Synthesis of a Spirocyclic Oxetane-Fused Benzimidazole

A new synthesis of 2-oxa-7-azaspiro[3.5]nonane is described. Spirocyclic oxetanes, including 2-oxa-6-azaspiro[3.3]heptane were converted into o-cycloalkylaminoacetanilides for oxidative cyclizations using Oxone® in formic acid. The expanded spirocyclic oxetane successfully gave the [1,2-a] ring-fused benzimidazole. X-ray crystal structure of the resultant new tetracyclic system, 1ʹ,2ʹ-dihydro-4ʹH-spiro[oxetane-3,3ʹ-pyrido[1,2-a]benzimidazole] and the azetidine ring-opened adduct, N-(2-acetamido-4-bromophenyl)-N-{[3-(chloromethyl)oxetan-3-yl]methyl}acetamide are disclosed.


Introduction
Oxetane is considered as a polar equivalent of the gem-dimethyl group ( Figure 1) [1][2][3]. Spirocyclic oxetanes such as 2-oxa-6-azaspiro [3.3]heptane (1a) and 2-oxa-7-azaspiro [3.5]nonane (1b) were proposed as valuable structural alternatives to ubiquitous morpholine in medicinal chemistry [4]. A drug discovery project within our group demanded a substituent that enabled higher binding affinities to the NAD(P)H:quinone oxidoreductase 1 (NQO1) active site [5]. NQO1 is an enzyme over-expressed in cancer cell lines. Our attention turned to oxetane due to its heralded metabolic robustness in comparison to carbonyl alternatives, while offering the hydrogen bonding capacity necessary for efficient binding to the His194 residue of NQO1 enabling more efficient reduction of benzimidazolequinone and imidazobenzimidazolequinone substrates. The following article describes

Synthesis of o-Cycloalkylaminoacetanilides
Nucleophilic aromatic substitution onto 1,4-dibromo-2-nitrobenzene using the oxalate salts of spirocyclic oxetanes 1a and 1b was possible in yields of >90% when using a large excess of potassium carbonate in DMF (Schemes 2 and 3). Reduction of the nitro group of 6a and 6b using iron and aqueous ammonium chloride yielded the corresponding anilines 7a and 7b in 95% yield. The acetylation was optimized with aniline 7a, where the initial attempt using acetyl chloride gave the desired acetamide 8a in 64% yield together with adduct 9a of ring-opening of the azetidine in 19% yield (Scheme 2). By-product 9a was confirmed by X-ray crystallography ( Figure 2). As pointed out by the Reviewer this may have occurred using the chloride ion liberated from the first acetylation. The ring-opening of azetidines by the nucleophilic attack of halide ions on azetidinium ions has been previously described [9]. Acetylations using acetic anhydride were in contrast regioselective giving diacetylated adduct 10a in 90% yield with neat acetic anhydride, while acetylation with acetic anhydride in methanol gave the desired product 8a in 81% yield. The latter conditions gave the analogous acetamide of 2-oxa-7-azaspiro [3.5]

Fusion of the Spirocyclic Oxetanes
In order to fuse the spirocyclic oxetane of 8a and 8b, the well-established oxidative cyclization reaction of acetamide onto the neighbouring cyclic amine was used [11][12][13]. Oxone ® in formic acid is as an expedient means of converting o-cycloalkylaminoacetanilides into [1,2-a] alicyclic ring-fused benzimidazoles and double annulated imidazobenzimidazoles in high yields [13]. Using the latter conditions (Scheme 4), acetanilide 8a gave a mixture of products indicating degradation of the spirocyclic oxetane and an absence of the desired benzimidazole 2a. In contrast the 2-oxa-7-azaspiro [3.5]nonane fused benzimidazole 2b was isolated in 74% yield at the end of the reaction of 8b without the requirement for chromatography by simple organic extraction from the basified aqueous mixture. The structure of the novel tetracyclic system 2b was confirmed by X-ray crystallography ( Figure 3).

General
All chemicals were obtained from commercial sources and used without further purification. Thin layer chromatography (TLC) was performed on TLC silica gel 60 F254 plates. Dry vacuum column chromatography was carried out on silica gel (Apollo Scientific ZEOprep 60/15-35 microns). Melting points were measured on a Stuart Scientific melting point apparatus SMP1. Infrared spectra were recorded using a Perkin-Elmer Spec 1 with ATR attached. 1 H-NMR spectra were recorded using a Joel ECX 400 MHz instrument equipped with a DEC AXP 300 computer workstation. The chemical shifts were recorded in ppm relative to tetramethylsilane. 13 C-NMR data were collected at 100 MHz with complete proton decoupling. High resolution mass spectra (HRMS) were carried out using ESI time-of-flight mass spectrometer (TOFMS). The precision of all accurate mass measurements were better than 5 ppm. NMR spectra of all compounds are available in the Supplementary Materials document accompanying this article.

Conclusions
The new spirocyclic oxetane fused system, 1ʹ,2ʹ-dihydro-4ʹH-spiro[oxetane-3,3ʹ-pyrido[1,2-a] benzimidazole] has been prepared through oxidative cyclization of the o-cycloalkylaminoacetanilide, where 2-oxa-7-azaspiro [3.5]nonane is the cycloamino substituent. Future work will focus on preparing quinone analogues for anti-cancer studies, as well as investigating conditions for the synthesis of the more strained tetracycle 2a. The acetanilide containing 2-oxa-6-azaspiro [3.3]heptane system proved to be unstable under the presented oxidative cyclization conditions giving an intractable mixture. Research is on-going in the group to establish milder conditions for oxidative cyclizations of o-cyclic amine substituted anilines [15] and acetamides to give ring-fused benzimidazoles.