[reaction: see text] A new protocol for the sequential allylic transfer reaction of a diene with two aldehydes in the construction of cyclic systems containing four stereogenic centers is achieved in a one-pot operation. Reaction of the diene-alehyde 1 with aldehyde in the presence of the diboronyl reagent catalyzed by a nickel complex produces products 2 and 3 depending on reaction conditions in high levels of diastereoselectivity. Extension of this method to the synthesis of six-membered rings is also investigated.
A highly diastereoselective synthesis of 3 is achieved from the reaction of 1 with hexamethylditin catalyzed by palladium complex in the presence of 20 mol % tris(pentafluorophenyl)borane as a Lewis acid additive for a reversal of diastereoselectivity, whereas 2 is formed in the absence of Lewis acid additive. The method described herein is successful with various substrates 1 in good yields and high levels of diastereoselectivity. [reaction: see text]
Stereocontrolled consecutive processes can offer advantages over stepwise transformations by increasing chemical efficacy and saving efforts as a result of a simple operation.[1] During the course of our research program aimed at developing new synthetic methods for the stereoselective construction of pyran rings through stepwise allylic transfer reactions, [2] we disclosed our investigations on transition-metal-catalyzed intramolecular allylations mainly between allene and carbonyl functionalities to afford cyclic compounds with high levels of stereoselectivity.[3] With these observations in hand, we became interested in designing a sequential allylic transfer reaction from 1 with two aldehydes to form acyclic 2-(1-stannylvinyl)-1,3-diols 2, which contain versatile functional groups (Scheme 1).It was envisaged that the sequential allylic transfer reaction starting from 1 could be realized by a three-step sequence as shown in Scheme 1. To provide direct access to 2, we considered the bis(stannyl) compound A as a crucial intermediate. The transformation involves propargylboration Scheme 1. General strategy for the synthesis of 2, and thus natural products 3 and 4, by a sequential approach. L* = chiral ligand.
Ultrasound is defined in terms of human hearing and it is a sound having a frequency higher than that to which the human ear can respond. The attempts to use the ultrasound at casting process have been carried out and there are a lot of study results on grain refinement on microstructure of casting alloys. However, these studies almost have been focused on mould vibration at solidifying melts of solid and liquid coexistence temperature. In this study, high intensity ultrasound was injected in aluminium full melts, especially A390 alloy, to evaluate the effect of ultrasound on microstructure and mechanical properties of castings. The effect of ultrasonic injection on melts could be summarised as follows: reduction of mean size of primary silicon, variation of phase distribution, improvement of solute homogeneity and mechanical properties.
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