Scholarly article on topic 'Utility of 4-hydroxythiocoumarin in organic synthesis'

Utility of 4-hydroxythiocoumarin in organic synthesis Academic research paper on "Chemical sciences"

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{4-Hydroxythiocoumarin / Synthesis / "Chemical reactivity" / Tautomerism / Reactions / Heterocycles}

Abstract of research paper on Chemical sciences, author of scientific article — Moaz M. Abdou

Abstract This review provides detailed methods for the synthesis, structures and chemical properties of 4-hydroxythiocoumarin and its most valuable bioactivities are mentioned. This compound represents easily accessible key educts for the synthesis of heterocyclic systems, which exhibit interesting biological activities in various fields.

Academic research paper on topic "Utility of 4-hydroxythiocoumarin in organic synthesis"

Arabian Journal of Chemistry (2014) xxx, xxx-xxx

King Saud University Arabian Journal of Chemistry

www.ksu.edu.sa www.sciencedirect.com

REVIEW

Utility of 4-hydroxythiocoumarin in organic synthesis

Moaz M. Abdou *

Egyptian Petroleum Research Institute, Nasr city, P.O. 11727, Cairo, Egypt Received 15 November 2013; accepted 2 June 2014

KEYWORDS

4-Hydroxythiocoumarin; Synthesis;

Chemical reactivity; Tautomerism; Reactions; Heterocycles

Abstract This review provides detailed methods for the synthesis, structures and chemical properties of 4-hydroxythiocoumarin and its most valuable bioactivities are mentioned. This compound represents easily accessible key educts for the synthesis of heterocyclic systems, which exhibit interesting biological activities in various fields.

© 2014 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

Contents

1. Introduction........................................................................................................................................................00

2. Molecular structures and spectral properties..........................................................................................................00

3. Tautomeric structure(s)........................................................................................................................................00

4. Chemical reactivity..............................................................................................................................................00

5. Synthesis............................................................................................................................................................00

5.1. Using acetophenones......................................................................................................................................00

5.2. Using thiophenol ..........................................................................................................................................00

5.3. Hydrolysis and decarboxylation of 3-carbethoxy-4-hydroxythiocoumarin............................................................00

6. Chemical reactions..............................................................................................................................................00

6.1. Reactions involving carbon—carbon bond formation........................................................................................00

6.1.1. C-C bond formation...............................................................00

6.2. Reactions involving carbon—heteroatom bond formation ..................................................................................00

6.2.1. C-N bond formation...............................................................00

6.2.2. C-O bond formation...............................................................00

* Tel.: +20 1000409279. E-mail address: moaz.chem@gmail.com Peer review under responsibility of King Saud University.

^jjfl I

Elsevier I Production and hosting by Elsevier

1878-5352 © 2014 King Saud University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.arabjc.2014.06.002

2 M.M. Abdou

6.2.3. Carbon—halogen bond formation............................................................................................................00

7. Synthesis of monocyclic heterocyclic compounds....................................................................................................00

7.1. Synthesis of five-membered systems with two heteroatoms................................................................................00

7.1.1. 1,2-Benzisothiazoles................................................................................................................................00

8. Synthesis of fused heterocyclic compounds............................................................................................................00

8.1. [5-6] Ring system..........................................................................................................................................00

8.1.1. Dihydrofuran and furocoumarins............................................................................................................00

8.1.2. Benzothiopyrano[4,3-d]oxazolone..............................................................................................................00

8.2. Fused [6-6] ring system..................................................................................................................................00

8.2.1. Pyrano benzothiopyran............................................................................................................................00

8.2.2. Synthesis of amino-substituted thiochromenopyranones ....................................... 00

8.2.3. Benzothiopyranooxazine-5-one................................................................................................................00

9. Conclusion..........................................................................................................................................................00

Acknowledgements..................................................................................................................................................00

References ..............................................................................................................................................................00

1. Introduction

The 4-hydroxythiocoumarin has aroused great interest in recent years owing to its wide variety of biological and pharmacological properties (Jung et al., 1999). Importance of it has been amplified because, not only there are significant synthetic end products, but also it constitutes the structural nucleus of many natural products (Jung et al., 2000, 2001).

