A Novel Intramolecular Photocyclization of N‐(2‐Bromoalkanoyl) Derivatives of 2‐Acylanilines via 1,8‐Hydrogen Abstraction

The photochemical reactions of different N‐(2‐acylphenyl)‐2‐bromo‐2‐methylpropanamides have been investigated. Irradiation of the N‐unsubstituted anilides 1a – 1c gave the corresponding dehydrobromination, cyclization, and bromo‐migration products 2, 3 , and 4 , respectively (Table 1). Irradiation of the N‐alkyl anilides 1e – 1g afforded the corresponding deacylation and cyclization products 5 and 6 , respectively, whereas irradiation of the N‐alkyl anilides 1i – 1k , carrying 2‐benzoyl groups on the aromatic rings, afforded the unexpected tricyclic lactams 7 (besides 2, 5 , and 6 ). The formation of the cyclization products 6 could be rationalized in terms of an electrocyclic ring closure of the 6π‐electron‐conjugated enamides 2 produced by dehydrobromination of 1 , followed by thermal 1,5‐acyl migration (Path B in the Scheme). The formation of the bridged lactams 7 probably follows a mechanism involving the 1,7‐diradical 8 generated by ζ‐H‐abstraction (1,8‐H transfer) by an excited acyl O‐atom (Path A).     

Photochemistry of N‐(2‐Acylphenyl)‐2‐methylprop‐2‐enamides: Competition between Photocyclization and Long‐Range Hydrogen Abstraction

The photochemical reactions of various ‘N‐methacryloyl acylanilides’ (=N‐(acylphenyl)‐2‐methylprop‐2‐enamides) have been investigated. Under irradiation, the acyl‐substituted anilides 1a – 1c and 1o afforded exclusively the corresponding quinoline‐based cyclization products of type 2 (Table 1). In contrast, irradiation of the benzoyl (Bz)‐substituted anilides 1e – 1h afforded a mixture of the open‐chain amides 4e – 4h and the cyclization products 2e – 2h . Irradiation of the para‐acyl‐substituted anilides 6a – 6e and 6h afforded the corresponding quinoline‐based cyclization products of type 5 as the sole products (Table 2). The formation of the cyclization products 2a – 2c and 2o can be rationalized in terms of 6π‐electron cyclization, followed by thermal [1,5] acyl migration, and that of compounds 3p, 5a – 5e , and 5h can be explained by a 6π‐electron cyclization only. The formation of the open‐chain amides 4e – 4h probably follows a mechanism involving a 1,7‐diradical, C and a spirolactam of type D (Scheme). Long‐range ζ‐H abstraction by the excited carbonyl O‐atom of the benzoyl group on the aniline ring is expected to proceed via a nine‐membered cyclic transition state, as proposed on the basis of X‐ray crystallographic analyses (Fig. 2).     

Photochemical Reactions of N‐(2‐Halogenoalkanoyl) Derivatives of Anilines

The photochemical reactions of 2‐substituted N‐(2‐halogenoalkanoyl) derivatives 1 of anilines and 5 of cyclic amines are described. Under irradiation, 2‐bromo‐2‐methylpropananilides 1a – e undergo exclusively dehydrobromination to give N‐aryl‐2‐methylprop‐2‐enamides (=methacrylanilides) 3a – e (Scheme 1 and Table 1). On irradiation of N‐alkyl‐ and N‐phenyl‐substituted 2‐bromo‐2‐methylpropananilides 1f – m , cyclization products, i.e. 1,3‐dihydro‐2H‐indol‐2‐ones (=oxindoles) 2f – m and 3,4‐dihydroquinolin‐2(1H)‐ones (=dihydrocarbostyrils) 4f – m , are obtained, besides 3f – m . On the other hand, irradiation of N‐methyl‐substituted 2‐chloro‐2‐phenylacetanilides 1o – q and 2‐chloroacetanilide 1r gives oxindoles 2o – r as the sole product, but in low yields (Scheme 3 and Table 2). The photocyclization of the corresponding N‐phenyl derivatives 1s – v to oxindoles 2s – v proceeds smoothly. A plausible mechanism for the formation of the photoproducts is proposed (Scheme 4). Irradiation of N‐(2‐halogenoalkanoyl) derivatives of cyclic amines 5a – c yields the cyclization products, i.e. five‐membered lactams 6a , b , and/or dehydrohalogenation products 7a , c and their cyclization products 8a , c , depending on the ring size of the amines (Scheme 5 and Table 3).     

Synthesis of [3,3′(4H,4′H)‐Bi‐2H‐1,3‐oxazine]‐4,4′‐diones and Their Hydrolysis

The [3,3′(4H,4′H)‐bi‐2H‐1,3‐oxazine]‐4,4′‐diones 3a – 3i were obtained by [2+4] cycloaddition reactions of furan‐2,3‐diones 1a – 1c with aromatic aldazines 2a – 2d (Scheme 1). So, new derivatives of bi‐2H‐1,3‐oxazines and their hydrolysis products, 3,5‐diaryl‐1H‐pyrazoles 4a – 4c (Scheme 3), which are potential biologically active compounds, were synthesized for the first time.     

