Synthesis of New 2-Aminocarbohydrate-1 , 4-Naphthoquinone Derivatives Promoted by Ultrasonic Irradiation

Neste trabalho nós descrevemos o uso do ultrassom na síntese de novas naftoquinonas, 6a-f e 7a-c, contendo na posição 2 do anel quinonoídico substituintes do tipo aminocarboidratos. Estas substâncias foram preparadas, em condições brandas, através da reação da 1,4-naftoquinona (8a) ou do metoxi-lapachol (8b) com diferentes aminocarboidratos 9a-d. As estruturas das substâncias foram confirmadas através das técnicas de RMN de H e de C-APT, unie bi-dimensionais (COSYH vs. H e HETCOR J CH ), e por espectrometria de massas de alta resolução com ionização por electrospray (ESI MS).


Introduction
The naphthoquinones are important natural substances that are widely distributed in different families of plants, fungi and some animals; 1 many of them play integral roles in vital biochemical processes.For example, vitamins K 1 and K 2 are natural naphthoquinones that are responsible for the human blood coagulation mechanism. 2Due to this involvement in metabolism, this class of substances has been extensively studied concerning their various biological properties.Many of them have ceased to be mere curiosities and have become very useful drugs for mankind. 3ecent reports in the literature 2,[4][5][6][7][8] point out that their mechanisms of action normally involve the generation of active oxygen species (e.g., superoxide radical anions, hydrogen peroxide and hydroxyl radicals) by redox cycling, intercalation in the DNA double helix or alkylation of biomolecules.In all of these cases, the biological activity requires bioreduction of the quinone nucleus as the first activating step. 6,80][11][12] The introduction of vicinal oxidizable groups, such as amino substituents, onto the quinone nucleus can exert influence on its redox properties, inducing oxidative stress in cells, and alkylation of cellular macromolecules, including lipids, proteins and DNA. 2,4Indeed, the literature to date clearly shows that aminoquinone frameworks are present in several important bioactive compounds that exhibit antitumor 3 1, antimalarial 13 2, antithrombotic 14 3, antifungal 9 4 and molluscicidal 15 activities 5 (Figure 1).
Although these methods previously described have been used for the synthesis of aminoquinones, there is a lack of synthetic studies for the preparation of sugaraminonaphthoquinones. Additionally, the development of a versatile method for their preparation in mild conditions is of great importance since it is possible that these new substances can constitute a new class of bioactive naphthoquinone derivatives.
As part of an ongoing research program on the synthesis of new biologically active naphthoquinones and on the basis of our experience in the field of the use of readily available carbohydrates 33,34 in organic reactions, herein we report the ultrasound-accelerated synthesis of new naphthoquinone derivatives 6a-d and 7a-c (Schemes 1 and 2) possessing an aminocarbohydrate chain at C-2 position of the quinone moiety.6][37] The ultrasound-accelerated 1 , 4 -a d d i t i o n o f a m i n o c a r b o h y d r a t e s 9 ac t o 1,4-naphthoquinone (8a) was first carried out in the presence of iodine at room temperature, following the procedure described in the literature 27 (Scheme 1, method A).The novel aminonaphthoquinones 6a-c were purified by flash column chromatography using 50% n-hexane/ ethyl acetate as eluent.The yields of these reactions are listed in Table 1.The reaction of D-glucosamine hydrochloride 9d using method A failed to produce the compound 6d.Thus, the reaction under ultrasound irradiation was also investigated in the absence of iodine (Scheme 1, method B).This method led to better yields of the aminonaphthoquinones 6a-c, as shown in the Table 1.In this case, the 1,4-addition reaction between naphthoquinone 8a and D-glucosamine hydrochloride 9d resulted in the compound 6d, as a mixture of two diastereoisomers, a:b (1:1) in 40% yield.The treatment of this mixture with acetic anhydride in pyridine in excess produced the desired tetraacetate derivative in 60% yield as a mixture of anomers (3:1), being 6e the major isomer.The yields of the quinones 6a-d were dependent on the structure of the aminocarbohydrates 9a-d in question.The reactions worked very well with primary amines 9a-c, affording the corresponding coupling products in good yields, while more hindered amine, e.g., 9d, led to the product 6d in moderate yield.

