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Janda, J. Srogl, I. Stibor, M. Nemec and P. Vopatrna, Tetrahedron Lett. Benati, C. Cammaggi and G. Zanardi, J. Muller, H. Kessler, H. Fricke and H. Suhr, Tetrahedron Lett. Schenck and R. Steimetz, Liebigs Ann. Jones, J. Lee and W. Herndon, J. Brion, Tetrahedron Lett. Gris et al. When the reaction was performed in refluxing HCl 2 M solution, the reaction time was 2. Oxazines Oxazines have attracted special attention as pharmaceutical drug candidates and are amply represented in patent literature. The reaction was carried out in microwave equipment for synthesis, and the reactants , , , and were used in a molar ratio of The reactions were also conducted under solvent conditions in refluxing toluene.
If the expected products were effectively present, this method was unsatisfactory in terms of yields and purity. The authors reported that N-alkylaminoethanol associated with aryl boronic acids gave the best results with a decrease in yield in the case of a primary aliphatic or aromatic amines compared to a secondary one. A similar observation was made with styrylboronic acid, while the presence of substituents was better tolerated in position 2 on the aminoalcohol than in position 1. Oxazinones The heterocyclic ring systems 1,3-oxazinones are present in many biologically important natural products, and they have been used as key intermediates in the synthesis of aminoalcohols and as chiral auxiliaries.
The reaction of with under varied conditions failed to give the desired spiro compounds The presence of C-6 methyl was considered crucial for the reaction to take place. To confirm this hypothesis, the reaction was carried out with different anthranilic acids , in order to study the effect of alkyl groups with different chain lengths and unsaturation sites at the C-6 position in anthranilic acid.
Because of their chemical and biological interest, syntheses of various 1,3-thiazines have been reported in the literature, but 1,3-thiazines incorporating a thiol function were hitherto unreported and were not accessible through any of the known synthetic routes for 1,3-thiazines. A diastereomeric ratio of was found for the reactions performed with MW activation, while that of was found for conventional thermal heating.
The authors suggested that the high diastereoselectivity in favor of cis Martins et al.
The wide synthetic versatility of five membered rings containing phosphorus
Triazines Triazines are compounds endowed with selective anticonvulsant, antibacterial, and antimalarial activity. However, Matikainen and Elo reported the synthesis of 3-amino1,2,4-triazines and from the heating of bis amidinohydrazone under solvent-free conditions under Vacuum Scheme The products were easily isolated, being sublimed off from the starting material and forming single crystals.
When unsymmetrical bis amidinohydrazones were used, two isomeric products were obtained; however, the authors did not inform the molar ratio. This method to synthesize 3-amino-1,2,4-triazines and using molecular solvents has not yet been described in the literature. Benzodiazepines Benzodiazepines are an important class of bioactive molecules and are widely used as anticonvulsant, antianxiety, analgesic, sedative, antidepressant, anti-inflammatory, and hypnotic agents.
Initially, a mixture of equimolar amounts of ethyl acetoacetate 3 and aromatic aldehydes 21 was irradiated in a domestic microwave oven to furnish the adduct Scheme The same reaction carried out by heating in ethanol gave only trace amounts of the product even after h of heating. Benzodiazepinediones Benzodiazepinediones are antithrombotics, antitumor and ethanol intoxication antagonists, and antibiotic agents with potential application as enzyme inhibitors and herbicides. Oxathiepinones Oxathiepinones, related isosterically to thiazepinones, are a very interesting class of compounds with a wide spectrum of biological activity.
One of the few synthetic methods available is via thiolysis of styrene oxide by thiosalicylic acid. The best result was obtained under solventfree conditions without using any Lewis acid catalyst or dehydrating agent. Organic solvents are high on the list of toxic or otherwise damaging compounds because of the large volumes used in the industry and difficulties in containing volatile compounds. The number of protocols for the solvent-free synthesis of a wide range of organic compounds and the number of reports of rapid, selective, and efficient transformations, with a high degree of conversion of reactants to products, grow daily.
This indicates that solvent-free reaction conditions may provide a tool for addressing waste-reduction and energy efficiency. Sustainability is an increasingly important issue in the wider context of population, health, the environment, energy, technology, and renewable resources, and, in chemical sciences, as an integral part of the rapidly emerging field called green chemistry.
Reaction Metrics for Green Chemistry Nowadays, it is widely accepted that no solvent is a priori green; rather, its greenness strongly depends on the specific application, its toxicological properties, and the environmental impact resulting not only from the production process but also from its whole life cycle. In order to evaluate the environmental impact of a synthetic organic process, a set of metrics that would enable an assessment of the initial developmental process and allow environmental improvements to be monitored during its developmental stages has been proposed by co-workers in the green chemistry area.
