We report an efficient and convergent strategy for the total synthesis of UCS1025A and its diastereomer tetra-epi-UCS1025A.Ucs1025A is a representative member of the naturally occurring pyrrolizidinone polyketides,from which members with potent antibacterial,antifungal,and anticancer activities have been identified.Our approach features a tandem carbonylative Stille cross coupling and Diels-Alder reaction to forge a key C-C bond and build the trans-decalin system.This tandem process utilizes carbon monoxide as a one-carbon linchpin to stitch a vinyl triflate and a vinylstannane together and form the desired enone moiety for the subsequent intramolecular Diels-Alder cyclization.Our synthesis also provides a versatile approach for the synthesis of other related pyrrolizidinone-containing polyketides.
Sandstone is widely distributed in cold regions and the freeze-thaw deterioration of them has caused many geological engineering disasters.As an important and direct index of frost resistance,the strength loss of sandstones under freeze-thaw actions should be investigated to provide a guidance for the stability assessment of geological engineering.In this research,the UCS(Uniaxial compressive strength)loss of six typical sandstones with different water contents after 0,20,40 and 60 freeze-thaw cycles was measured in the laboratory.The experimental results indicated that the freeze-thaw damage was more serious in sandstones containing high water contents,and the critical saturations for causing a significant loss of UCS under freeze-thaw were 60%-80%for these sandstones.Below this critical saturation,the UCS loss of the sandstones was mainly caused by water weakening rather than freeze-thaw damage.Besides,a developed strength prediction model was proposed by combining the exponential decay function and multiple linear regression method.The initial porosity,elastic modulus and tensile strength of fresh sandstones were a good parameter combination to accurately determine the decay constant in this developed model.The main novelty of this model is that it can accurately and easily estimate the UCS loss of sandstones after any freeze-thaw cycle only using the initial parameters of fresh sandstones,but it does not need to perform freeze-thaw and mechanical strength experiments.This study not only provides an accurate prediction model of UCS under freeze-thaw,but also makes a contribution to better understanding the frost resistance mechanism of sandstones.
