Exploring the reaction of multifunctional allylic bromides with N , S - dinucleophiles: isothiuronium salts and analogs as useful motifs to assemble the 1,3-thiazine core

The reactivity profile of allylic bromides (derived from the Morita-Baylis-Hillman reaction) towards thiourea derivatives and further transformations of the resulting isothiuronium bromides are described. Isothiuronium salts, prepared in near quantitative yields, undergo a selective acetylation or a base-promoted intramolecular cyclization to give 2-amino-1,3-thiazin-4-ones in good overall yields. Besides the simplicity of the reaction, conditions, and purification steps, the methods presented here furnished high-purity products. The structural characterization of representative compounds was unequivocally confirmed by X-ray diffraction analysis


Introduction
Isothiuronium salts have attracted the attention of the scientific community due to their unique chemical and biological properties. 1 They have been explored as prodrugs of the alcoholdeterrent cyanamide, 2 as agonists of GABA-type receptors, 3 as local anesthesiophores, 4 and as cationic detergents for the template-assisted assembly of plasmid DNA into stable nanomeric particles. 5Isothiuronium-containing molecules are also widely employed in the selective recognition of anions, 6 particularly oxo-anions such as phosphates, sulfates and carboxylates. 7sothiuronium salts are typically obtained through the displacement reaction of alkyl halides with thiourea and serve as convenient starting materials for the preparation of thiols due to their propensity to undergo cleavage of the amidino group by base-mediated solvolysis (Scheme 1a). 8t is known that the rate of the cleavage of the isothiuronium group is highly pH-dependent and occurs very fast even under mild alkaline conditions. 5However, the fate of the isothiuronium salt as an electrophile in aqueous alkaline medium can be dramatically changed if other competitive electrophilic sites are available.For instance, the well-studied synthesis of 2-aminothiazoles 9 from -bromoketones and thiourea derivatives involves the intermediacy of reactive isothiuronium salts that undergo cyclization even in the absence of a base 10 (Scheme 1b).
While five-membered rings can be made available using this methodology, the homologous cyclization to afford the 1,3-thiazine framework has been rather underexplored, studies being restricted to simple -haloacids such as 3-chloropropionic acid 11 (Scheme 1c).

Scheme 1
As part of our research interest in synthetic transformations 12 involving Morita-Baylis-Hillman adducts 13 1, we recently reported that reaction of diverse allylic bromides 2 with thiourea in aqueous acetone and subsequent treatment of the intermediate isothiuronium salts with aqueous bicarbonate furnished 2-amino-1,3-thiazin-4-ones 3 in high yields under mild conditions 14 (Scheme 2).

Scheme 2
Herein, we present a full account of the preparation of isothiuronium salts and 2-amino-1,3thiazin-4-ones derived from Morita-Baylis-Hillman adducts, their selective acetylation in aqueous alkaline medium and further transformations wherein the thioureido unit plays a direct role.

Results and Discussion
Preparation of isothiuronium salts Treatment of allylic bromides 2a-h (readily available from the bromination of Morita-Baylis-Hillman adducts 1 with LiBr in acidic medium 12a,c ) with 0.95 equiv of thiourea 4a (Figure 1) in aqueous acetone at room temperature furnished, in near quantitative yields, the corresponding isothiuronium salts 5a-h, which were easily isolated in pure form after an operationally simple work-up consisting of evaporating the solvent followed by trituration of the residue with a mixture of ether/CH2Cl2 (Scheme 3 and Table 1).Salts 5a-h are stable crystalline solids that can be stored for months at 0 o C without any noticeable decomposition.Considering that thiourea is an ambident nucleophile, either the sulfur or one of the nitrogen atoms is able to participate in a nucleophilic attack on one of the several electrophilic sites available in the allylic bromides 2. Therefore, caution must be taken before assuming any product structure, especially due to the frequency with which the reactions of thiourea with electrophilic reagents have led to products whose structures have been subsequently revised. 15esides their full spectroscopic characterization, mainly based upon the characteristic signals for the SC(=NH)NH and CH2S groups (> 170 and < 30 ppm, respectively) in the 13 C NMR spectra of all purified products, the structural elucidation of isothiuronium salts 5a-h was further confirmed by X-ray crystallographic analysis 16 after the isolation of isothiuronium 5b as a single crystal from ethanol (Figure 2).
The reactivity profile of allylic bromides 2a-h was further studied with a non-symmetric S,Ndinucleophile, namely N-phenylthiourea 4b.In this case, the use of aqueous acetone as the solvent system did not lead to acceptable results.However, a smooth conversion to the expected S-alkylated salts 6 was achieved in acetonitrile, which also facilitated the work-up because the products precipitated out and were separated by simple filtration, requiring no further purification (Table 1).In spite of the mild conditions and high yields obtained with thioureas 4a,b, allylic bromides 2 were not reactive in the presence of the N-benzoylated thioureas 4c or 4d (Figure 1), probably due to their lower nucleophilicity.