The measures are a mixture of qualitative and quantitative assessments of inputs and outputs for a particular process. The main parameters are briefly discussed as follows: i Atom economya,b This parameter is the ratio of the molecular weight of the target molecule to the sum total of the molecular weights of all the substances produced in the stoichiometric equation for the reaction involved. It allows for the amount of the reactants incorporated into the end product.
For other reactions e. The main use of this parameter is to adapt reaction sequences such that transformations with low atom economy are limited to a minimum. Waste is defined as everything produced in the process except the desired product. It takes the chemical yield Martins et al. In order to arrive at a more meaningful prediction, the E-factor is multiplied by an environmentally hazardous quotient Q. For example, a Q value of 1 can be attributed to NaCl, while heavy metals can be assigned a value between based on their toxicity.
It introduces the important issue of eco toxicity. It takes into account the yield, stoichiometry, solvent, and the reactants used in synthesis. Also, a few unified metrics have been developed that combine some of the above-mentioned individual parameters with relevant factors for specific purposes. These include process parameters, raw material cost, yield, throughput time, throughput volume, number of steps in synthetic sequence, special equipment requirements, reproducibility, tolerance to abuse, linearity of sequence, environmental abuse potential, potential occupational health and safety hazards, raw material availability, susceptibility to regulatory changes, and patent protection.
It consists of three domains: the analysis of the starting material, the analysis of the impact, and the analysis of the improvements. Within each of these parameters, individual penalty points of various relative weights are assigned that take into account all possible situations when setting up an organic chemistry experiment. The penalty points are cumulative for all components of the preparation.
More recently, Reinhardt et al. The above analyses often show that the cost of waste, including effluent treatment, waste disposal, loss of raw materials, etc. Environmental Protection Agency and corporate initiatives to develop their own set of qualitative and semiquantitative green parameters. For example, GlaxoSmithKline has published a set of metrics including carbon efficiency CE and reaction mass efficiency RME , which enables the assessment of batch processes in terms of waste, energy usage, and chemistry efficiency. Another new reaction metric, the stoichiometric factor SF , assesses reactions run under nonstoichiometric conditions.
E-Factor The E-factor will be discussed in detail in this review due to our novel proposal to evaluate it on a laboratory scale. The aim was to use this data to drive the greening of the pharmaceutical industry.
Five-Membered Heterocycles with One Heteroatom
This alternative metric offers an advantage over the E-factor as it gives a mental picture of how wasteful a process is. The E-factor is the actual amount of waste produced in the process, defined as everything besides the desired product. For example, when considering an aqueous waste stream, only the inorganic salts and organic compounds contained in the water are counted, while the water itself is excluded.
Note that this method of calculation will automatically exclude water used in the process but not water formed. Inclusion of water Chemical Reviews, , Vol. E-factor by industry segment. In theory, the E-factor is derived directly from atom efficiency. It can be easily calculated from the quantity of tons of raw material purchased and tons of product sold, for a particular product or a production site or even a whole company, as shown in eqs Thus, the E-factor regularly decreases as the yearly production volume of the given chemical industry increases, in the order: pharmaceutical industry, lowtonnage chemical industry, basic chemical synthesis industry, petrochemical industry Figure 5.
In fact, the production of pharmaceuticals, which demands a greater degree of advanced technologies, nevertheless generates the largest amount of wastes per production unit. This is especially due to the need for multiple-step processes for synthesis and purification with significant volumes of solvents, which then generally cannot be reused due to the high demands made on the purity of the end product.
Figure 6. Synthetic procedure steps to obtain product. Workup of Solvent-free Reactions For the preparation of an organic product to be considered acceptable, it must both be relatively efficient and offer easy workup and purification steps. Although many reactions performed in solvent-free conditions do not require the extraction of the product, they do require extensive purification, especially in catalyzed reactions. One topic that is often neglected in the context of solvent-free synthesis is the effort necessary to remove the reactants or products during the workup procedure, generally consuming a large amount of organic solvent and energy, respectively.
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The solvent extraction volume can exceed the volume of water by factors of up to This deficiency in the literature prevents the complete application of the set of metrics that evaluate the actual environmental impact of any given synthetic organic process. Cyclocondensation Reactions under Solvent-free Conditions As discussed above, solvents play a large role in the waste generated from synthetic processes, leading to negative environmental impacts. In order to evaluate this impact, a set of metrics has been proposed.