In spite of the fact that the chemistry of 4-hydroxythiocoum-arin has undergone a development in the last three decades, no review has so far been published concerning the chemistry of this compound. In the review presented, we attempted to correlate the published material relating to methods of synthesis, properties, and reactions of 4-hydroxythiocoumarin.

2. Molecular structures and spectral properties

The structures of 4-hydroxythiocoumarin have been assigned by UV, IR, NMR and MS spectroscopies (Jung et al., 2000, 2001; Nakazumi and Kitao, 1977; Lau et al., 1987; Nakazumi et al., 1983). The UV spectrum of 4-hydroxythio-coumarin revealed two intense absorption bands at 232 and 320 nm (Nakazumi and Kitao, 1977). These absorption spectra have been interpreted with the aid of PPP-type calculation (Rath and Rajagopal, 1971). Also, the striking feature of its infrared spectrum is the strong bands of double bonds of the cyclic system (1580, 1550 and 1520 cm-1) and stretching vibrations of the carbonyl group (1620 cm-1) Nakazumi et al., 1983.

The proton NMR spectrum of 4-hydroxythiocoumarin (Jung et al., 2001) revealed only one signal is observed as a singlet at 6.26 ppm, typical chemical shift for hydrogens on non-aromatic double bonds and no other signal is observed with the exception of the four aromatic hydrogens (H-5, H-6, H-7, H-8) at 7.83-7.63 and 8.32 ppm.

The mass spectrum (Nakazumi and Kitao, 1977) of 4-hydroxythiocoumarin (Fig. 1) indicated that the base peak was due to M-28. The main fragmentation of it involved the ejection of carbon monoxide followed by the loss of methyl radical and carbon monoxide. The sequence may be rationalized as in Scheme 1. There are two fragmentation pathways. One is the loss of a neutral ketene by the retro-Diels-Alder reaction, followed by the loss of carbon monoxide (1a fi 1b fi 1c). The other is the loss of carbon monoxide from the molecular ion, which gives 1d as the base beak of the spectrum (1a fi 1d fi m/e 121 or 105).

3. Tautomeric structure(s)

4-Hydroxythiocoumarin can exist in three tautomeric keto-enol forms namely, 4-hydroxy-2-thiochromenone (A), 2,4-thiochro-mandione (B), and 2-hydroxy-4-thiochromenone (C) (Fig. 2). These three possible prototropic transformations have been intensively examined by various chemical reactivity, spectral, and computational methods (Jung et al., 2000, 2001; Nakazumi and Kitao, 1977; Lau et al., 1987; Nakazumi et al., 1983; Rath and Rajagopal, 1971; Arjunan et al., 2013).

S ^O 1

S ^O 1a

-CHO or -CHS

m/e 121 or 105

Figure 1 Mass spectrum of 4-hydroxythiocoumarin.

4. Chemical reactivity

It is evident from the topography of 4-hydroxythiocoumarin (Fig. 3) that it possesses both electrophilic and nucleophilic properties. The most significant reactivity is the nucleophilic-ity of the carbon atom at the third position, because of the influence of the hydroxyl group with electron-donating properties and electron-withdrawing effects of the carbonyl oxygen atom at the second place. These factors make the third position in the thiocoumarin ring very convenient for many reactions. The oxygen atom of the hydroxyl group however remains the main site for attack by alkylating agents. It seemed that hard nucleophiles attack preferentially

-CH3CO

Utility of 4-hydroxythiocoumarin

nrMe + RAR -!fn.-

R R toluene

S ^O 1

3 a b c d e f g h i j

R OEt OEt OEt OEt OMe OMe OMe OMe Cl Cl

Base NaOEt NaH NaAPA KAPA KAPA NaAPA NaH NaOEt NaH NaOEt

Yield (%) 81 86 55 62 58 61 81 74 66 62

Scheme 1

Figure 2 Possible tautomeric structures of 4-hydroxythiocoum-arin 1 (A-C).