Synthesis of Derivatives of the Optically Active β‐Amino Acids from (+)‐Car‐2‐ene

The reaction of (+)‐car‐2‐ene ( 4 ) with chlorosulfonyl isocyanate (=sulfuryl chloride isocyanate; ClSO₂NCO) led to the tricyclic lactams 6 and 8 corresponding to the initial formation both of the tertiary carbenium and α‐cyclopropylcarbenium ions (Scheme 2). A number of optically active derivatives of β‐amino acids which are promising compounds for further use in asymmetric synthesis were synthesized from the lactams (see 16, 17 , and 19 – 21 in Scheme 3).     

One‐Pot Synthesis of Functionalized 2H‐Chromene (=2H‐1‐Benzopyran) Derivatives via a Three‐Component Reaction between a CH‐Acid,a Dialkyl Acetylenedicarboxylate,and Methyl Chloroglyoxylate or Benzyl Carbonochloridate Mediated by Triphenylphosphine

An effective route to functionalized 2H‐chromene (=2H‐1‐benzopyran) derivatives 4 is described (Scheme 1). This involves the reaction of a 1,1‐diactivated alkene, resulting from the reaction of dimedone (=5,5‐dimethylcyclohexane‐1,3‐dione; 1a ) with methyl chloroglyoxylate (ClC(O)COOMe), benzyl carbonochloridate (ClC(O)OCH₂Ph) or 3,5‐dinitrobenzoyl chloride (3,5‐(NO₂)₂C₆H₃C(O)Cl), and a dialkyl acetylenedicarboxylate (=dialkyl but‐2‐ynedioate) in the presence of Ph₃P which undergo intramolecular Wittig reaction to produce 2H‐chromene derivatives (Scheme 1).     

Efficient Synthesis of 1‐Arylquinoxalin‐2(1H)‐ones via Cyclocondensation of N‐Aryl‐Substituted 2‐Nitrosoanilines with Functionalized Alkyl Acetates

N‐Aryl‐substituted 2‐nitrosoanilines (=2‐nitrosobenzenamines) 1 , readily available by nucleophilic substitution of the ortho‐H‐atom in nitroarenes with arenamines, react with 2‐substituted acetic acid esters in the presence of a weak base giving 1‐arylquinoxalin‐2(1H)‐ones (Scheme 2). This cyclocondensation allows for the synthesis of compounds 2 – 4 , unsubstituted at C(3) or substituted by alkyl, aryl, ester, amide, and keto groups, in good to excellent yields (Tables 1–4).     

An Approach to the Synthesis of 7‐Amino‐6‐imino‐9‐phenyl‐6H‐benzo[c]chromene‐8‐carbonitrile Derivatives via a Three‐Component Reaction under Ultrasonic Irradiation

An efficient synthesis of 7‐amino‐6‐imino‐9‐phenyl‐6H‐benzo[c]chromene‐8‐carbonitrile derivatives 3 by a three‐component reaction of salicylaldehydes (=2‐hydroxybenzaldehydes) 1 , malononitrile (=propanedinitrile), and 2‐(1‐arylethylidene)malononitrile 2 under ultrasonic irradiation in EtOH is reported. Good yields, short reaction times, and easy purification are the main advantages of the present method. The structures were confirmed spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS) and by elemental analyses. A plausible mechanism for this reaction is proposed (Scheme 2).     

A Novel and Efficient Synthesis of 3‐[(4,5‐Dihydro‐1H‐pyrrol‐3‐yl)carbonyl]‐2H‐chromen‐2‐ones (=3‐[(4,5‐Dihydro‐1H‐pyrrol‐3‐yl)carbonyl]‐2H‐1‐benzopyran‐2‐ones)

An efficient one‐pot synthesis of 3‐[(4,5‐dihydro‐1H‐pyrrol‐3‐yl)carbonyl]‐2H‐chromen‐2‐one (=3‐[(4,5‐dihydro‐1H‐pyrrol‐3yl)carbonyl]‐2H‐1‐benzopyran‐2‐one) derivatives 4 by a four‐component reaction of a salicylaldehyde 1 , 4‐hydroxy‐6‐methyl‐2H‐pyran‐2‐one, a benzylamine 2 , and a diaroylacetylene (=1,4‐diarylbut‐2‐yne‐1,4‐dione) 3 in EtOH is reported. This new protocol has the advantages of high yields (Table), and convenient operation. The structures of these coumarin (=2H‐1‐benzopyran‐2‐one) derivatives, which are important compounds in organic chemistry, were confirmed spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS) and by elemental analyses. A plausible mechanism for this reaction is proposed (Scheme 2).     

