Ultrasonic-Assisted Synthesis of Two t-Butoxycarbonylamino Cephalosporin Intermediates on SiO 2

Herein, we describe a facile and high efficient strategy for the synthesis of two forms of the 7β-t-butoxycarbonylamino3-chloromethyl-3-cephem-4-carboxylates using ultrasonic irradiation. By SiO 2 as weak Lewis acid catalyst, 4-methoxybenzyl 7β-t-butoxycarbonylamino-3-chloromethyl-3-cephem-carboxylate (Boc-ACLE) and benzhydryl 7β-t-butoxycarbonylamino-3chloromethyl-3-cephem-4-carboxylate (Boc-ACLH) were successfully synthesized through the efficient protection of the N-tbutoxycarbonyl (N-Boc), and the reactions occurred at low temperature requiring short reaction times and exhibiting excellent isolated yields (96% and 96.2%, resp.). The advantages of this reaction route including the usage of economical reagents and mild reaction conditions and high isolated yield make the two significant t-butoxycarbonylamino cephalosporin intermediates possible in large-scale production.


Introduction
7-t-Butoxycarbonylamino-3-chloromethyl-3-cephem-4carboxylates serving as the extremely important intermediates are employed to synthesize the commonly used antibiotics, cephalosporin.As shown in Figure 1, the chemical structure of cephalosporin intermediate contains a chemically active 3-chloromethyl-group at 3-side chain, which can be easily coupled with drug-active functional groups forming cephalosporin derivatives, especially the fourth-generation cephalosporins Cefoselis sulfate [1,2].However, owing to their presence in various biological environments, the amine functionality at 7 positions needs to be protected which is one of the most challenging issues in this synthetic chemistry.Among the current amine protection methods, the t-butoxycarbonyl (Boc) protection has been considered as the useful one because of its excellent stability regarding catalytic hydrogenation and extreme resistance to underlying basic or nucleophilic reactions [3].
In the past decade, multiple processes have been available to introduce the Boc protecting group using Boc 2 O (di-t-butyl dicarbonate) to synthesize 4-methoxybenzyl 7-t-butoxycarbonylamino-3-chloromethyl-3-cephem-carboxylate (Boc-ACLE).The synthesis of Boc-ACLE has been reported by Lee et al. [4] that they dissolved 4-methoxybenzyl 7-amino-3-chloromethyl-3-cephem-carboxylate (ACLE⋅ HCl) into CH 2 Cl 2 solvent first.Next, the mixture was reacted with di-t-butyldicarbonate (Boc 2 O) in the presence of a catalyst of N-(trimethylsilyl) acetamide (NSA).However, the catalyst of NSA has numerous drawbacks for the practical reaction such as its high costs, sensitivity to moisture, the low yield (51%), and other further troubles from processing (number 1, Table 1).Recently, Du [5] used tetrabutylammonium bromide (TEBA) as phase transfer catalyst to synthesize Boc-ACLE compound, where ACLE.HCl was reacted with Boc 2 O in weak basic aqueous solution at room temperature.However, Boc 2 O was easily decomposed under basic aqueous solution environment in which Boc 2 O was largely lost, and the reaction required long reaction times (10 h) giving a yield of 80% (number 2, Table 1).In 2010, Lin [6] reported that the Boc-ACLE could be synthesized via a catalyst-free method starting from the reaction of ACLE⋅HCl and Boc 2 O in tetrahydrofuran (THF).Using triethylamine as acid binding agent, they finally got a high yield of 87.21%.Unfortunately, Scheme 1: Synthetic route of Boc-ACLH.the reproducibility seems not good and we determined that the yield was only 15% (number 3, Table 1).
Another t-butoxycarbonylamino cephalosporin intermediate, benzhydryl 7-t-butoxycarbonylamino-3-chloromethyl-3-cephem-4-carboxylate (Boc-ACLH), was typically synthesized by Dai's group using 7-aminocephalosporanic acid (7-ACA) as starting material following the approach outlined in Scheme 1 [7].Key intermediate 1 was obtained when 7-ACA was hydrolyzed in basic solution at −30 ∘ C. Sequentially, compound 1 reacted with Boc 2 O to protect amine group catalyzed by TEBA in NaCO 3 -H 2 O-acetone solution to produce compound 2, which was then treated with diphenyldiazomethane in n-hexane forming compound 3.In the last step, the hydroxyl group of compound 3 was substituted by chlorine in presence of phosphorus pentachloride and pyridine in dichloromethane at −45 ∘ C to get the desired product 4b (Boc-ACLH).It is apparent that this synthetic route includes tedious steps, complex experimental operations, and harsh conditions.Alternative synthetic strategies with advantages such as short steps, mild conditions, and high efficiency are keen to be developed.
It can be deduced from the above-mentioned cases that the choice of raw materials, catalyst, solvent, and the reaction conditions play significant roles in synthesizing the two tbutoxycarbonylamino cephalosporin intermediates.Generally, in terms of producing these N-Boc derivatives, there are two kinds of catalytic methods (i.e., using base catalyst and Lewis acid catalyst).There have been plenty of base-catalyzed Boc-protection studies reported by using Boc 2 O/DMAP system; however, the high toxicity of DMAP was not neglectable [8].What is worse, the base-catalyzed reactions reported often resulted in the generation of byproducts like isocyanate, urea, and N,N-di-Boc derivatives [9,10].Other catalytic approaches involving Lewis acids, such as La(NO sulfamic acid, I 2 , and hexafluoroisopropanol (HFIP), have been attempted [11][12][13][14][15].As the increasing demand of mild reaction conditions encourages the development of greener route to achieve these significant synthetic works in recent years, the sonochemistry has offered a solution that a large variety of organic/medical transformations succeeded using this more efficient and facile method [16].For instance, Amira et al. [17] have applied ultrasonic irradiation technology for the N-Boc protection which not only eliminated the harsh reaction conditions but also assisted in impressive yield.Dighe and Jadhav [18] found microwave assisted chemoselective method could shorten the reaction time for the generation of N-Boc products with excellent isolated yield.It is worth mentioning that solvent-free synthetic strategies using ionic liquid catalyst have emerged which show great potential for N-Boc formation [19,20].
In this paper, we aim at proposing an optimum route for the synthesis of the two t-butoxycarbonylamino cephalosporin intermediates based upon the synergetic effect of ultrasonic assistance and SiO 2 catalyst.The efficiency of our route for N-Boc formation of our targeted product was analyzed and the mechanism of the proposed ultrasonic-assisted strategy for N-Boc protection of amines was investigated.In addition, the products were confirmed by spectrometric methods.In comparison to conventional methods, our route may offer a highly efficient and methodologically simple catalytic process to introduce N-Boc protecting group into our desired products.and Boc 2 O were added to the mixture for the appropriate time of bath ultrasonication at 0-5 ∘ C (Table 2).After completion of the reaction indicated by thin-layer chromatography (TLC), the mixture was poured into water and the organic phase was separated followed by washing with brine water and drying with anhydrous sodium sulfate under vacuum evaporation.The crude product was purified by column chromatography over silica gel to yield the desired compounds.and 13 C NMR refer to Tables 3 and 4
To overcome the exothermicity of the reaction while carrying out the reaction on a large-scale operation, the reaction temperature should be controlled and determining an appropriate solvent which depends on substrate reactivity and solubility seems very important [14].As the exothermic phenomenon was found in our N-acylation reaction, therefore, to maintain the stability of thermos-sensitive cephalosporin compounds, we chose dichloromethane as solvent and kept the reaction temperature below 10 ∘ C.  The effect of SiO 2 and ultrasound on the yields of the formation of Boc-ACLE was recorded in Table 2.The proposed route without both catalyst and ultrasonic irradiation took 6 hours to achieve the reaction but the resulting isolated yield was only 15%.When Lewis acid catalyst SiO 2 was added to the route, the yield of the reaction was improved up to 35%.To find the synergic effect of ultrasound and SiO 2 , another control reaction using ultrasound alone was performed under the same reaction time without SiO 2 that a low yield of 40% was found.Notably, when the Lewis acid catalyst of SiO 2 and ultrasonic power were utilized together, an overwhelming yield of 96% was discovered after 30 mins.The similar impressive result also happened to synthesize Boc-ACLH (the yield is 96.2%) using both ultrasound and SiO 2 which implied these results were attributed to the synergy effect of ultrasonic irradiation and SiO 2 .To explain this mechanism, we believe that, as a result of the electronic and steric molecular structure of 7-amino-3-chloromethyl-3-cephem-4-carboxylates, the amine group is low nucleophilic which could be activated by the weak Lewis acid support body of SiO 2 under ultrasonic irradiation.Figure 2 describes the structure of SiO 2 surface.Tran et al. [21] believed that the adjacent hydroxyl and double hydroxyl groups of SiO 2 surface showing weak acidity could activate the carbonyl compounds.We hypothesize that the surface of SiO 2 support body formed the polyhydroxyl matrix and the (Boc) 2 O was attacked by the free base (ACLE or ACLH) on the matrix which was consistent with the conclusion of Sunseri et al. [22].Ultrasonic vibrations enable the formation of intermolecular extrusions and confusions which has a significant influence on carbon dioxide generated from mono-t-butyl carbonate (compound 6).We believe there are two reasonable mechanisms of facilitating the nucleophilic attack of amine group on the carbonyl group involved during the process; the former is that with the bubble formation and breakdown; the cavitation contributes to the efficient generation of the N-Boc amide (Boc-ACLE or BOC-ACLH); the latter is based on the reaction equilibrium that the continuous escape of CO 2 results in the reaction preferring a forward tendency (Scheme 3).

