Bromopentane Synthesis Essay

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1. Introduction

Melanin is produced in melanocytes by melanogenesis and determines skin and hair color. It also plays an important role in skin protection from UV radiation. However, excessive accumulation by pathological and environmental factors induces pigmentation problems and has become a critical issue in the cosmetic field [1,2]. Therefore, melanogenesis inhibitors have become important constituents in cosmetic products for depigmentation [3,4,5].

Melanogenesis is a complex biosynthetic process regulated by enzymatic and chemical reactions. Exposure of the skin to UV radiation stimulates keratinocytes to secrete α-melanocyte stimulating hormone (α-MSH). α-MSH then binds to melanocortin 1 receptor (MC1R) on melanocyte and induces melanogenesis. Microphthalmia-associated transcriptional factor (MITF) is a key transcriptional factor in melanogenesis that involved in the regulation of melanogenic enzymes such as tyrosinase, tyrosinase-related protein (TRP)-1 and TRP-2. Tyrosinase catalyzes the conversion of l-tyrosine to dopaquinone, the first rate-limiting step in the melanogenesis. TRP-1 and TRP-2 are also involved in the major steps of melanin synthesis. Therefore, tyrosinase, TRP-1 and TRP-2 have become important targets in the development of depigmenting agents, such as skin-whitening cosmetics [6,7,8,9].

Resveratrol, 3,4′,5-trihydroxy-trans-stilbene, has diverse biological activities, such as antioxidant, anti-inflammatory and cardiovascular protective effects [10,11,12]. Resveratrol also has been reported to reduce melanin synthesis [13,14,15]. However, its utilization and development in products are limited due to the chemical stability, poor solubility, and low bioavailability [16,17]. Therefore, attempts to increase the bioavailability of resveratrol have been made in many fields. Drug delivery systems using new formulations, such as encapsulation are suggested to enhance bioavailability [18,19,20]. The resveratrol derivatives, such as resveratrol dimers or synthetic derivatives, also have been reported to have better biological activity with fewer drawbacks [21,22,23,24,25]. Recently, the acetylated derivative of resveratrol has been synthesized with more efficient anti-melanogenic activity and better stability [26]. For the purpose of developing anti-melanogenesis inhibitory resveratrol derivatives with better bioavailability, nine resveratrol derivatives, five alkyl ether derivatives and four alkyl ester derivatives by chemical reaction and were synthesized in the present study. Effect on melanogenesis and the mode of action were also investigated.

2. Results and Discussion

2.1. Chemistry

The resveratrol ether derivatives were synthesized as shown in Scheme 1. Resveratrol (1) was converted to the corresponding ether derivatives (2a2e) in the presence of corresponding alkyl bromides. The synthesis of resveratrol ester derivatives is shown in Scheme 2. Resveratrol ester derivatives (3a3d) were synthesized by the addition of corresponding acyl chlorides. The structures of synthetic derivatives were confirmed by spectroscopic analysis including NMR, IR and MS analysis (Figure 1).

Scheme 1. Synthesis of resveratrol ether derivatives 2ae.

Scheme 1. Synthesis of resveratrol ether derivatives 2ae.

Reagents and conditions: (i) DMF, NaOH, 10 min; (ii) 1-bromoethane, 1-bromobutane, 1-bromopentane, 1-bromohexane, or 1-bromooctane, 40 °C, 24 h.

Scheme 2. Synthesis of resveratrol ester derivatives 3ad.

Scheme 2. Synthesis of resveratrol ester derivatives 3ad.

Reagents and conditions: (i) CH2Cl2, 10 °C, 5 min; (ii) TEA, DMAP, 10 °C; (iii) acetyl chloride, 3,3-dimethylacryloyl chloride, 2-ethylhexanoyl chloride, or octanoyl chloride, 10 °C; 1 h.

Figure 1. Chemical structure of resveratrol (1) and its derivatives (2ae and 3ad).

Figure 1. Chemical structure of resveratrol (1) and its derivatives (2ae and 3ad).

2.2. Effect on Melanogenesis

2.2.1. Effect on Melanin Content in B16F10 Melanoma Cells

The effect of resveratrol derivatives on melanogenesis and cell viability was first investigated using B16F10 melanoma cells. Stimulation of B16F10 melanoma cells with 100 nM α-MSH for 72 h significantly increased the melanin synthesis. Resveratrol derivatives dose-dependently reduced the melanin content concentration from 5 to 20 µg/mL without any cytotoxicity (Figure 2A,B). Although the inhibition was slightly increased compared to resveratrol in some derivatives, there was no significant difference among resveratrol and synthetic derivatives. In addition, all the synthetic derivatives showed similar inhibition regardless of side chains.

Figure 2. Effects of resveratrol derivatives on (A) melanin content and (B) cell viability in B16F10 melanoma cells. NC: vehicle treated normal control; PC: α-MSH stimulated positive control. * p < 0.05 compared with PC group.