O/ 90 °C O ^ I y AorB rfVS 4 h * ho^^s"^ '

A : Eaton's reagent, 70 °C. B : 116% PPA, 120 °C.

Scheme 3

Figure 3 Chemical reactivity of 4-hydroxythiocoumarin.

the oxygen atom, while soft ones attack preferentially the carbon atom (Fig. 3).

| ^O HCl, EtOH /H2O , '' L Heating, 2h " 0

Scheme 4

1 (91 %)

5. Synthesis

Several syntheses were reported in the literature (Jamkhandi and Rajagopal, 1967; Vishnyakova et al., 1979; Ruwet et al., 1970; Szell et al., 1969; Szell, 1967). Most of these methods are based on the Friedel-Crafts acylation of thiophenol in which Lewis acids such as ZnCl2, AlCl3 and POCl3 are used. These Lewis acids form an intractable solid mass during the reaction which makes stirring of the reaction mixture and product isolation considerably difficult. As a consequence, they suffer from drawbacks such as tedious work-up, poor yield and use of expensive reagents. However, many synthetic approaches that furnished 4-hydroxythiocoumarin in high yields have been reported.

5.1. Using acetophenones

+ Ph^Ph

HCl, 130 °C

Scheme 5

S O 10

R: 4-OMe, H, 4-Cl, 4-NO2

S O O S 12

The reaction of 2-mercaptoacetophenone 2 with acylating agents 3 such as phosgene, dimethylcarbonate, or diethylcar-bonate in the presence of stoichiometric amounts of base in

r^y "Me NaH, DMF. 4

|| Me NaH, DMF- ||

^J reflux, 18 h L

Scheme 2

Scheme 6

anhydrous toluene or xylene afforded 4-hydroxythiocoumarin 1 in variable yields (Scheme 1). It was found that sodium hydride was the most effective base among sodium ethoxide, sodium metal, freshly prepared sodium 3-aminopropylamide (NaAPA), and potassium 3-aminopropylamide (KAPA) Jung et al., 2001.

On the other hand, base-catalyzed cyclization of S-(2-acet-ylphenyl)dimethyl thiocarbamate 4 with sodium hydride in dimethylformamide (DMF) in the absence of air furnished 4-hydroxythiocoumarin 1 (Lau et al., 1987) (Scheme 2).

M.M. Abdou

Ph—N 11

// Ph _

-PhNH2

s' ~O 14

S' *0 15

Scheme 7

HC(OEt)3 k /-PrOH, reflux

16,17 rnh2 time (hr) Yield (%)

a n-Butylamine 18 99

b Ethanolamine 5 97

c W,W-Dimethylethylenediamine 19 32

d Aniline 5 72

e 2-Trifluoromethylaniline 21 72

f 2-Bromoaniline 3 73

g 3-Bromoaniline 3 69

h 4-Bromoaniline 3 78

i 4-Aminophenol 2 65

j 1-Aminonaphthalene 4 72

k 2-Benzylamine 6 98

l 4-Methoxybenzylamine 6 70

m Glycine 20 90

n Urea 2 78

o Ethylurea 2 70

P Methyl carbamat 11 52

q Ethyl carbamate 23 32

S ^O 18

Scheme 8

HNO3 H2SO4 / AcOH*

Scheme 9

1 19 20 (55%)

Scheme 10

5.2. Using thiophenol

Lee et al. developed a simple and efficient modification of Pechmann condensation leading to 4-hydroxythiocoumarin 1. Thus, treatment of thiophenol 5 with Meldrum acid 6 under solvent-free conditions afforded phenylsulfonylcarbonyl acetic

S ^O 1

^COß/acetone heating

Scheme 11

S ^O 22

osA\ f

// VS-ci-^asvfy^

■■ stirring, r.t. L^

O - ^ -S'

1 23 24

Base: pyridine or triethylamine in dichloromethane

Scheme 12

acid 7 in 67% isolated yield, that transformed to 4-hydroxy-thiocoumarin 1 upon treatment with Eaton's reagent or polyphosphoric acid (PPA) Park et al., 2007 (Scheme 3).

