-tetra­hydro-9H-xanthene-1,8(2H,7H)-dione

In the title compound, C23H26O3, the three six-membered rings of the xanthene system are non-planar, having total puckering amplitudes, Q T, of 0.443 (2), 0.202 (2) and 0.449 (2) Å. The central ring adopts a boat conformation and the outer rings adopt sofa conformations. The crystal structure is stabilized by van der Waals inter­actions

9-(4-Methoxy­phen­yl)-3,3,6,6-tetra­methyl-3,4,6,7-tetra­hydro-2H-xanthene-1,8(5H,9H)-dione

In the mol­ecule of the title compound, C24H28O4, the three six-membered rings of the xanthene system are not planar, having envelope, boat and envelope conformations. In the crystal structure, C—H⋯O hydrogen bonds link the mol­ecules, generating centrosymmetric R 2 2(12), R 4 4(28) and R 2 2(16) ring motifs and forming a three-dimensional network

9-(3,4-Dimeth­oxy­phen­yl)-3,3,6,6-tetra­methyl-4,5,6,9-tetra­hydro-3H-xanthene-1,8(2H,7H)-dione

The asymmetric unit of the title xanthene compound, C25H30O5, contains two mol­ecules in which the pyran ring and the dimeth­oxy­phenyl ring are nearly perpendicular to one another [dihedral angles = 86.81 (8) and 84.45 (9)°]. One of the meth­oxy groups in one mol­ecule is twisted away from the phenyl ring [C—O—C—C torsion angle = −103.40 (16)°]. The pyran ring adopts a boat conformation whereas the two fused cyclo­hexane rings adopt envelope conformations in both mol­ecules. In the crystal, mol­ecules are linked into a three-dimensional network by C—H⋯O hydrogen bonds

A highly efficient green synthesis of 1, 8-dioxo-octahydroxanthenes

SmCl3 (20 mol%) has been used as an efficient catalyst for reaction between aromatic aldehydes and 5,5-dimethyl-1,3-cyclohexanedione at 120°C to give 1,8-dioxo-octahydroxanthene derivatives in high yield. The same reaction in water, at room temperature gave only the open chain analogue of 1,8-dioxo-octahydroxanthene. Use of eco-friendly green Lewis acid, readily available catalyst and easy isolation of the product makes this a convenient method for the synthesis of either of the products

Effects of large dams on the aquatic food web along a coastal stream with high sediment loads

The contribution of two basal energy sources – detrital organic matter and primary producers – as part of aquatic food webs varies typically along river continua. A host of barriers to river flow increase the water residence time and sediment and nutrient retention in reservoirs worldwide, and potentially alter the balance between detritus-based and algae-based energy pathways in the downstream food webs. We explored this issue on the Sélune River (Normandy, France), a small coastal stream that drains an agricultural catchment with high sediment runoff. Seasonal measurements of the following parameters were compared upstream and downstream of the reservoirs of two large dams (16 m and 36 m high): sediment fluxes, nutrient and chlorophyll a concentrations, algal communities in the epilithic biofilm (taxonomic composition, biomass and growth), and benthic invertebrate communities (abundance and trophic guild structure). As anticipated, annual sediment fluxes were much lower downstream of the reservoirs, where significant decreases in water turbidity, phosphate and silicate concentrations were recorded. A higher chlorophyll a concentration in water and a higher contribution of pelagic algae taxa to the photosynthetic biofilm suggested drifting and deposition of reservoir-borne phytoplankton downriver. Photosynthetic biofilm growth was higher downstream of the reservoirs in spring and fall, and so was the abundance of herbivores in the invertebrate community, notably scrapers and algae eaters. Energy pathways within riverine food webs were traced using stable isotope analyses of carbon (C) and nitrogen in the tissues of aquatic consumers (invertebrates and fish). Mixing models revealed a discontinuity in the origin of the C entering the food webs along the river continuum, confirming a greater contribution of algal C to aquatic consumers downstream of the reservoirs. These results illustrate mechanisms whereby large reservoirs can modulate C flow in food webs along a small coastal river with high sediment loads, and make it possible to anticipate the effects of dam removal on the future river ecosystem