Conclusions
In summary, the ultrasonic-assisted approach catalyzed by SiO 2 for the synthesis of two t-butoxycarbonylamino compounds at low temperature was investigated.By means of the synergetic effect of the ultrasound and SiO 2 , the designed reaction system showed a high efficiency on the desired products without any harsh conditions.Additionally, compared with existing production processing of the two tbutoxycarbonylamino cephalosporin intermediates, the work described in this study allows for stabilization of the realtime reaction temperature and low cost for the reaction.
Importantly, this study greatly advances not only the practical cephalosporin production field but also the field of other synthetic drugs and their derivatives requiring Boc protection of amines.

2 Scheme 3 :Figure 2 :
Scheme 3: The proposed ultrasonic-assisted synthetic mechanism of N-Boc protection for amines.

Table 1 :
Comparison of three methods for the synthesis of Boc-ACLE.

Table 2 :
The effect of SiO 2 and ultrasound on the yields of the Boc-ACLE.
[4]4a: Solid, m.p. 63-65 ∘ C. The yield was 96.0%.HPLC assay confirmed the purity of 4a was 98.5%.The column oven temperature was set at 295 K.The mobile phase consisted of methanol and deionized water (75 : 25, v/v) and flowed through the column in 15 min with the flow rate of 1.0 mL/min.Photodiode array detection was used to detect the Boc-ACLE at the wavelength of 254 nm. 1 H a Data taken from[4]; b this work.