5.3. Hydrolysis and decarboxylation of 3-carbethoxy-4-hydroxythiocoumarin

Another elegant approach to attain 4-hydroxythiocoumarin 1 in high yields was reported by Jung and coworkers via the acid-catalyzed hydrolysis and decarboxylation of 3-carbeth-oxy-4-hydroxythiocoumarin 8 (Jung et al., 2000) (Scheme 4).

6. Chemical reactions

6.1. Reactions involving carbon—carbon bond formation 6.1.1. C—C bond formation

6.1.1.1. C3-Benzylation. Ziegler et al. reported that direct C3-benzylation of 4-hydroxythiocoumarin 1 with diphenyl-methanol 9 was achieved using 1,1,2,2-tetrachloroethane containing a catalytic amount of hydrogen chloride (Ziegler and Roßmann, 1957) (Scheme 5).

Reaction of 4-hydroxythiocoumarin 1 with Schiff bases 11 in acetic acid under reflux yielded the corresponding benzyli-dene-bis-4-hydroxythiocoumarin derivatives 12 (Merchant and Martyres, 1983) (Scheme 6).

It may be postulated that this reaction is likely to involve the initial formation of benzaldehyde 13 via "hydramine" cleavage of 11 that is followed by Aldol condensation of benzaldehyde with 1 to provide an intermediate hydroxyl compound 14 which loses water to afford 15. Michael addition of 15 on 1 gives the expected product 12 (Scheme 7).

6.1.1.2. Condensation with amines. The one-pot synthesis of 3-aminomethylenethiochroman-2,4-dione derivatives 17 was

reported by Park and Lee (Park and Lee, 2004) upon condensation of 4-hydroxythiocoumarin 1 with not only primary amines but also primary amino group-containing substrates, such as a-aminoacids, carbamates, and ureas 16, in the presence of triethyl orthoformate. Whereas the reaction proceeded smoothly with the substrate containing the primary amino group, the condensation with secondary amine was fruitless. It is also noteworthy that triethyl orthoacetate, in contrast to orthoformate, did not undergo a three-component condensation even with primary amines (Scheme 8).

6.2. Reactions involving carbon—heteroatom bond formation

6.2.1. C—N bond formation

6.2.1.1. Nitration reaction. Nitration of 4-hydroxythiocouma-rin 1 with fuming nitric acid in a mixture of glacial acetic acid and concentrated sulfuric acid afforded 3-nitro-4-hydroxythio-coumarin 18 (Nakazumi et al., 1983; Peinhardt and Reppel, 1973) (Scheme 9).

6.2.2. C—O bond formation

6.2.2.1. O-Alkylation reaction. 4-Allyloxythiocoumarin 20 was obtained by the reaction of 1 with allylic bromide 19 in acetone containing catalytic amounts of anhydrous potassium carbonate (Majumdar et al., 1989) (Scheme 10).

Also, Majumdar et al. have succeeded in preparing thio-coumarin-4-yl-prop-2-ynyl ether 22 via refluxing of 1 with propargyl bromide 21 in dry acetone and potassium carbonate (Majumdar et al., 1989) (Scheme 11).

1 35 36 (88 %)

Scheme 19

6.2.2.2. Tosylation reaction. Several groups have developed general methodologies for the one step formation of 4-(p-tolu-enesulfonyloxy)thiocoumarin 24 via tosylation reaction of 4-hydroxythiocoumarin 1 with tosyl chloride 23 in the presence of base at room temperature (Majumdar et al., 2006; Soltau et al., 2005; Valente and Kirsch, 2011; Inhulsen et al., 2011; Majumdar and Pal, 2009) (Scheme 12).

