Inverse-electron-demand Diels−Alder reactions have attracted intense research focus. However, enolate and enamine are the most employed intermediates to realize such transformation, and the use of dienolate intermediate remains elusive. Reported herein is the asymmetric inverse-electron-demand oxa-Diels−Alder reaction between allyl ketones and alkenyl 1,2-diketones using a bifunctional thiourea catalyst. The reaction afforded various highly functionalized dihydropyrans with good to excellent enantioselectivities under mild conditions, and further novel transformations on the products have also been realized.

The discovery of new reaction modes mediated by easily available substrates is an important research topic in organic synthesis. Allyl ketones and related compounds have been demonstrated to undergo γ- or α-selective additions to different electrophiles. We disclose here the Brønsted base-catalyzed reaction of allyl ketones and alkynyl 1,2-diketones, which undergo a unique α-selective addition/intramolecular aldol-type annulation/C-C bond cleavage process, and a series of 2-acyloxycyclopent-3-enones can be obtained under very mild conditions.

Different from other upper-tropospheric jet stream that owns the single jet axis, the tropical easterly jet (TEJ) generally splits into two branches over the tropical western Pacific where the TEJ inflows from June to September, with the north branch locating near 20 N and the south branch appearing near the equator. However, how these double easterly jets change from year to year and whether they have effects on the tropical cyclones (TCs) over the tropical western Pacific are still unknown. Observational evidence has demonstrated that the double easterly jets in the TEJ's inflow region exhibit an out-of-phase relationship in the interannual time scale and is significantly related to the TCs genesis. Corresponding to a stronger south branch and a weaker north branch, the accelerated (decelerated) easterly wind for the south (north) branch would induce a northerly (southerly) ageostrophic wind, thus triggering an uppertropospheric divergence between the double easterly jets. This divergence can further lead to anomalous ascending flows over the western North Pacific, which turns out to be the primary cause that increases the TCs number over there. The results highlight that the out-of-phase variation of the double easterly jets could dynamically modulate the occurrence of the TCs over the western North Pacific by inducing the secondary vertical circulation.

An objective detection and tracking algorithm based on relative vorticity at 850 hPa using National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) Reanalysis I data was applied to track cyclones in the Southern Hemisphere during austral winters from 1948 to 2017. The climatological characteristics of extratropical cyclones, including track density, frequency, intensity, lifetime, and their related variabilities, are discussed. The frequency and average lifetime of cyclones have substantially decreased. The average maximum intensity of cyclones has shown an increasing trend over the 70 year study period. The cyclone track density shows a decreasing trend in lower latitudes, consistent with the region where the upper troposphere zonal wind weakens. Baroclinicity can explain the increase in cyclone intensity: when a cyclone moves to higher latitudes and enters the region with greater baroclinicity, it strengthens. As there is no discernible increase in cyclogenesis in the medium latitudes (45°–70°S), but significantly less cyclogenesis in lower and higher latitudes, it is hypothesized that there is no clear poleward cyclogenesis shift over the Southern Hemisphere.