These green metrics furnish information to assist chemists in order to choose between alternative routes and obtain cleaner and more sustainable processes. Quantitative and qualitative assessments of input and output for a particular process have been defined by Sheldon et al. As already discussed in this review, the E-factor is designated as the weight of waste generated per weight of product, whereas waste is defined as everything produced in the process except the desired product eqs In this review, the E-factor was applied to evaluate the magnitude of waste generated in cyclocondensation reactions at a laboratorial scale.
TRIAZOLES: A VERSATILE THERAPEUTIC AGENT
In order to calculate the E-factor, it was necessary to consider all the data involved in each synthetic procedure, such as amount of reactants and volume of solvents used in the synthesis or in the workup. After analyzing the papers considered in this review, it was observed that most procedures entailed a workup step.
Percentage of procedures using different synthetic steps for cyclocondensation reactions. This distribution is illustrated in Figure 7. Although the majority of procedures used some workup step, it is important to note that a great many of the synthetic procedures did not inform the amount of solvent used in their protocols. Table 3 compares the Procedure E-factors from protocols carried out under solvent-free conditions and using molecular solvents for the same reaction and the same product. Table 4 shows the Procedure E-factors for all procedures described in this review.
The E-factor calculation was performed by using eq 3 defined by Sheldon with adaptations in accordance with the data available. The E-factor was performed for each individual compound, and the Procedure E-factor was obtained as an average value for a series of individual E-factors.
Compr. Heterocyclic Chem. III Vol. 6 Other Five-membered Rings with Three or more Heteroatoms
Therefore, some compounds that presented values outside of these parameters were excluded from the E-factor calculations. In general, in protocols using water as solvent, the amount of water was excluded from the E-factor calculations, while the amount of water formed in the reaction was normally computed. The data obtained showed that the exclusion of the mass of water from E-factors for cyclocondensation reactions that led to the formation of water in the reaction medium brought about a reduction in the range of 0.
It can thus be concluded that, while the calculation of water formed during the reaction did not significantly alter the E-factor values in protocols using molecular solvent These ranges were based on the data presented in Figure 5. From the data presented in Figure 8, it can be concluded that all E-factors from solvent-free conditions showed values in the range of 0. These elevated values are similar to those for fine chemical and pharmaceutical industries and have been attributed to the classical use of stoichiometric reactants with low atom utilization.
Interestingly, only solvent-free procedures that used solvent in the isolation step showed E-factors higher than On the other hand, in those protocols that either did not carry out an isolation step PIS or did not inform the amount of solvent used in PIS, the E-factors were dramatically reduced to a range of 0. This denotes the power of solvents in the waste generated in the reactions carried out at the laboratorial scale, since the mass of solvent is much larger than the mass of products obtained. In addition, the low E-factors obtained for cyclocondensation reactions are not related with the low volume employed in the reaction or with the low atom utilization but with the absence of solvents in the reaction medium.
Thus, it can be said that most researchers did not use a quality system that accounted for environmental and economic impacts of synthetic procedures. Another important issue is related with the kind of solvent used for isolation and the kind of purification method. Nevertheless, the lack of data concerning the Chemical Reviews, , Vol.
Table 4. Nb E-factor refs SYS 0. Table 5. Percentage of procedures performed under solvent-free and solvent conditions considering the synthesis step SYS. Therefore, the E-factor is a powerful tool for quantitative assessments of input and output for a given process and has played a major role in calling the attention of the worldwide chemical industry to the problem of waste generation in chemical manufacturing.
In this review, this green metric was used to determine the waste generated in cyclocondensation reactions at a laboratory scale, both under solvent- Solvent-Free Heterocyclic Synthesis free conditions and when possible also in the presence of solvents. An alarming finding was that most of the publications did not give a complete report informing all of the data involved in the synthetic procedures. For the workup, little information was reported, a fact that hindered the assessment of the E-factor and the determination of the greenness of a given protocol.
In addition, many solventfree protocols involved the use of organic solvents in the workup step, leading to elevated E-factors. However, if we truly intend to work toward the sustainability of all chemical processes developed, an urgent change is needed in the manner that we report findings. Solvent-Free Conditions and Economic Perspectives Chemical products are generally created using energyintensive processes and nonrenewable, petroleum-based resources as feedstocks in which solvents play a great role.
The synthesis of fine chemicals or pharmaceuticals, widely carried out in batch processes, implies many successive steps: reaction, separation, isolation, and purification. The solvent is chosen according to the reactants and the reaction characteristics.
Each reaction thus has a given optimal solvent that satisfies at the same time objectives of selectivity and solubility, safety constraints, and economic and environmental criteria. Therefore, the solvent generally differs from one reaction step to another. Consequently, the solvent often has to be switched before beginning a new reaction step.
