S‐Functionalized Cysteine: Powerful Ligands for the Labelling of Bioactive Molecules with Triaquatricarbonyltechnetium‐99m(1+) ([99mTc(OH2)3(CO)3]+)

S‐Alkylated cysteines are used as efficient tridentate N,O,S‐donor‐atom ligands for the fac‐[M(CO)₃]⁺ moiety (M=^99mTc or Re). Reaction of (Et₄N)₂[ReBr₃(CO)₃] ( 3 ) with the model S‐benzyl‐L ‐cysteine ( 2 ) leads to the formation of [Re( 2′ )(CO)₃] ( 4 ) as the exclusive product ( 2′ =C‐terminal anion of 2 ). The tridentate nature of the alkylated cysteine in 4 was established by X‐ray crystallography. Compound 2 reacts with [^99mTc(OH₂)₃(CO)₃]⁺ under mild conditions (10⁻⁴ M , 50°, 30 min) to afford [^99mTc( 2′ )(CO)₃] ( 5 ) and represents, therefore, an efficient chelator for the labelling of biomolecules. L ‐Cysteine, S‐alkylated with a 3‐aminopropyl group (→ 7 ), was conjugated via a peptide coupling sequence with Coα‐[α‐(5,6‐dimethyl‐1H‐benzimidazolyl)]‐Coβ‐cyanocobamic b‐acid ( 6 ), the b‐acid of cyanocob(III)alamin (vitamin B₁₂) (Scheme 3). More convenient was a one‐pot procedure with a derivative of vitamin B₁₂ comprising a free amine group at the b‐position. This amine 15 was treated with NHS (N‐hydroxysuccinimide)‐activated 1‐iodoacetic acid 14 to introduce an I‐substituent in vitamin B₁₂. Subsequent addition of unprotected L ‐cysteine resulted in nucleophilic displacement of the I‐atom by the S‐substituent, affording the vitamin B₁₂ alkylated cysteine fragment 17 (Scheme 4). The procedure was quantitative and did not require purification of intermediates. Both cobalamin–cysteine conjugates could be efficiently labelled with [^99mTc(OH₂)₃(CO)₃]⁺ ( 1 ) under conditions identical to those of the model complex 5 . Biodistribution studies of the cobalamin conjugates in mice bearing B10‐F16 melanoma tumors showed a tumor uptake of 8.1±0.6% and 4.4±0.5% injected dose per gram of tumor tissue after 4 h and 24 h, respectively (Table 1).     

Synthesis of (±)‐Kempa‐6,8‐dien‐3‐ol (=(2aRS,3SR,4aSR,7RS,7aSR,10bSR,10cSR)‐2,2a,3,4,4a,5,6,7,7a,8,10b,10c‐Dodecahydro‐2a,7,10,10c‐tetramethylnaphth[2,18‐cde]azulen‐3‐ol)

The synthesis of kempa‐6,8‐dien‐3β‐ol ( 4a ), as a synthetic leading model of the natural product 4b , was carried out starting from intermediate 12 , the synthetic route of which has been developed previously (Scheme 1). The conversion of 12 to the model compound 4a involved the elaboration of three structure modifications by three processes, Tasks A, B, and C (see Scheme 2). Task A was achieved by epoxy‐ring opening of 41 with Me₃SiCl (Scheme 9), and Task B being performed by oxidation at the 13‐position, followed by hydrogenation, and then epimerization (Schemes 4 and 5). The removal of the 2‐OH group from 12 (Task C) was achieved via 30b according to Scheme 6, whereby 30b was formed exclusively from 30a / 31a 1 : 1 (Scheme 7). In addition, some useful reactions from the synthetic viewpoint were developed during the course of the present experiments.     