Acetylation of isothiuronium salts
The unusual behavior of isothiuronium salts and their application as key intermediates in various transformations for the preparation of sulfur-containing compounds provided the motivation for further studies of the reactivity of allylic salts 5 and 6.Recently, mono-and diacylation of isothiuronium salts in basic medium has led to useful building blocks and Nacylimidocarbamates of biological importance. 17,18ue to the high propensity of isothiuronium salts 5a-h to readily cyclize to 2-amino-1,3thiazin-4-ones 3 in basic medium (see below), acetylation was carried out by mixing 5 and an excess of acetic anhydride in CH2Cl2 followed by quenching the mixture by the addition of aqueous NaHCO3.This resulted in a smooth transformation wherein a pre-formed isothioureido intermediate was intercepted by Ac2O to give N,N'-diacetylated products 7a-h in excellent yields (Scheme 4 and Table 2).No signal of cyclization products was detected.It is worth noting that the collection of the 13 C NMR spectroscopic data for the diacetylated products 7 in CDCl3 was not straightforward due to the absence of the characteristic signals for the acetyl groups at 25.0-29.0(CH3) and 179.0-185.0(C=O) ppm.This suppression was probably a manifestation of stereo-electronic effects related to the nitrogen inversion barrier. 19Nevertheless, this problem was conveniently solved by acquiring all spectra in DMSO-d6.A single crystal of 7b was collected from ethanol and was analyzed by X-ray methods, confirming the proposed structure (Figure 3a).Other reliable conditions for the diacetylation with Ac2O in basic medium include the use of pyridine in excess or a combination of DBU in CH3CN, although the conditions with aqueous NaHCO3 gave cleaner reactions.On the other hand, employing a 2 M NaOH solution led not only to the formation of diacetylated products 7 but also the concurrent hydrolysis of the S-CN bond to give 20,21 a mixture of thiol 8 and disulfide 9 as by-products in 10-25% combined yield after chromatography (Scheme 5).However, efforts to increase the formation of the thiol/disulfide mixture by changing the reaction conditions, the work-up, or the starting materials 5 or 7 were unsuccessful.

Scheme 5
Acetylation of N-phenylisothiuronium salts 6 under the aqueous conditions described above (Ac2O/CH2Cl2-NaHCO3/H2O) led to somewhat different results.In the three cases studied, the major product observed was the monoacetyl isothiourea 10 (Scheme 4 and Table 2, entries 9-11).The corresponding 4-methoxy-and 4-nitro-substituted isothioureas 10b and 10d were easily separated from the unidentified minor by-products by recrystallization from ethyl acetate or by chromatography.The unequivocal determination that the unsubstituted NH group was selectively acetylated and not the N-aryl-substituted one was achieved by single crystal X-ray methods after recrystallization of each monoacetylated product 10b and 10d from ethyl acetate (Figure 3b,c).Unfortunately, all attempts to purify the phenyl-substituted derivative 10a, either by crystallization or column chromatography, failed due to extensive decomposition.Other basic conditions were also tested without success.
Unexpectedly, running the acetylation of the nitro-substituted salt 6d with acetic anhydride under basic medium at room temperature for longer periods (3-15 days) allowed the formation of the rearranged thiourea 11d (Figure 4) in up to 31% yield, together with the expected monoacetylated isothiourea 10d as the major product (ca.60-68% yield).All spectral data are consistent with the proposed structure for the by-product 11d, mainly based upon the appearance of typical signals for the terminal olefinic moiety (CH2=) at around 6.0-6.5 ppm in the 1 H NMR spectrum, as well as the existence of a pair of doublets (J = 8.4 Hz), which establishes the connectivity between C3-H (6.78 ppm) and the NH group (12.64 ppm, D2O-exchangeable).The unexpected aza-Morita-Baylis-Hillman adduct 11d may be formed by a 3,3-sigmatropic rearrangement of the monoacetylated isomer A (Fig. 4), which could be generated by a competitive acetylation of the isothiuronium salt 6d at the less preferred N-phenyl site.Evidence for the formation of intermediate A was found in the 1 H and 13 C NMR analysis of the crude reaction in the early stages, although its isolation was not possible due to a low stability.In addition, all attempts to convert the major product 10d into the rearranged 11d (by a phenyl or acetyl group migration from one nitrogen atom to the other) failed, but further studies will be necessary to unravel the mechanistic process associated with the formation of by-product 11d.