In the industry, solvent replacements are usually carried out by evaporation or distillation operations in the batch reactor used during the reaction. The detailed procedure depends on the reactor equipment in terms of the overhead distillation column and control loops but generally imitates the laboratory methodology developed by the chemist. Consequently, the industry can count on procedures that are robust and reliable but that are also slow and consume large amounts of solvent.
Cost distribution studies have shown that the operating costs, which include mainly time, energy, equipment, and personnel, represent the main part of the global cost for an industrial process. In addition, by avoiding the generation of pollution, industries can reduce or eliminate the costs of regulation, reduce materials use, and significantly reduce the risks associated with manufacturing and the use of chemical processes and products.
Two examples illustrate this tendency. The first is the redesign of the sertraline manufacturing process. Additionally, the process changes eliminated the need to use, distill, and recover four toxic solvents dichloromethane, tetrahydrofuran, toluene, and hexane from the original synthesis, since ethanol became the sole solvent. It is worth noting that the redesign of the sertraline manufacturing process received a Presidential Green Chemistry Challenge Award in In regard to costs involved in laboratory-scale syntheses, it has been shown that conventional solvents are easy to obtain and relatively cheap, making the global costs mainly defined by the supply of the starting materials and dependent on the performance of the reaction media.
Larger companies budget close to U. Conclusions After having examined all the cyclocondensation reactions described in this review, it is necessary to return to some central issues: i whether solvent-free conditions are better than molecular solvents for cyclocondensation reactions; ii whether the chemical mechanistic aspects of solvent-free reactions discussed in the literature are associated with cyclocondensation reactions; iii whether solvent-free cyclocondensation reactions are truly green. We hope to have shed some light on these issues.
The data from the papers reviewed here clearly show that the use of solvent-free conditions in heterocyclic synthesis generally leads to similar or higher yields and a reduction in the reaction time when compared to the same reaction performed in the presence of molecular solvents. In many of the reactions, microwave equipment was used as the activation mechanism, and this raises several polemic issues: the type of equipment used for reaction, the device used to measure the reaction temperature, the reproducibility of results, and the existence of nonthermal microwave effects.
We concluded that all reactions performed in MW, both domestic oven and synthesis equipment, presented better yields and shorter reaction times than those not carried out in MW. However, the advantages of microwave domestic ovens are undermined by their poor reproducibility. In terms of temperature measurements and the existence of nonthermal effects, we believe that more research is necessary, and at present we consider that the existence of nonthermal effects cannot be excluded. In regard to the environmental aspects of cyclocondensation reactions under the solvent-free conditions described in this review, some of the reports described as solvent-free protocols used organic solvents in the isolation step, and it Martins et al.
The E-factor for solvent-free reactions presented in this review showed values in a range of 0. This leads us to conclude that, even considering only the synthetic steps, the use of solvent-free conditions contributed to the reduction of total waste generated during a given synthetic process. Only a few of the papers presented information about the exact amount of solvents used in product isolation. None of the papers informed the amount of solvent expenditure in the purification of compounds.
This deficiency impedes a complete evaluation of the actual environmental impact of specific synthetic organic processes, denoting that most researchers did not operate with a quality system that encompassed the environmental impact of synthetic procedures. In this sense, there is an urgent need for changes in the manner that papers are reported to avoid the risk of green chemistry and solvent-free becoming nothing more than buzzwords for publishing papers in the academic medium.
From an economic perspective, the use of solvent-free conditions may lead to a decrease in the total cost of synthetic processes. Industrially speaking, one of the main costs involved in a process is related to the use of solvents. Moreover, reducing the amount of solvent usage can reduce the costs involved in waste treatment. In addition, by reducing the use of solvents, industries may reduce or eliminate the cost of regulation and material use and significantly reduce the risks associated with the manufacture and use of chemicals, giving rise to the production of clean products or processes in a cost-competitive manner.
Moreover, considering the importance of the cyclocondensation reaction, the main reaction employed in heterocyclic synthesis, the information presented in this review clearly illustrates the substantial advances achieved over the past decade in solvent-free reactions, good reactivities and selectivities, simplified workup steps, and generally cleaner, more economical, and safer protocols.
An investigation on the Web of Science showed a proliferation of papers in which the topic solvent-free appeared. This demonstrates the increase in new researchers in the area.
We hope to have given a clear idea of the applicability of solvent-free conditions in cyclocondensation reactions to obtain heterocyclic compounds. We would like to conclude with an optimistic view for the future expansion of synthetic procedures in solvent-free media. This positive outlook comes from the certainty that the results reported here will be but the beginning of a great advance in this promising field in the near future. The authors thank Anna Clara L. Martins for many stimulating discussions and for her help in the cover art conception.
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