Enantioselective Synthesis of Ethyl 4,5,7,8,9‐Penta‐O‐acetyl‐2,6‐anhydro‐ 3‐deoxy‐D‐erythro‐L‐gluca‐nononate: a 2‐Monodeoxygenated Derivative of `2‐Keto‐3‐deoxy‐D‐glycero‐D‐galacto‐nononic Acid'

A study aimed at developing an enantioselective synthesis of the title compound 23 , a 2‐monodeoxy analogue of the naturally occurring (+)‐2‐keto‐3‐deoxy‐D ‐glycero‐D ‐galacto‐2‐nononic acid (KDN), is reported. From D ‐mannose as starting material, the chiral 1,3‐diene 10 , activated by a silyloxy substituent at C(2), was prepared in six steps (Scheme 1). However, the intermediates were often contaminated with varying amounts of by‐products arising from overoxidation during cleavage with periodic acid. An alternative route starting from the inexpensive and readily available D ‐isoascorbic acid ( 12 ), though a little longer than the first, satisfactorily circumvented the purification problem and led to the desired dienes 17 in good yields (scheme 2). The [Co^II(S,S)‐(+)‐salen]‐catalyzed hetero‐Diels‐Alder reactions of the aforementioned dienes with ethyl glyoxylate proceeded smoothly at room temperature, giving the dihydropyrano adducts 18 in moderate yields (Scheme 3). Dihydroxylation of 18a followed by reduction of the keto function gave the desired 4,5‐trans dihydroxy moiety of the KDN framework (Scheme 4, see 21 ). The spectroscopic data of the penta‐O‐acetylated 2‐deoxy‐KDN ethyl ester 23 were consistent with those reported for the corresponding methyl ester derived from natural KDN.     

Fused 1,2‐Dithioles. Part VII

The 1,2‐dithiolosultam derivative 14 was obtained from the (α‐bromoalkylidene)propenesultam derivative 9 (Scheme 1). Regioselective cleavage of the two ester groups (→ 1b or 2b ) allowed the preparation of derivatives with different substituents at C(3) in the dithiole ring (see 27 and 28 ) as well as at C(6) in the isothiazole ring (see 17 – 21 ; Scheme 2). Curtius rearrangement of the 6‐carbonyl azide 21 in Ac₂O afforded the 6‐acetamide 22 , and saponification and decarboxylation of the latter yielded ‘sulfothiolutin’ ( 30 ). Hydride reductions of two of the bicyclic sultams resulted in ring opening of the sultam ring and loss of the sulfonyl group. Thus the reduction of the dithiolosultam derivative 14 yielded the alkylidenethiotetronic acid derivative 33 (tetronic acid=furan‐2,4(3H,4H)‐dione), and the lactam‐sultam derivative 10 gave the alkylidenetetramic acid derivative 35 (tetramic acid=1,5‐dihydro‐4‐hydroxy‐2H‐pyrrol‐2‐one) (Scheme 3). Some of the new compounds ( 14, 22, 26 , and 30 ) exhibited antimycobacterial activity. The oxidative addition of 1 equiv. of [Pt(η²‐C₂H₄)L₂] ( 36a , L=PPh₃; 36b , L=1/2 dppf; 36c , L=1/2 (R,R)‐diop) into the S? S bond of 14 led to the cis‐(dithiolato)platinum(II) complexes 37a – c . (dppf=1,1′‐bis(diphenylphosphino)ferrocene; (R,R)‐diop={[(4R,5R)‐2,2‐demithyl‐1,3‐dioxolane‐4,5‐diyl]bis(methylene)}bis[diphenylphosphine]).     