Figure 4
Cyclization to thiazinones 1,3-Thiazines constitute an important class of compounds in organic chemistry, and are common structural motifs in numerous bioactive molecules. 22Thiazines decorated with reactive functional groups, such as 2-amino-1,3-thiazin-4-ones, have recently attracted much attention due to their versatile applications as synthetic intermediates. 23According to our preliminary report, 14 the one-step reaction of allylic bromides 2a with thiourea and subsequent addition of NaHCO3 as an acid scavenger gave 2-amino-1,3-thiazin-4-ones 3 as insoluble solids (Scheme 6).The results supported by NMR (taken in deuterated trifluoroacetic acid due to the very low solubility of thiazinones 3 in the majority of usual NMR solvents) were further validated by X-ray analysis after the isolation of 1,3-thiazin-4-one 3a as a single crystal. 14Therefore, in the absence of an external electrophile (such as Ac2O) an intramolecular attack of the nucleophilic isothioureido group on the carboxylate moiety takes place that ultimately produces the cyclized product 3 exclusively.Under these conditions, no product of alkaline hydrolysis (thiols, disulfides) was detected in the crude reactions (checked by NMR).

Scheme 6
This practical and straightforward reaction was extended to a variety of allylic bromides 2 and similar results were observed (Table 3).NaHCO3 was the base selected, but other bases such as NaOH and Et3N were also suitable promoters of this transformation.
Although providing high yields and a simple work-up for most of the cases studied, the onestep procedure did not give a clean conversion to the naphthyl derivative 3i for which an acceptable match was not found in the elemental analysis.On the other hand, a two-step protocol, consisting of isolating the isothiuronium intermediate 5i prior to the basic treatment, not only produced the product 3i with analytical purity but also in higher overall yield (entry i).In a few other cases studied, the two-step procedure also proceeded cleanly to give the expected thiazinones 3 in high purity, although the yields were usually lower than in the one-step strategy.
Unfortunately, the presence of the phenyl groups in the N-phenylisothiuronium salts 6 was detrimental to the intramolecular cyclization process and no expected thiazinone was observed after several trials under a variety of basic procedures.Because of the high reactivity associated with N2-arylated 2-amino-1,3-thiazin-4-ones in aqueous medium, 24 harsher conditions led to a complex mixture of products.

Conclusions
Reactions of allylic bromides 2 with thiourea (or N-phenylthiourea) in aqueous medium offer a smooth, efficient, and clean protocol to obtain multifunctionalized isothiuronium salts 5 (or 6) which were able to participate in selective acetylation reactions in the presence of Ac2O to give diacetylated products 7 (or monoacetylated derivatives 10), according to the substitution pattern of the thioureido framework.Isothiuronium salts 5 also participate in a base-promoted intramolecular cyclization to 2-amino-1,3-thiazin-4-ones 3.One of the most remarkable aspects of these methodologies is their simplicity in terms of setting up the reaction as well as the subsequent work-up and purification steps, affording in most cases high-purity products in nearly quantitative yields.The potential applicability of these multifunctionalized allylic products derived from the Morita-Baylis-Hillman reaction in further synthetic transformations is currently under research.

