Abstract:Preferential¯ow pathways, such as soil pipes, are usually present in the soil of slopes. Subsurface¯ow through the soil pipes aects the subsurface drainage system and is responsible for sediment removal from slopes. However, a record of the inner structure of soil pipes has rarely been reported for slopes. A ®brescope examination of the morphology and¯ow phases in soil pipes in hillslopes underlain by a Quaternary sand± gravel formation provided the following information: the main pores of the soil pipes ran mostly parallel with the slope gradient; the cross-sections of the soil pipes were approximately circular; and occurred on a few occasions; with some triple junctions being present. In addition, both full¯ow and partly full-depth conditions occurred simultaneously in the soil pipe. The full¯ow condition has long been used in hydrological studies to model the pipe¯ow mechanism. Both the full¯ow condition and the partly full-depth condition, however, must be examined closely in order to evaluate the subsurface hydrology in heterogeneous soil and the hydrogeomorphological processes of subsurface hydraulic erosion.
The Diels–Alder reaction,
which is a traditional [4 + 2]
cycloaddition with two carbon–carbon bond formations, is one
of the most powerful tools to synthesize versatile and unique six-membered
rings. We show that chiral supramolecular U-shaped boron Lewis acid catalysts promote the unprecedented multiselective
Diels–Alder reaction of propargyl aldehyde with cyclic dienes.
Independent from the substrate control, enantio-, endo/exo-, π-facial-, regio-, site-, and substrate-selectivities could
be controlled by the present U-shaped catalysts. The obtained reaction
products could access the concise synthesis of chiral diene ligands
and a key intermediate of (+)-sarkomycin. The results presented here
might partially contribute to the development of artificial enzyme-like
supramolecular catalysts for multiselective reactions, which will
be able to target organic compounds that have thus far eluded synthesis.
In the enantioselective inverse-electron-demand hetero-Diels-Alder reaction, simple α,β-unsaturated aldehydes (i. e., acroleins) are still challenging substrates, unlike α,β-unsaturated carbonyl compounds containing electronwithdrawing groups. In the present study, the reaction of α-aryl-β-alkyl-substituted acroleins with ethyl vinyl sulfide was developed with the use of bulky chiral supramolecular Brønsted acid catalysts, such as tris(pentafluorophenyl) borane-assisted chiral phosphoric acid catalysts. As a result, cis-cycloadducts as optically active 3,4-dihydro-2H-pyrans were exclusively obtained in high yields with high enantioselectivities via the favored endo orbital approach. An obtained optically active cis-isomer could be transformed into the corresponding trans-isomer without a loss of enantiopurity by O,S-acetal epimerization. Moreover, transformations to synthetically useful optically active epoxide and δ-valerolactone are also demonstrated.[a] Dr.
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