Synthesis and Characterization of Ruthenium Complexes with Substituted Pyrazino[2,3‐f][1,10]‐phenanthroline (=Rppl; R=Me,COOH, COOMe)

A series of substituted pyrazino[2,3‐f][1,10]‐phenanthroline (Rppl) ligands (with R=Me, COOH, COOMe) were synthetized (see 1 – 4 in Scheme 1). The ligands can be visualized as formed by a bipyridine and a quinoxaline fragment (see A and B ). Homoleptic [Ru(R¹ppl)₃](PF₆)₂ and heteropleptic [Ru(R¹ppl){(R²)₂bpy}₂](PF₆)₂ (R¹=H, Me, COOMe and R²=H, Me) metal complexes 5 – 7 and 8 – 13 , respectively, based on these ligands were also synthesized and characterized by conventional techniques (Schemes 2 and 3, resp.). In the heteroleptic complexes, the R¹‐ppl ligand reduces at a less‐negative potential than the bpy ligand, reflecting the acceptor property conferred by the quinoxaline moiety. The potentiality of some of these complexes as solar‐cell dyes is discussed.     

Oxidative Fragmentations of the 5α‐ and 5β‐Hydroxy‐B‐norcholestan‐ 3β‐yl Acetates

Oxidations of 5α‐hydroxy‐B‐norcholestan‐3β‐yl acetate ( 8 ) with Pb(OAc)₄ under thermal or photolytic conditions or in the presence of iodine afforded only complex mixtures of compounds. However, the HgO/I₂ version of the hypoiodite reaction gave as the primary products the stereoisomeric (Z)‐ and (E)‐1(10)‐unsaturated 5,10‐seco B‐nor‐derivatives 10 and 11 , and the stereoisomeric (5R,10R)‐ and (5S,10S)‐acetals 14 and 15 (Scheme 4). Further reaction of these compounds under conditions of their formation afforded, in addition, the A‐nor 1,5‐cyclization products 13 and 16 (from 10 ) and 12 (from 11 ) (see also Scheme 6) and the 6‐iodo‐5,6‐secolactones 17 and 19 (from 14 and 15 , resp.) and 4‐iodo‐4,5‐secolactone 18 (from 15 ) (see also Scheme 7). Oxidations of 5β‐hydroxy‐B‐norcholestan‐3β‐yl acetate ( 9 ) with both hypoiodite‐forming reagents (Pb(OAc)₄/I₂ and HgO/I₂) proceeded similarly to the HgO/I₂ reaction of the corresponding 5α‐hydroxy analogue 8 . Photolytic Pb(OAc)₄ oxidation of 9 afforded, in addition to the (Z)‐ and (E)‐5,10‐seco 1(10)‐unsaturated ketones 10 and 11 , their isomeric 5,10‐seco 10(19)‐unsaturated ketone 22 , the acetal 5‐acetate 21 , and 5β,19‐epoxy derivative 23 (Scheme 9). Exceptionally, in the thermal Pb(OAc)₄ oxidation of 9 , the 5,10‐seco ketones 10, 11 , and 22 were not formed, the only reaction being the stereoselective formation of the 5,10‐ethers with the β‐oriented epoxy bridge, i.e. the (10R)‐enol ether 20 and (5S,10R)‐acetal 5‐acetate 21 (Scheme 8). Possible mechanistic interpretations of the above transformations are discussed.     

Stabilized Carbenium Ions as Latent,Z‐type Ligands

Controlling the reactivity of transition metals using secondary, σ‐accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺ ([ 3 ]⁺; Acr=9‐N‐methylacridinium) and [(o‐Ph₂P(C₆H₄)Xan)AuCl]⁺ ([ 4 ]⁺; Xan=9‐xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [ 4 ]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative ( 7 ) in which the metal atom is covalently bound to the former carbocationic center. This anion‐induced Au^I/Au^III oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [ 4 ]⁺ into an X‐type ligand in 7 . We conclude that the carbenium moiety of this complex acts as a latent Z‐type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.     

Stabilized Carbenium Ions as Latent,Z‐type Ligands

Controlling the reactivity of transition metals using secondary, σ‐accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺ ([ 3 ]⁺; Acr=9‐N‐methylacridinium) and [(o‐Ph₂P(C₆H₄)Xan)AuCl]⁺ ([ 4 ]⁺; Xan=9‐xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [ 4 ]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative ( 7 ) in which the metal atom is covalently bound to the former carbocationic center. This anion‐induced Au^I/Au^III oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [ 4 ]⁺ into an X‐type ligand in 7 . We conclude that the carbenium moiety of this complex acts as a latent Z‐type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.     

