Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Summary

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.

Abstract

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.

Introduction

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 activities^2,3,4,5, enzyme inhibition^6,7,8, antibiotic activity^9,10, fluorescent tagging

Protocol

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 Linker

NOTE: The duration of this step is 25 min for the set-up and 24 h of reaction time.

1. Add 1 g (1 equiv.) of....

Results

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 ster.......

Discussion

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 .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Chemicals
REM Resin	Nova Biochem	8551010005	Solid Polymer Support; 1.1 mmol/g loading
Furfurylamine	Acros Organics	119800050	Reagent
Dimethylformamide (DMF)	Sigma-Aldrich	227056	Solvent
Dichloromethane (DCM)	Sigma-Aldrich	270997	Solvent
Methanol	Sigma-Aldrich	34860	Solvent
trans-4-bromo-β-nitrostyrene	Sigma-Aldrich	400017	Nitro-olefin solid
trans-3,4-dimethoxy-β-nitrostyrene	Sigma-Aldrich	S752215	Nitro-olefin solid
trans-2,4-dichloro-β-nitrostyrene	Sigma-Aldrich	642169	Nitro-olefin solid
trans-β-nitrostyrene	Sigma-Aldrich	N26806	Nitro-olefin solid
Triethylamine (TEA)	Sigma-Aldrich	T0886	Solvent
Trimethylsilyl chloride (TMSCl)	Sigma-Aldrich	386529	Reagent; CAUTION - highly volatile; creates HCl gas
Tetra-n-butylammonium fluoride (TBAF) in Tetrahydrofuran (THF)	Sigma-Aldrich	216143	Reagent
Tetrahydrofuran (THF)	Sigma-Aldrich	401757	Reagent
1-Bromooctane	Sigma-Aldrich	152951	Alkyl-halide
Iodomethane	Sigma-Aldrich	289566	Alkyl-halide
Allylbromide	Sigma-Aldrich	337528	Alkyl-halide
Benzylbromide	Sigma-Aldrich	B17905	Alkyl-halide
Glassware/Instrumentation
25 mL solid-phase reaction vessel	Chemglass	CG-1861-02	Glassware with filter
Thermo Scientific Nicole iS5	Thermo Scientific	IQLAADGAAGFAHDMAZA	Instrument
AVANCE III NMR Spectrometer	Bruker	N/A	Instrument; 300 MHz; Solvents: CDCl3 and CD3OH
Wrist-Action Shaker Model 75	Burrell Scientific	757950819	Instrument