6.2.3. Carbon—halogen bond formation

Halogenoheteroarenes are useful intermediates for the syntheses of bioactive natural products and pharmaceutical drugs.

6.2.3.1. Bromination. The direct bromination of 4-hydroxy-thiocoumarin 1 with bromine in acetic acid (MacKenzie and Thomson, 1982) at low temperatures yields 3-bromo-4-hydroxythiocoumarin 25 (Scheme 13).

6.2.3.2. Chlorination. Majumdar and Samanta have demonstrated the synthesis of 4-chlorocoumarin 26 can be succesfully accomplished via treatment of 1 with phosphorous oxychloride under reflux (Majumdar and Samanta, 2002) (Scheme 14).

S^O 40

Scheme 21

S^O Cl

O -HCl

Dry THF fY^T OH

V r- un, * L II

S O 42

Scheme 22

7. Synthesis of monocyclic heterocyclic compounds

7.1. Synthesis of five-membered systems with two heteroatoms 7.1.1. 1,2-Benzisothiazoles

The Posner reaction between 1 and hydroxylamine in refluxing ethanolic solution represents one of the most successful strategies to attain 1,2-benzisothiazol-3-acetic acid 27, which is known as analogs of heteroauxin (Giannella et al., 1971) (Scheme 15).

The possible mechanism could account for the formation of product 62 via the formation of 1:3 coumarin-hydroxylamine "adduct" 28, which in turn gives 1:2 adduct 29. The intermediate 29 undergoes an intramolecular cyclization to afford 30, which on hydrolysis during the reaction affords the final product 27 (Scheme 16).

8. Synthesis of fused heterocyclic compounds

8.1. [5-6] Ring system

8.1.1. Dihydrofuran and furocoumarins

Furocoumarins are an important class of heterocyclic compounds possessing anticoagulant, insecticide, anthelminthic,

1 1 +Ar-CHO+ Ph-NH2 TTT^ Li,

1 48 49 50

48,50 a b c d e

Ar Ph 4-Cl-Ph 4-CH3-Ph 4-OCH3-Ph 2,4-Cl2-Ph

Yield % 67 62 68 67 62

Scheme 25

hypnotic, antifungal, and HIV protease inhibition activities (Eicher and T., 2004).

8.1.1.1. [4 + 1] Cycloaddition reaction followed by a [1,3]H shift. Asymmetric domino Michael-S2N reaction of 1 with bromonitroalkene 31 in aqueous sodium acetate and tetrabu-tylammonium bromide (TBAB) gave the corresponding 2,3-dihydrofuran 32 (Xie et al., 2011; Fan et al., 2010) (Scheme 17). This reaction can also be catalyzed by using N,N-diisopropylethylamine (DIPEA) as an additive (Lau et al., 1987).

8.1.1.2. Using of electrochemical routes. Laccase (Agaricus bisporus)-catalyzed domino reaction of 4-hydroxythiocouma-rin 1 with catechols 33 using aerial oxygen as the oxidant delivers for the synthesis of 8,9-dihydroxy-5-thiocoumestans 34 as single regioisomers with yields ranging from 55% to 96% (Hajdok et al., 2009) (Scheme 18). Some of the compounds described here have been made accessible in the presence of potassium ferricyanide as an oxidizing agent in aqueous solution (Reddy and Darbarwar, 1985).

8.1.2. Benzothiopyrano[4,3-d]oxazolone

One of the most successful strategies for constructing benzo[l]thiopyrano[4,3-d]oxazol-4-one 36 is the condensation and oxidative cyclization of 4-hydroxythiocoumarin 1 with formamide 35 at 155-160 0C (Ray and Paul, 2004) (Scheme 19).