Stereoselective Synthesis of [5‐[4,4,4,4′,4′,4′‐Hexafluoro‐N‐(2‐hydroxyethoxy)‐D‐valine]]‐ and [5‐[4,4,4,4′,4′,4′‐Hexafluoro‐N‐(2‐hydroxyethoxy)‐L‐valine]cyclosporin A

Addition of various amines to the 3,3‐bis(trifluoromethyl)acrylamides 10a and 10b gave the tripeptides 11a – 11f , mostly as mixtures of epimers (Scheme 3). The crystalline tripeptide 11f ₂ was found to be the N‐terminal (2‐hydroxyethoxy)‐substituted (R,S,S)‐ester HOCH₂CH₂O‐D ‐Val(F₆)‐MeLeu‐Ala‐O^tBu by X‐ray crystallography. The C‐terminal‐protected tripeptide 11f ₂ was condensed with the N‐terminus octapeptide 2b to the depsipeptide 12a which was thermally rearranged to the undecapeptide 13a (Scheme 4). The condensation of the epimeric tripeptide 11f ₁ with the octapeptide 2b gave the undecapeptide 13b directly. The undecapeptides 13a and 13b were fully deprotected and cyclized to the [5‐[4,4,4,4′,4′,4′‐hexafluoro‐N‐(2‐hydroxyethoxy)‐D ‐valine]]‐ and [5‐[4,4,4,4′,4′,4′‐hexafluoro‐N‐(2‐hydroxyethoxy)‐L ‐valine]]cyclosporins 14a and 14b , respectively (Scheme 5). Rate differences observed for the thermal rearrangements of 12a to 13a and of 12b to 13b are discussed.     

In‐ and Out‐of‐Cavity Interactions by Modulating the Size of Ruthenium Metallarectangles

Cationic (arene)ruthenium‐based tetranuclear complexes of the general formula [Ru₄(η⁶‐p‐cymene)₄(μ‐N∩N)₂(μ‐OO∩OO)₂]⁴⁺ were obtained from the dinuclear (arene)ruthenium complexes [Ru₂(η⁶‐p‐cymene)₂(μ‐OO∩OO)₂Cl₂] (p‐cymene=1‐methyl‐4‐(1‐methylethyl)benzene, OO∩OO=5,8‐dihydroxy‐1,4‐naphthoquinonato(2?), 9,10‐dihydroxy‐1,4‐anthraquinonato(2?), or 6,11‐dihydroxynaphthacene‐5,12‐dionato(2?)) by reaction with pyrazine or bipyridine linkers (N∩N=pyrazine, 4,4′‐bipyridine, 4,4′‐[(1E)‐ethene‐1,2‐diyl]bis[pyridine]) in the presence of silver trifluoromethanesulfonate (CF₃SO₃Ag) (Scheme). All complexes 4 – 12 were isolated in good yield as CF₃SO salts, and characterized by NMR and IR spectroscopy. The host–guest properties of the metallarectangles incorporating 4,4′‐bipyridine and (4,4′‐[(1E)‐ethene‐1,2‐diyl]bis[pyridine] linkers were studied in solution by means of multiple NMR experiments (1D, ROESY, and DOSY). The largest metallarectangles 10 – 12 incorporating (4,4′‐[(1E)‐ethene‐1,2‐diyl]bis[pyridine] linkers are able to host an anthracene, pyrene, perylene, or coronene molecule in their cavity, while the medium‐size metallarectangles 7 – 9 incorporating 4,4′‐bipyridine linkers are only able to encapsulate anthracene. However, out‐of‐cavity interactions are observed between these 4,4′‐bipyridine‐containing rectangles and pyrene, perylene, or coronene. In contrast, the small pyrazine‐containing metallarectangles 4 – 6 show no interaction in solution with this series of planar aromatic molecules.     

