Scheme 3.1: Schematic representation of conversion of cyano moiety to the tetrazole ring
3.5. Application of Tetrazole as Nucleosidic Base Analouges and Linkers
After the discovery of ribavirin,21a shodomycin,21b-f pyrazomycin21g-h containing a heterocyclic ring as nucleobase which have shown significant biological activity, a great deal of research activity have been put forth for the synthesis of nucleosides containing 5-membered nitrogen containing heterocycles as a nucleobase surrogate or as a replacement of phosphate backbone. As for example, a number a triazole compounds have been synthesized with this aim as has been described in Chapter 2.
Since the tetrazole ring is biologically important, so advances in the field of synthesis of tetrazole nucleosides is going on in a rapid manner.
The first example in the field of synthesis of tetrazole nucleosides and the study of antiviral property was reported by Pooniam et al.22 in 1974. The synthesis of
1-ß-D-ribofuranosyltetrazole 3.23 and its 5-carboxamide 3.24 and 5-acetamide 3.25 derivatives has been described by them. These tetrazole N-nucleosides have been tested against influenza A2/Asian/J-305 virus infection in mice and have been found to be inactive at high doses.
N N N
N
O HO
H
OH
N N N
N
O HO
CONH2
OH
NN N
N
O HO
CH2CONH2
OH 3.23 3.24 3.25
Figure 3.3: Structures of 1-ß-D-ribofuranosyltetrazole and its 5-carboxamide and 5- acetamide derivatives.
Tetrazole-related C-nucleosides also have been reported by Popsavin et al.23 and their biological activity has been evaluated. The two tetrazole C-nucleosides synthesized are 2-benzamido-2-deoxy-ß-D-ribofuranose (Figure 3.4, 3.26) and 3- azido-3-deoxy-ß-D-xylofuranose (Figure 3.4, 3.27) from D-glucose out of which compound 3.27 shows moderate inhibitory activity against in-vitro growth of both N2a and BHK 21 tumor cell lines.
O BzO
NH N N N
NHBz OH
O BzO
NH N N N
OH N3 3.26 3.27
Figure 3.4: Structures of 2-benzamido-2-deoxy-ß-D-ribofuranose and 3-azido-3- deoxy-ß-D-xylofuranose.
As a part of their investigation on the reactivity of ditetrazole compounds with electrophilic sugar moiety, Filichev et al.24 have synthesised mono- and bis-3′- substituted thymidine derivatives containing 1,5-bis(tetrazol-5-yl)- 3-oxapentane (Figure 3.5, 3.28) as a linker. These compounds possess interesting properties of inhibition of DNA chain elongation and as antisense agents. Pedersen et al.25 have reported the synthesis of the thymidine dimers in which 2,5-disubstituted tetrazole ring substituted the natural phosphodiester linkage (Figure 3.5, 3.29). These thymidine dimers have been incorporated into oligodeoxynucleotides (ODNs) and the thermal stability of the duplexes formed by the modified oligonucleotides have been investigated by thermal denaturation study. It has been observed that the replacement of the phosphodiester linkage with tetrazole results in lowering of the thermal melting stability of the DNA duplexes by about 5.8 oC in thermal melting temperature (Tm).
O TrO
N NH O
O
N N N
N
O N
NH O
O
OH O
HO N
NH O
O
NN N
N
O HO
N NH O
O
NN N
N O
3.28 3.29
Figure 3.5: Stuctures of tetrazole used as linkers in thymidine dimers.
Thymidine- and AZT-linked 5-(1,3-dioxoalkyl)tetrazoles have also been designed and synthesized by Bosch et al.26 by the condensation of nucleoside-derived 2- oxonitriles with the lithium salt of 5-acetyl-1-(4-fluorobenzyl)tetrazole (Figure 3.6).
The evaluation of the biological activity of 3.31 revealed that it could serve as a lead compound for the treatment of AIDS.
