Synthesis and cellular profiling of diverse organosilicon small molecules.

Small-molecule synthesis coupled with cellular profiling using multidimensional screening has been used to assess the role of stereochemical and skeletal variation on assay outcomes, albeit within a limited structure space.1 Here, we provide an extension of this approach as a framework to assess the role of a main-group element on assay outcomes. We considered that incorporating, for example, boron, silicon, selenium, or germanium into small molecules could provide new structures where the unique properties of these elements may contribute to new biological activities.2 The success of bortezomib (Velcade), a boron-containing proteasome inhibitor, highlights the use of a main-group element in a therapeutic context.3 As a first step, a synthesis pathway was developed leading to organosilicon products2,4,5 having silicon placed within a chiral environment (Scheme 1). This pathway allows the diversification of stereochemistry at nearby stereogenic centers and the variation of neighboring functional groups. In order to profile the biological activity of these organosilanes, multidimensional screening was performed, providing a first quantitative glimpse of the rich activities of silicon-containing small molecules within varying stereochemical and appendage contexts. Our syntheses of test, silicon-containing small molecules relies on the stereoselective annulation pathways available for allylsilanes with π-electrophiles.6 Crotylsilanes containing hydrogen-bond donors and acceptors (2-4) were derived from the common ester crotylsilane precursor 1 (Figure 1A).7 Each of the (R)and (S)enantiomers of 2-4 was prepared to facilitate the synthesis of single enantiomers for biological evaluation. The anisyl crotylsilanes were selected based on their stability, enhanced nucleophilicity in the annulation reaction (relative to Ph), and ease of protodesilylation.8 Spiro-oxindoles represent an attractive framework for synthesis due to their precedent for biological activity.9 Therefore, we developed a general method for Lewis acid-mediated annulations of isatins10 (5) using macrobead-bound crotylsilanes 2-4.11 This method yields spirocyclic annulation products as single stereoisomers as judged by LCMS and 1H NMR spectroscopy (Figure 1B). Use of 2,6-di-tert-butyl-4-methylpyridine (DTBMP) as a protic acid scavenger was essential to prevent Si-O deprotection or cleavage from the macrobead support. No protodesilylation or elimination of the silyl group was observed when HF/py (5% in THF) was used to cleave the Si-O linker, demonstrating the complementary reactivity of the Si-O bond and the Si-C bond under these conditions. Relative stereochemistry was confirmed by X-ray crystallographic analysis (6l). In addition to incorporating functionality in the crotylsilane component, the modular placement of aryl iodide functional groups in the isatin component can be used in appending processes for further substitution and follow-up chemistry, for example, to facilitate target identification. The conversion of the aryl iodide to various amido functionalities was accomplished using the Buchwald amidation (for example, Figure 1C).12 Ninety representative compounds resulting from the annulation pathway were next profiled using 41 diverse assays to evaluate the potential of silicon-containing small molecules to modulate cellular processes. These small-molecule screens use whole cells (mammalian cell lines), whole organisms (yeast, bacteria, plasmodium), and pure proteins that are relevant to cancer, developmental biology, infectious disease, and psychiatric disease.13 To normalize the Scheme 1