Synthesis of Bis‐Heterocyclic 1H‐Imidazole 3‐Oxides from 3‐Oxido‐1H‐imidazole‐4‐carbohydrazides

The reaction of 1H‐imidazole‐4‐carbohydrazides 1 , which are conveniently accessible by treatment of the corresponding esters with NH₂NH₂?H₂O, with isothiocyanates in refluxing EtOH led to thiosemicarbazides (=hydrazinecarbothioamides) 4 in high yields (Scheme 2). Whereas 4 in boiling aqueous NaOH yielded 2,4‐dihydro‐3H‐1,2,4‐triazole‐3‐thiones 5 , the reaction in concentrated H₂SO₄ at room temperature gave 1,3,4‐thiadiazol‐2‐amines 6 . Similarly, the reaction of 1 with butyl isocyanate led to semicarbazides 7 , which, under basic conditions, undergo cyclization to give 2,4‐dihydro‐3H‐1,2,4‐triazol‐3‐ones 8 (Scheme 3). Treatment of 1 with Ac₂O yielded the diacylhydrazine derivatives 9 exclusively, and the alternative isomerization of 1 to imidazol‐2‐ones was not observed (Scheme 4). It is important to note that, in all these transformations, the imidazole N‐oxide residue is retained. Furthermore, it was shown that imidazole N‐oxides bearing a 1,2,4‐triazole‐3‐thione or 1,3,4‐thiadiazol‐2‐amine moiety undergo the S‐transfer reaction to give bis‐heterocyclic 1H‐imidazole‐2‐thiones 11 by treatment with 2,2,4,4‐tetramethylcyclobutane‐1,3‐dithione (Scheme 5).     

7‐Decarboxymethyl‐cobyrinates: Vitamin B12‐Derivatives that Lack the c‐Side Chain

The synthesis of cobyrinic acid derivatives by reduction of dehydrocobyrinates is largely unexplored. It is, however, a rational path to B₁₂ analogues that lack specific substituents of the corrin moiety of natural B₁₂ derivatives. The partial syntheses of four epimeric 7‐decarboxymethyl‐cobyrinates is described, which is achieved by reduction of Δ7‐dehydro‐7‐de[carboxymethyl]‐cobyrinate with zinc or with the ‘prebiotic’ reducing agent formic acid. A direct and remarkably efficient route was found to 7‐decarboxymethyl‐cobyrinates, which are cobyrinic acid derivatives in which the c‐side chain at ring B of vitamin B₁₂ is missing. The structures of the hexamethyl‐7‐decarboxymethyl‐cobyrinates were characterized and the stereochemical and conformational properties at their newly saturated ring B were analyzed. The stereochemical outcome of the reduction was found to depend strongly on the reaction conditions. In 7‐decarboxymethyl‐cobyrinates, both peripheral carbon centres of ring B carry a hydrogen atom, and the characteristic quaternary carbon centre at C7 of the cobyrinic acid moiety of vitamin B₁₂ is lacking. The still highly substituted 7‐decarboxymethyl‐cobyrinates are readily dehydrogenated in the presence of dioxygen, furnishing 7‐de[carboxymethyl]‐Δ⁷‐dehydro‐cobyrinate as the common, unsaturated oxidation product. The noted stability of vitamin B₁₂ and of other Co^III‐cobyrinates in the presence of air is a consequence of their highly substituted corrin macrocycle, a finding of interest in the context of chemical rationalizations of the B₁₂ structure.     

