Published: February 6th, 2019
Here we present a protocol to demonstrate an efficient method for the synthesis of spirocyclic heterocycles. The five-step process utilizes solid-phase synthesis and regenerating Michael linker strategies. Generally difficult to synthesize, we present a customizable method for the synthesis of spirocyclic molecules otherwise inaccessible to other modern approaches.
A convenient synthetic route for spirocyclic heterocycles is well sought after due to the molecule's potential use in biological systems. By means of solid-phase synthesis, regenerating Michael (REM) linker strategies, and 1,3-dipolar cycloaddition, a library of structurally similar heterocycles, both with and without a spirocyclic center, can be constructed. The main advantages of the solid-support synthesis are as follows: first, each reaction step can be driven to completion using a large excess of reagents resulting in high yields; next, the use of commercially available starting materials and reagents keep the costs low; finally, the reaction steps are easy to purify via simple filtration. The REM linker strategy is attractive because of its recyclability and traceless nature. Once a reaction scheme is completed, the linker can be reused multiple times. In a typical solid-phase synthesis, the product contains either a part of or the whole linker, which can prove undesirable. The REM linker is "traceless" and the point of attachment between the product and the polymer is indistinguishable. The high diastereoselectivity of the intramolecular 1,3-dipolar cycloaddition is well documented. Limited by the insolubility of the solid support, the reaction progression can only be monitored by a change in the functional groups (if any) via infrared (IR) spectroscopy. Thus, the structural identification of intermediates cannot be characterized by conventional nuclear magnetic resonance (NMR) spectroscopy. Other limitations to this method stem from the compatibilities of the polymer/linker to the desired chemical reaction scheme. Herein we report a protocol that allows for the convenient production of spirocyclic heterocycles that, with simple modifications, can be automated with high-throughput techniques.
Despite recent discoveries using highly-functionalized spirocyclic heterocycles in a number of biological systems1, a convenient pathway is still necessary for their easy manufacture. Such systems and uses for these heterocycles include: MDM2 inhibition and other anticancer activities2,3,4,5, enzyme inhibition6,7,8, antibiotic activity9,10, fluorescent tagging
CAUTION: Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in these syntheses are acutely toxic and carcinogenic. Please use all appropriate safety practices when performing the following reactions, including the use of engineering controls (fume hood and IR and NMR spectrometers) and personal protective equipment (safety goggles, gloves, lab coat, full-length pants, and closed-toe shoes).
1. Michael Addition of Furfurylamine to the REM Link.......
As outlined in the procedure above, the synthetic route to spirocyclic oximes (see Figure 1) begins with the Michael addition of furfurylamine to compound 1, the REM linker, to afford 2. A subsequent Michael addition and 1,3-dipolar cycloaddition of the support 2 using various β-nitrostyrene derivatives yield the tricyclic compound 3, an N-silyloxy isoxazolidine with four unique.......
In a typical REM linker/solid-phase synthetic strategy, prior to the release of an amine from the solid support, it is critical to form a quaternary ammonium salt, as described in section 4 of the protocol39. Due to the steric hindrance of the tricyclic system and bulky R2 groups (benzyl and octyl halides), only small alkylating reagents (methyl and allyl halides) could be utilized in this reaction46. With a simple modification, allowing for the addition and use .......
This work was funded by a grant from the Faculty Research Council to K.S. Huang (Azusa Pacific University - United States). C.R. Drisko is a recipient of the John Stauffer Scholarship and the Gencarella Undergraduate Research Grant. S.A. Griffin received an S2S Undergraduate Research Fellowship from the Department of Biology and Chemistry.
Authors (left to right) Cody Drisko, Dr. Kevin Huang and Silas Griffin conducted the experiments and prepared the manuscript. Cody Drisko is a John Stauffer Fellow and a recipient of the Gencarela Research Grant. Silas is a S2S Azusa Pacific Univers....
|Solid Polymer Support; 1.1 mmol/g loading
|Trimethylsilyl chloride (TMSCl)
|Reagent; CAUTION - highly volatile; creates HCl gas
|Tetra-n-butylammonium fluoride (TBAF) in Tetrahydrofuran (THF)
|25 mL solid-phase reaction vessel
|Glassware with filter
|Thermo Scientific Nicole iS5
|AVANCE III NMR Spectrometer
|Instrument; 300 MHz; Solvents: CDCl3 and CD3OH
|Wrist-Action Shaker Model 75
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