Experimental Section
General.All chemicals were of reagent grade and were used as received.Melting points were determined using a Microquímica MQPF301 apparatus and are uncorrected.Infrared spectra were acquired with a Perkin-Elmer FT-IR 1600 spectrometer (range 4000-400 cm -1 ) using KBr for solids and film for liquid samples. 1 H NMR (400 MHz) and 13 C NMR (100 MHz, fully decoupled) spectra were recorded with a Varian AS-400 spectrometer.Samples were prepared in an appropriate deuterated solvent (DMSO-d6, CDCl3, D2O, or TFA-d1).Chemical shifts are reported in parts per million ().Coupling constants (J) are measured in Hertz (Hz) and coupling patterns are designated as s (singlet); d (doublet); t (triplet); q (quartet); m (multiplet); brs (broad signal).X-ray analysis was carried out with an automatic diffractometer for monocrystals ENRAF NONIUS CAD-4.Elemental analyses were conducted in a CHNS-O Carlo Erba EA-1110 by UFSC-Central Analítica, Departamento de Química, Florianópolis, SC, Brazil.Purifications by column chromatography were performed with silica gel (Aldrich, 100-200 mesh particle size).The allylic bromides 2 12a-c and thiourea derivatives 25 4b, 4c and 4d were prepared according to the described methods.Detailed characterization of thiazin-4-ones 3a, 3g and 3j was previously presented. 14pical procedure for the synthesis of isothiuronium bromides (5) To a stirred solution of allylic bromide 2 (1.0 mmol) in 5.0 mL of acetone/H2O (4:1 v/v) at 25 ºC was added 0.95 mmol of thiourea.After stirring for 1 h, the solution was concentrated under reduced pressure and the resulting solid was crushed with ethyl ether/CH2Cl2 (4:1) and filtered to give pure isothiuronium salt 5.

Typical procedure for the synthesis of N-phenylisothiuronium bromides (6)
To a stirred solution of allylic bromide 2 (1.0 mmol) in 5.0 mL of acetonitrile at 25 ºC was added N-phenylthiourea (0.95 mmol).After stirring for 1 h, the insoluble solid formed was separated by filtration and washed with acetonitrile to give pure N-substituted isothiuronium salts 6.

Typical procedure for the acetylation of isothiuronium bromides (5)
To a stirred suspension of isothiuronium bromide 5 (1.0 mmol) in 3.0 mL of CH2Cl2 at 25 ºC was added acetic anhydride (3.0 mmol) followed by sat.NaHCO3 (3.0 mL).After stirring for 15 min, the final mixture was diluted with CH2Cl2 and the organic extract was washed with H2O and brine, dried over Na2SO4, filtered and concentrated under reduced pressure to give the corresponding acetylated products 7.

Typical procedure for the acetylation of isothiuronium bromides (6)
To a stirred suspension of the isothiuronium bromide 6 (1.0 mmol) in 3.0 mL of CH2Cl2 at 25 ºC was added acetic anhydride (3.0 mmol) followed by sat.NaHCO3 (3.0 mL).After stirring for 20 min, the final mixture was diluted with CH2Cl2 and the organic extract was washed with H2O

Synthesis of 2-amino-1,3-thiazin-4-ones (3)
From the allylic bromides (2).To a stirred solution of allylic bromide 2 (1.0 mmol) in 4.0 mL of acetone/H2O (3:1 v/v) at 25 ºC was added 2.0 mmol of thiourea.After stirring for 1-3 h, the final mixture was extracted with CH2Cl2 and the organic extract was discarded.The aqueous phase was then treated with 5.0 mL of sat.NaHCO3 and the remaining basic solution was immediately extracted with CH2Cl2.The organic extract was concentrated under reduced pressure and the resulting solid was washed with H2O, EtOH and acetone to give the corresponding 2-amino-1,3-thiazin-4-one 3.
From the isothiuronium bromides (5).To a stirred suspension of isothiuronium salt 5 (1.0 mmol) in 3.0 mL of CH2Cl2 was added sat.NaHCO3 (3.0 mL).After stirring for 15 min, the final mixture was diluted with CH2Cl2 and the organic extract was washed with H2O and concentrated under reduced pressure.The solid formed was crushed with CH2Cl2 and filtrated to give the corresponding thiazin-4-one 3. (5Z)-2-Amino-

Table 1 .
Synthesis of isothiuronium salts 5 and 6 a Isolated yields.
brine, dried over Na2SO4, filtered and concentrated under reduced pressure.The resulting residue was purified by recrystallization with ethyl acetate to give the corresponding products 10.