Reactions of Imidoyl Isoselenocyanates with Aromatic 2‐Amino N‐Heterocycles and 1‐Methyl‐1H‐imidazole

The reaction of N‐phenylimidoyl isoselenocyanates 1 with 2‐amino‐1,3‐thiazoles 10 in acetone proceeded smoothly at room temperature to give 4H‐1,3‐thiazolo[3,2‐a] [1,3,5]triazine‐4‐selones 13 in fair yields (Scheme 2). Under the same conditions, 1 and 2‐amino‐3‐methylpyridine ( 11 ) underwent an addition reaction, followed by a spontaneous oxidation, to yield the 3H‐4λ⁴‐[1,2,4]selenadiazolo[1′,5′:1,5] [1,2,4]selenadiazolo[2,3‐a]pyridine 14 (Scheme 3). The structure of 14 was established by X‐ray crystallography (Fig. 1). Finally, the reaction of 1‐methyl‐1H‐imidazole ( 12 ) and 1 led to 3‐methyl‐1‐(N‐phenylbenzimidoyl)‐1H‐imidazolium selenocyanates 15 (Scheme 4). In all three cases, an initially formed selenourea derivative is proposed as an intermediate.     

Synthesis of Imidazole Derivatives Using 2‐Unsubstituted 1H‐Imidazole 3‐Oxides

The reaction of 1,4,5‐trisubstituted 1H‐imidazole 3‐oxides 1 with Ac₂O in CH₂Cl₂ at 0 – 5° leads to the corresponding 1,3‐dihydro‐2H‐imidazol‐2‐ones 4 in good yields. In refluxing Ac₂O, the N‐oxides 1 are transformed to N‐acetylated 1,3‐dihydro‐2H‐imidazol‐2‐ones 5 . The proposed mechanisms for these reactions are analogous to those for N‐oxides of 6‐membered heterocycles (Scheme 2). A smooth synthesis of 1H‐imidazole‐2‐carbonitriles 2 starting with 1 is achieved by treatment with trimethylsilanecarbonitrile (Me₃SiCN) in CH₂Cl₂ at 0 – 5° (Scheme 3).     

Reactions of Stable α‐Chlorosulfanyl Chlorides with CS‐Functionalized Compounds

The smooth reaction of 3‐chloro‐3‐(chlorosulfanyl)‐2,2,4,4‐tetramethylcyclobutanone ( 3 ) with 3,4,5‐trisubstituted 2,3‐dihydro‐1H‐imidazole‐2‐thiones 8 and 2‐thiouracil ( 10 ) in CH₂Cl₂/Et₃N at room temperature yielded the corresponding disulfanes 9 and 11 (Scheme 2), respectively, via a nucleophilic substitution of Cl^? of the sulfanyl chloride by the S‐atom of the heterocyclic thione. The analogous reaction of 3‐cyclohexyl‐2,3‐dihydro‐4,5‐diphenyl‐1H‐imidazole‐2‐thione ( 8b ) and 10 with the chlorodisulfanyl derivative 16 led to the corresponding trisulfanes 17 and 18 (Scheme 4), respectively. On the other hand, the reaction of 3 and 4,4‐dimethyl‐2‐phenyl‐1,3‐thiazole‐5(4H)‐thione ( 12 ) in CH₂Cl₂ gave only 4,4‐dimethyl‐2‐phenyl‐1,3‐thiazol‐5(4H)‐one ( 13 ) and the trithioorthoester derivative 14 , a bis‐disulfane, in low yield (Scheme 3). At ?78°, only bis(1‐chloro‐2,2,4,4‐tetramethyl‐3‐oxocyclobutyl)polysulfanes 15 were formed. Even at ?78°, a 1 : 2 mixture of 12 and 16 in CH₂Cl₂ reacted to give 13 and the symmetrical pentasulfane 19 in good yield (Scheme 5). The structures of 11, 14, 17 , and 18 have been established by X‐ray crystallography.     