Figure 2. Effects of resveratrol derivatives on (A) melanin content and (B) cell viability in B16F10 melanoma cells. NC: vehicle treated normal control; PC: α-MSH stimulated positive control. * p < 0.05 compared with PC group.

2.2.2. Effect on Tyrosinase Activity

Inhibition of melanin synthesis can be achieved either by inhibiting tyrosinase activity or by reducing melanogenic enzyme expression [8,9]. Therefore, the effect of resveratrol derivatives on tyrosinase activity and the expression of melanogenic enzymes were investigated.

Tyrosinase catalyzes the first rate-limiting step in the melanogenesis and plays a pivotal role in melanin synthesis [6,7]. The effect of resveratrol derivatives on tyrosinase activity was first evaluated in vitro using mushroom tyrosinase. Although resveratrol effectively inhibited the tyrosinase activity, both alkyl ether (2a2e) and ester derivatives (3a3d) showed little inhibition (Figure 3). These results suggest that free hydroxyl groups of resveratrol are important for the inhibition of tyrosinase activity, which is consistent with previous reports [27].

Figure 3. Effects of resveratrol derivatives (100 µg/mL) on tyrosinase activity. NC: vehicle treated normal control. * p < 0.05 compared with NC group.

Figure 3. Effects of resveratrol derivatives (100 µg/mL) on tyrosinase activity. NC: vehicle treated normal control. * p < 0.05 compared with NC group.

2.2.3. Effect on Melanin Synthesis in B16F10 Melanoma Cells

Melanin synthesis is also regulated by the expression of melanogenic enzymes. Tyrosinase and TRP-1 are key enzymes involved in the major steps of melanin synthesis [8,9]. Therefore, the effect of the resveratrol derivative 2a on the expressions of tyrosinase and TRP-1 was determined. The expression of tyrosinase was dramatically reduced by the treatment of compound 2a (Figure 4). Treatment of 2a also inhibited the expression of TRP-1 expression. These results suggest that 2a efficiently inhibited the melanogenic enzyme expression.

Figure 4. Effect of resveratrol derivative 2a on the expression of tyrosinase and TRP-1 in B16F10 melanoma cells. NC: vehicle treated normal control; PC: α-MSH stimulated positive control.

Figure 4. Effect of resveratrol derivative 2a on the expression of tyrosinase and TRP-1 in B16F10 melanoma cells. NC: vehicle treated normal control; PC: α-MSH stimulated positive control.

2.3. Discussion

Botanical ingredients are good sources of medicine, functional foods and cosmetics. They provide numerous compounds with diverse skeletons and biological activities. However, their applications are often limited due to their small amounts, poor bioavailability, etc. Resveratrol is well known for its potential biological activities. As a cosmetic ingredient, it has antioxidant and melanogenesis inhibitory activities. However, it has limitations for cosmetic development, such as chemical instability and low solubility. In addition, the hydroxyl moiety of resveratrol contributes to poor skin absorption. Many attempts have been made to overcome its limitations, and the synthetic derivatives of resveratrol have been suggested as effective in increasing stability and bioavailability [26,28,29,30].

In our present study, we synthesized nine resveratrol derivatives, including five ether derivatives (2a2e) and four ester derivatives (3a3d) and then evaluated melanogenesis inhibitory activity. Our present study showed that all the synthetic ether and ester derivatives of resveratrol inhibited melanin synthesis in melanoma cells. Further study also showed that resveratrol derivative 2a inhibited melanin synthesis in melanoma cells by inhibiting the expression of melanogenic enzymes, tyrosinase and TRP-1 (Figure 2 and Figure 4). However, it showed little effect on tyrosinase activity (Figure 3). Taken together, we suggest that 2a reduced melanin synthesis by the inhibition of melanogenic enzyme expressions rather than direct inhibition on tyrosinase activity.

Skin is a barrier of our body with lipophilic membrane. Therefore, skin absorption of chemical is determined by its physicochemical properties. Lipophilicity is one of the important factors that affect the skin permeation process. Melanocytes and keratinocytes exist in epidermis, the outermost layers of the skin. Thus, lipophilic compounds penetrate deeper in the skin layers [31]. Our synthetic resveratrol derivatives have alkyl or acyl chains instead of free hydroxyl moiety of resveratrol. They exhibit more lipophilic properties than resveratrol, thus these synthetic derivatives are expected to show better bioavailability in skin application. Consistent with our suggestion, triacetyl resveratrol has been reported to have anti-melanogenic activity with better stability [26]. Moreover, resveratrol triacetate reduced hyperpigmented spots in human skin models without any skin irritation [28]. Although the exact mechanism of our synthetic derivatives needs to be confirmed in further animal study, these derivatives might be promising candidates for skin-whitening cosmetics with better practical applications.

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