Utility of 4-hydroxythiocoumarin

8.2. Fused [6-6] ring system

8.2.1. Pyrano benzothiopyran

The Pechmann-Duisberg reaction was employed by Merchant et al. to synthesize cyclopenta[3',4']pyrano-[3,2-c][l] benzothiopyran-4,l1-dione 38 via the condensation of 1 with ethylcyclopentanone-2-carboxylate 37 in the presence of anhydrous potassium carbonate (Merchant et al., 1981) (Scheme 20).

In a similar manner, condensation of 4-hydroxythiocouma-rin 1 with ethyl-2,3-dihydro-3-oxobenzofuran-2-carboxylate 39 afforded the corresponding 6H,12H-benzofuro [2',3';4,5]pyr-ano[3,2-c]thiobenzopyran-6,12-dione 40 (Mulwad et al., 1999) (Scheme 21).

Nejadshafiee and Saidi have recently reported the synthesis of 4-hydroxy-3-phenylthiochromeno[4,3-b]pyrane-2,5-dione 42 through the reaction of (chlorocarbonyl)phenyl ketene 41 with 4-hydroxythiocoumarin 1 (Nejadshafiee and Saidi, 2013) (Scheme 22).

A plausible mechanism for the formation of 42 involves that the OH group of the enol form of 1 will attack the acyl chloride of ketene 43 followed by ring closure to produce product 42 (Nejadshafiee and Saidi, 2013) (Scheme 23).

8.2.2. Synthesis of amino-substituted thiochromenopyranones 8.2.2.1. Bi component condensation (with unsaturated nitriles). Kislyi et al. noted that the Michael cycloaddition reaction of 1 with phenylidenenitroacetonitrile 46 in ethanolic triethyl-amine under reflux afforded 2-amino-3-nitro-4-phenylthio-chromeno[4,3-b]pyran-5-one 47 (Kislyi et al., 1999) (Scheme 24).

8.2.3. Benzothiopyranooxazine-5-one

Three component condensation of aromatic aldehydes 48, 4-hydroxythiocoumarin 1 and aniline 49 in acetic acid medium at room temperature for 1 h is disclosed for the synthesis of corresponding benzothiopyranooxazine-5-ones 50 in good yields at room temperature within one hour (Reddy and Darbarwar, 1986) (Scheme 25).

9. Conclusion

The studies reviewed above clearly demonstrate that 4-hydroxythiocoumarin is readily obtainable and a valuable building block in organic synthesis. It is sure that, as it has been until now, the use of 4-hydroxythiocoumarin will show a continuous flow of applications in the next years and will continue to be an indispensable synthetic tool in organic chemistry.

Acknowledgements

I am very much indebted to my capable and enthusiastic members and co-workers whose names appear in the list of references. The Academy of Scientific Research and Technology, ASRT, Egypt is acknowledged for their continuous financial support. Finally, I would like to thank Professor El-Sayed I. El-Desoky, and referees for their helpful suggestions.

References

Arjunan, V., Santhanam, R., Sakiladevi, S., Marchewka, M.K.,

Mohan, S., 2013. J. Mol. Struct. 1037, 305-316. Eicher, T., Hauptmann, S., Speicher, A., 2004. Five-Membered Heterocycles: Sections 5.1-5.21. The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications, second ed. John Wiley & Sons, 52-121. Fan, L.P., Li, P., Li, X.S., Xu, D.C., Ge, M.M., Zhu, W.D., Xie, J.W.,

2010. J. Org. Chem. 75, 8716-8719. Giannella, M., Gualtieri, F., Melchiorre, C., 1971. Phytochemistrty 10, 539-544.

Hajdok, S., Conrad, J., Leutbecher, H., Beifuss, U., Strobel, S.,

Schleid, T., 2009. J. Org. Chem. 74, 7230-7237. Inhulsen, I., Chin, K., Gowert; M., Margaretha, P., 2011. Helv. Chem.

Acta 94, 1030-1034. Jamkhandi, P.S., Rajagopal, S., 1967. Arch. Pharmuz. 300, 561-566. Jung, J.C., Kim, J.C., Park, O.S., Jang, B.S., 1999. Arch. Pharm. Res. 22, 302-305.

