SUMMARYNew explicit dynamic computational developments in conjunction with finite element formulations for impact problems are described in this paper. The proposed methodology is based on employing a variational inequality for dynamic problems involving Coulomb friction with the so-called forward incremental displacement-central difference method specially formulated in this paper for this class of problems. To enforce the constraints on the contact boundary, a linear complementary equation is established by means of a minimization problem subjected to constraints, which is equivalent to discretization of the variational inequality of the dynamic problem. In conjunction with these developments, a new conjugate gradient based explicit solution strategy is described for effectively solving the linear complementary equations. With the motivation for providing effective computational procedures suitable for vectorization and parallel computations, the proposed developments not only provide a fundamentally sound and robust theoretical basis but also serve to be ideally suited for impact problems involving frictional contact on high speed computing environments.
In this article, a process to prepare the key chiral intermediate (R)-7-(benzyloxy) -3,4,12,12a-tetrahydro-1H-[1,4] oxazino[3,4c]pyrido[2,1-f][1,2,4]triazine-6,8-dione 6-R of influenza antiviral drug baloxavir marboxil is described. The process includes a novel preparation of 2-(2,2-dimethoxyethoxy) ethanamine 13 with more convenient and safer manipulation, and a method to convert the mother liquor of the chemical resolution to the racemate 7-(benzyloxy)- 3,4,12,12atetrahydro-1H-[1,4]oxazino[3,4-c]pyrido[2,1-f][1,2,4]triazine-6,8dione 6-rac. This new process has been successfully demonstrated on the 50 g scale of 13 with 66.4 % yield in 3 steps, and 10 g scale of 6-R via 9 steps with 99.9 % purity, > 99 % ee and 11.0 % overall yield. All the steps just involve simple purifications without chromatography.
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