O
HO N
N O
O
N3
O
NN N N
F
O H
O
HO N
N O
O O
NN N N
F
O H
HO 3.30 3.31
Figure 3.6: Structures of thymidine- and AZT-linked 5-(1,3-dioxoalkyl)tetrazoles.
Muller et al.27 have reported the synthesis of the tetrazole nucleosides from Hoffer’s chloro sugar (Scheme 3.4) and tested the metal ion coordination property of the compounds. It has been observed that tetrazole nucleosides do not form any complexes with the metal ion under any experimental conditions as the basicity of tetrazole nucleoside is far too low to allow any complexation.
N
HN N
N NaH
O OTol TolO
Cl
O OTol
TolO N
NN N
resolution
O OTol
TolO N
NN N
+
O OTol TolO
N
NNN NH3 / MeOH O OH
HO N
NN N
+ O
OH HO
N NNN 3.32
2.91
3.33
3.34 3.35 3.36 3.37
Scheme 3.4: Synthesis of tetrazole nucleosides; Tol = p-toluoyl). Additional isomeric products with N2-glycosidic bonds were also obtained.
Pedrosa et al.28 have synthesized mono- and disubstituted tetrazoles from methyl D-glucopyranoside anomers (Scheme 3.5) wherein the tetrazole is exploited as a linker.
Scheme 3.5: Structures of mono- and di-substituted tetrazoles of methyl D- glucopyranoside.
Aldhoun et al.29 have designed and synthesized C-glycosyl R-amino acids which contain the tetrazole ring holding the carbohydrate and glycinyl moiety. Since numerous mechanisms of carbohydrate action in glycoproteins are, at present, poorly
understood so natural and unnatural glycopeptides with a well-defined structure and composition can serve as synthetic probes in the studies for investigating the role of carbohydrate domain on the biological activity of glycoprotein. Hence, efforts are being made towards the development of synthetic methodologies of natural O- and N- linked glycosyl amino acids and glycopeptides30 as well as of unnatural C-linked analogues31 to be incorporated into peptides.The synthesis of S-linked glycosyl amino acids and thioglycopeptides has also received attention in the past.32 Thus, two series of compounds have been prepared, one containing C-galactosyl and C-ribosyl O- tetrazolyl serine while the other containing S-tetrazolyl cysteine derivatives (Figure 3.7). In both the cases, the first step involved the thermal cycloaddition of a sugar azide with p-toluensulfonyl cyanide (TsCN) to give a 1-substituted 5-sulfonyl tetrazole which was developed by Demko et al. and the second step constituted the replacement of the tosyl group with a serine or cysteine residue.
O BnO
OBn OBn N N N N
O COOMe NHBoc
O BnO
OBn OBn N N N N
O COOMe NHBoc
O AcO
OAc OAc N N N N
S COOMe NHBoc
O AcO
OAc OAc N N N N
S COOMe NHBoc O
BnO BnO BnO
OBn
N N NN
O COOMe NHBoc
O AcO AcO
AcO OAc
N N N N
S COOMe NHBoc
3.43 3.44
3.45 3.46
3.47 3.48
Figure 3.7: Structures of C-galactosyl and C-ribosyl O-tetrazolyl serines and C- galactosyl and C-ribosyl S-tetrazolyl serines.
Davis et al.33 have reported the synthesis of bicyclic tetrazole derivatives of D- mannofuranose and D-rhamnofuranose and L-rhamnofuranose (Figure 3.8). Since the tetrazoles of pyranoses (3.49) possessed great inhibitory potential towards the glycosidase enzymes and other sugar possessing enzymes so their furanose tetrazole counterparts were synthesized. The key step to their synthesis involved an intramolecular [1,3]- dipolar cycloaddition of azide and nitrile moieties.
Figure 3.8: Structures of pyranose tetrazole (3.49), D-manno tetrazole (3.50), D- rhamno tetrazole (3.51) and L-rhamnofuranose tetrazole (3.52).