Jung, J.C., Kim, J.C., Park, O.S., 2000. Synth. Commun. 30, 1193-1203. Jung, J.C., Jung, Y.J., Park, O.S., 2001. Synth. Commun. 31, 1195-1200. Kislyi, V.P., Nesterov, V.N., Shestopalov, A.M., Semenov, V.V., 1999.

Russ. Chem. Bull. 48, 1135-1138. Lau, C.K., Belanger, P.C., Dufresne, C., Scheigetz, J., 1987. J. Org.

Chem. 52, 1673-1680. MacKenzie, N.E., Thomson, R.H., 1982. J. Chem. Soc., Perkin Trans. 1, 395-402.

Majumdar, K.C., Pal, A.K., 2009. J. Sulfur Chem. 30 (5), 481-489. Majumdar, K.C., Samanta, S.K., 2002. Synthesis, 121-125. Majumdar, K.C., Choudhury, P.K., Khan, A.T., 1989. Synth.

Commun. 19, 3249-3258. Majumdar, K.C., Chattopadhyay, S.K., Mukhopadhyay, P.P., 2006.

Synth. Commun. 36, 1291-1297. Merchant, J.R., Martyres, G., 1983. Indian J. Chem., Sect. B: Org.

Chem. Incl. Med. Chem. 22, 35-36. Merchant, J.R., Koshti, N.M., Bakre, K.M., 1981. J. Heterocycl.

Chem. 18, 1655-1658. Mulwad, V.V., Hegde, A.S., Suryanarayan, V., 1999. Indian J. Chem.,

Sect. B: Org. Chem. Incl. Med. Chem. 38, 148-151. Nakazumi, H., Kitao, T., 1977. Bull. Chem. Soc. Jpn. 50, 939-944. Nakazumi, H., Ueyama, T., Kitaguchi, T., Kitao, T., 1983. Phosphorus. Sulfur Silicon Relat. Elem. 16, 59-66. Nejadshafiee, P., Saidi, A.J., 2013. Iran. Chem. Soc. 10 (2), 237-241. Park, H., Lee, K.I., 2004. Synth. Commun. 34, 2053-2062. Park, S.J., Lee, J.C., Lee, K.I., 2007. Bull. Korean Chem. Soc. 28 (7), 1203-1205.

Peinhardt, G., Reppel, L., 1973. Pharmazie 28, 729-733. Rath, P.C., Rajagopal, K., 1971. Indian J. Chem. 9, 91-93. Ray, S., Paul, S., 2004. J. Indian Chem. Soc. 81, 488-491. Reddy, B.S., Darbarwar, M., 1985. Indian J. Chem., Sect. B: Org.

Chem. Incl. Med. Chem. 24, 556-559. Reddy, B.S., Darbarwar, M., 1986. J. Indian Chem. Soc. 63, 323-325. Ruwet, A., Draguet, C., Renson, M., 1970. Bull. Soc. Chem. Belg. 79, 639-644.

Soltau, M., Gowert, M., Margaretha, P., 2005. Org. Lett. 7, 5159-5161. Szell, T., 1967. J. Chem. Soc. C, 2041-2044.

Szell, T., Kovacs, K., Zarandy, M.S., Erdohely, A., 1969. Helv. Chem.

Acta 52, 2636-2641. Valente, S., Kirsch, G., 2011. Tetrahedron Lett. 52, 3429-3432. Vishnyakova, G.M., Smirnova, T.V., Perina, A.I., Sugrobova, L.V., 1979. Izv. Vyssh. Uchebn. Zaved Khim. Khim. Tekhnol. 22, 283-286. Xie, J.W., Li, P., Wang, T., Zhou, F.T., 2011. Tetrahedron Lett. 52, 2379-2382.

Ziegler, E., Roßmann, U., Litvan, F>, 1957. Monatsh. Chem. 88, 587-591.