Results and Discussion
The characterization of the aminonaphthoquinone structures 6a-c and 6e was performed by using one-and two-dimensional NMR techniques and by HR ESI MS.][40][41] The 1 H NMR spectrum of compound 6a showed the anomeric proton (H-1') as a singlet at d 4.98.The protons H-2' and H-3' appeared as doublets at d 4.59 The absence of vicinal coupling indicates a trans relationship between protons H-1' and H-2' and the b-anomeric configuration for the carbohydrate. 41The envelope conformations O E and 4 E for compounds 6a and 6c, respectively, were established on the basis of X-ray crystallographic data of 5-modified analogous of methyl 2,3-O-isopropylidene-b-ribofuranoside 42 and 5-deoxy-1,2-O-isopropylidene-a-D-xylofuranose. 43he twist-boat conformation of the D-galactose ring in 6b was confirmed by the 1H-1H vicinal coupling constants values J H-1',H-2' , J H-2',H-3' , J H-3',H-4' and J H-4',H-5' (5.0, 2.4, 8.0 and 1.5 Hz, respectively) of the ring protons. 38he a:b ratio of D-glucosamine derivatives 6e and 6f was determined by the areas of the H-1' peaks of the respective anomers in the 1 H NMR spectra.The a-configuration at position C-1' of the major isomer 6e is supported by the small coupling constant between H-1' and H-2' (d, J H-1', H-2' = 4.0 Hz).The structures of the two possible chair conformations ( 4 C-1 and 1 C-4) of the a-pyranose form are shown in Figure 2. The large vicinal coupling constants for the protons 3 J H-2', H-3' and 3 J H-3', H-4' (10.0 and 9.5 Hz, respectively) are in agreement with trans-diaxial arrangements for the protons H2'-H3' and H3'-H4', suggesting a 4 C-1 conformation for the pyran ring.
For comparison purposes, the reactions of amine 9a with naphthoquinone 8a or methoxylapachol 8b were carried out under conventional heating.In cases, no conversion into products was observed after a 2 h reaction, indicating that the ultrasound irradiation is essential for the activation and performing of these coupling reactions.The effects of sonication on reactions can be related to high temperature and pressure resulting from acoustic cavitation. 44he structures proposed for compounds 7a, 7b and 7c are supported by their HR ESI MS and NMR spectroscopic data ( 1 H, APT, HETCOR 1 J CH and n J CH , n = 2,3).)))

Experimental
Chemical reagents and all solvents used in this study were purchased from Merck AG (Darmstadt, Germany) and VETEC LTDA.The synthesis of new aminonaphthoquinones 6a-d and 7a-c was performed in an ultrasonic bath (40 KHz) model USC 1400, ultrasonic cleaner, UNIQUE.Melting points were determined with a Fisher-Johns instrument and are uncorrected.Infrared (IR) spectra were recorded on Perkin-Elmer FT-IR model 1600 spectrophotometer, in KBr pellets.NMR spectra, unless otherwise stated, were obtained in deuterated CDCl 3 using a Varian Unity Plus 300 MHz spectrometer.Chemical shifts (d) are expressed in ppm and the coupling constants (J) in Hertz.High-resolution electrospray ionization mass spectrometry (HR ESI MS) was performed in positive ion mode on a Q-Tof Micro instrument.The progress of all reactions was monitored by TLC performed on 2.0 cm × 6.0 cm aluminum sheets precoated with silica gel 60 (HF-254, E. Merck) to a thickness of 0.25 mm.The developed chromatograms were viewed under ultraviolet light at 254 nm.Merck silica gel (60-200 mesh) was used for column chromatography.

Synthesis General procedure for the preparation of the novel aminonaphthoquinones 6a-d and 7a-c
A solution of naphthoquinone 8a or 8b (0.5 mmol) and aminocarbohydrate 9 (0.5 mmol) in methanol (MeOH) (2 mL) was sonicated for 2 h at r.t..The solvent was removed under reduced pressure.The residue was purified by flash column chromatography (gradient elution, 10-30% AcOEt in hexane) to afford the desired compounds 6a-c and 7a-c.

Conclusions
We have developed a method for the synthesis of new series of substituted-2-aminonaphthoquinones 6a-d and 7a-c by the reaction of 1,4-naphtoquinone (8a) or 2-methoxylapachol (8b) with different aminocarbohydrates 9a-d, under ultrasonic irradiation, which accelerate the 1,4-addition, in the absence of iodine (method B) being a faster and cleaner direct access for the synthesis of new sugar-based quinone compounds.Further efforts are being dedicated towards the evaluation of the biological profiles of this novel type of 1,4-naphthoquinone derivatives.

Figure 1 .
Figure 1.Some examples of bioactive products with the aminonaphthoquinone moiety.