Nucleotides. Part LXXVIII

Several N(‐hydroxyalkyl)‐2,4‐dinitroanilines were transformed into their phosphoramidites (see 5 and 6 in Scheme 1) in view of their use as fluorescence quenchers, and modified 2‐aminobenzamides (see 9, 10, 18 , and 19 in Scheme 1) were applied in model reactions as fluorophors to determine the relative fluorescence quantum yields of the 3′‐Aba and 5′‐Dnp‐3′‐Aba conjugates (Aba=aminobenzamide, Dnp=dinitroaniline). Thymidine was alkylated with N‐(2‐chloroethyl)‐2,4‐dinitroaniline ( 24 ) to give 25 which was further modified to the building blocks 27 and 28 (Scheme 3). The 2‐amino group in 29 was transformed by diazotation into the 2‐fluoroinosine derivative 30 used as starting material for several reactions at the pyrimidine nucleus (→ 31, 33 , and 35 ; Scheme 4). The 3′,5′‐di‐O‐acetyl‐2′‐deoxy‐N²‐[(dimethylamino)methylene]guanosine ( 37 ) was alkylated with methyl and ethyl iodide preferentially at N(1) to 43 and 44 , and similarly reacted N‐(2‐chloroethyl)‐2,4‐dinitroaniline ( 24 ) to 38 and the N‐(2‐iodoethyl)‐N‐methyl analog 50 to 53 (Scheme 5). The 2′‐deoxyguanosine derivative 53 was transformed into 3′,5′‐di‐O‐acetyl‐2‐fluoro‐1‐{2‐[(2,4‐dinitrophenyl)methylamino]ethyl}inosine ( 54 ; Scheme 5) which reacted with 2,2′‐[ethane‐1,2‐diylbis(oxy)]bis[ethanamine] to modify the 2‐position with an amino spacer resulting in 56 (Scheme 6). Attachment of the fluorescein moiety 55 at 56 via a urea linkage led to the doubly labeled 2′‐deoxyguanosine derivative 57 (Scheme 6). Dimethoxytritylation to 58 and further reaction to the 3′‐succinate 59 and 3′‐phosphoramidite 60 afforded the common building blocks for the oligonucleotide synthesis (Scheme 6). Similarly, 30 reacted with N‐(2‐aminoethyl)‐2,4‐dinitroaniline ( 61 ) thus attaching the quencher at the 2‐position to yield 62 (Scheme 7). The amino spacer was again attached at the same site via a urea bridge to form 64 . The labeling of 64 with the fluorescein derivative 55 was straigthforward giving 65 . and dimethoxytritylation to 66 and further phosphitylation to 67 followed known procedures (Scheme 7). Several oligo‐2′‐deoxynucleotides containing the doubly labeled 2′‐deoxyguanosines at various positions of the chain were formed in a DNA synthesizer, and their fluorescence properties and the T_ms in comparison to their parent duplexes were measured (Tables 1–5).     

Heterospirocyclic N‐(2H‐Azirin‐3‐yl)‐L‐prolinates: New Dipeptide Synthons

The synthesis of methyl N‐(1‐aza‐6‐oxaspiro[2.5]oct‐1‐en‐2‐yl)‐L ‐prolinate ( 1e ) has been performed by consecutive treatment of methyl N‐[(tetrahydro‐2H‐pyran‐4‐yl)thiocarbonyl]‐L ‐prolinate ( 5 ) with COCl₂, 1,4‐diazabicyclo[2.2.2]octane (DABCO), and NaN₃ (Scheme 1). As the first example of a novel class of dipeptide synthons, 1e has been shown to undergo the expected reactions with carboxylic acids and thioacids (Scheme 2). The successful preparation of the nonapeptide 16 , which is an analogue of the C‐terminal nonapeptide of the antibiotic Trichovirin I 1B, proved that 1e can be used in peptide synthesis as a dipeptide building block (Scheme 3). The structure of 7 has been established by X‐ray crystal‐structure analysis (Figs. 1 and 2).     

Synthesis and Characterization of N,N‐Di‐substituted Acylthiourea Copper(II) Complexes

A series of six N,N‐di‐substituted acylthiourea ArC(O)NHC(S)NRR′ ligands (denoted as HLⁿ) [Ar = 1‐Naph: NRR′ = NPh₂, HL¹ ( 1 ); N(iPr)Ph, HL² ( 2 ). Ar = Mes: NRR′ = NPh₂, HL⁴ ( 3 ); N(iPr)Ph, HL⁵ ( 4 ); NEt₂, HL⁶ ( 5 ). Ar = Ph: NRR′ = N(iPr)Ph, HL⁸ ( 6 )] were synthesized and characterized. These ligands were deprotonated to form Cu^II complexes through metathesis or combined redox reaction with copper halides. The structures of the complexes were investigated with single‐crystal X‐ray diffraction. The reaction of the 1‐naphthalene derivative HL¹ ( 1 ) with CuBr in the presence of sodium acetate produced cis‐CuL¹₂ ( 7 ), where the deprotonated ligand is bound to the Cu^II atom in a bidentate‐(O, S) coordination mode. Similarly treatment of HL² ( 2 ) with NaOAc and CuCl resulted in the formation of the cis‐arranged product [cis‐CuL²₂ ( 8 )]. The reaction of mesityl derivative HL⁴ ( 3 ) and CuBr with and without the addition of NaOAc gave the cis‐CuL⁴₂ ( 9 ) and cis‐(HL⁴)₂CuBr ( 10 ), respectively. In contrast, reaction of HL⁵ ( 4 ) and CuI in the presence of NaOAc resulted in trans‐CuL⁵₂ ( 11 ). Alternatively trans‐CuL⁶₂ ( 12 ) was obtained by the reaction of diethyl‐substituted HL⁶ ( 5 ) with CuCl₂ in the absence of a base.     

The Syntheses of Pyrimido‐pyridazinone and Pyrrolidino‐pyrimidinone 2′‐Deoxynucleoside Derivatives

Nucleosides that have ambivalent tautomeric properties have value in a variety of nucleic‐acid hybridisation applications and as mutagenic agents. We describe here synthetic studies directed to stable derivatives based on N⁴‐aminocytosine. Treatment of the 5‐(chloroethyl)‐4‐(triazol‐1‐yl)pyrimidine‐nucleoside derivative 1 with benzylhydrazine leads to the formation of the 6,6‐bicyclic pyrimido‐pyridazin‐7‐one 6 , in addition to the 5,6‐bicyclic derivative 7 . The 6,6‐bicyclic benzyl derivative 6 was converted to its 5′‐triphosphate for studies with DNA polymerases. Reaction of the triazole 1 with hydrazine, followed by acetylation, led to the desired acetylated 6,6‐bicyclic derivative 12 . However, the latter compound undergoes acyl migration followed by ring contraction to the 5,6‐bicyclic compound 13 on treatment with base.     

Synthesis of (2′S)‐2′‐Deoxy‐2′‐C‐methylpurine Nucleosides and Their Phosphoramidites

We describe the stereoselective synthesis of (2′S)‐2′‐deoxy‐2′‐C‐methyladenosine ( 12 ) and (2′S)‐2′‐deoxy‐2′‐C‐methylinosine ( 14 ) as well as their corresponding cyanoethyl phosphoramidites 16 and 19 from 6‐O‐(2,6‐dichlorophenyl)inosine as starting material. The methyl group at the 2′‐position was introduced via a Wittig reaction (→ 3 , Scheme 1) followed by a stereoselective oxidation with OsO₄ (→ 4 , Scheme 2). The primary‐alcohol moiety of 4 was tosylated (→ 5 ) and regioselectively reduced with NaBH₄ (→ 6 ). Subsequent reduction of the 2′‐alcohol moiety with Bu₃SnH yielded stereoselectively the corresponding (2′S)‐2′‐deoxy‐2′‐C‐methylnucleoside (→ 8a ).