Synthesis of Polyalkylphenyl Prop‐2‐ynoates and Their Flash Vacuum Pyrolysis to Polyalkylcyclohepta[b]furan‐2(2H)‐ones

A new method for the smooth and highly efficient preparation of polyalkylated aryl propiolates has been developed. It is based on the formation of the corresponding aryl carbonochloridates (cf. Scheme 1 and Table 1) that react with sodium (or lithium) propiolate in THF at 25 – 65°, with intermediate generation of the mixed anhydrides of the arylcarbonic acids and prop‐2‐ynoic acid, which then decompose almost quantitatively into CO₂ and the aryl propiolates (cf. Scheme 11). This procedure is superior to the transformation of propynoic acid into its difficult‐to‐handle acid chloride, which is then reacted with sodium (or lithium) arenolates. A number of the polyalkylated aryl propiolates were subjected to flash vacuum pyrolysis (FVP) at 600 – 650° and 10⁻² Torr which led to the formation of the corresponding cyclohepta[b]furan‐2(2H)‐ones in average yields of 25 – 45% (cf. Scheme 14). It has further been found in pilot experiments that the polyalkylated cyclohepta[b]furan‐2(2H)‐ones react with 1‐(pyrrolidin‐1‐yl)cyclohexene in toluene at 120 – 130° to yield the corresponding 1,2,3,4‐tetrahydrobenz[a]azulenes, which become, with the growing number of Me groups at the seven‐membered ring, more and more sensitive to oxidative destruction by air (cf. Scheme 15).     

Synthesis of 9‐Halogenated 9‐Deazaguanine N7‐(2′‐Deoxyribonucleosides)

The syntheses of N⁷‐glycosylated 9‐deazaguanine 1a as well as of its 9‐bromo and 9‐iodo derivatives 1b , c are described. The regioselective 9‐halogenation with N‐bromosuccinimide (NBS) and N‐iodosuccinimide (NIS) was accomplished at the protected nucleobase 4a (2‐{[(dimethylamino)methylidene]amino}‐3,5‐dihydro‐3‐[(pivaloyloxy)methyl]‐4H‐pyrrolo[3,2‐d]pyrimidin‐4‐one). Nucleobase‐anion glycosylation of 4a – c with 2‐deoxy‐3,5‐di‐O‐(p‐toluoyl)‐α‐D ‐erythro‐pentofuranosyl chloride ( 5 ) furnished the fully protected intermediates 6a – c (Scheme 2). They were deprotected with 0.01M NaOMe yielding the sugar‐deprotected derivatives 8a – c (Scheme 3). At higher concentrations (0.1M NaOMe), also the pivaloyloxymethyl group was removed to give 7a – c , while conc. aq. NH₃ solution furnished the nucleosides 1a – c . In D₂O, the sugar conformation was always biased towards S (67–61%).     

A Novel,One‐Pot Four‐Component Route to 2′‐Thioxo‐2′,3′‐dihydrospiro[indole‐3,6′‐[1,3]thiazin]‐2‐one Derivatives

An efficient route to 2′,3′‐dihydro‐2′‐thioxospiro[indole‐3,6′‐[1,3]thiazin]‐2(1H)‐one derivatives is described. It involves the reaction of isatine, 1‐phenyl‐2‐(1,1,1‐triphenyl‐λ⁵‐phosphanylidene)ethan‐1‐one, and different amines in the presence of CS₂ in dry MeOH at reflux (Scheme 1). The alkyl carbamodithioate, which results from the addition of the amine to CS₂, is added to the α,β‐unsaturated ketone, resulting from the reaction between 1‐phenyl‐2‐(1,1,1‐triphenyl‐λ⁵‐phosphanylidene)ethan‐1‐one and isatine, to produce the 3′‐alkyl‐2′,3′‐dihydro‐4′‐phenyl‐2′‐thioxospiro[indole‐3,6′‐[1,3]thiazin]‐2(1H)‐one derivatives in excellent yields (Scheme 2). Their structures were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS) and by elemental analyses.     

A One‐Pot Biginelli Synthesis of 6‐Unsubstituted 5‐Aroylpyrimidin‐2(1H)‐ones and 6‐Acetyl‐1,2,4‐triazin‐3(2H)‐ones

The three‐component Biginelli‐like cyclocondensation reaction of enamines 1 , urea, and aldehydes in dioxane/acetic acid efficiently afforded the corresponding 6‐unsubstituted 3,4‐dihydropyrimidin‐2(1H)‐ones 2 in good yields (Scheme 1, Table). The corresponding reaction of azaenamine (=hydrazone) 7 with benzaldehyde and urea afforded 6‐acetyl‐1,2,4‐triazin‐3(2H)‐ones in good yields (Scheme 3).     

Unexpected Thermal Transformation of Aryl 3‐Arylprop‐2‐ynoates: Formation of 3‐(Diarylmethylidene)‐2,3‐dihydrofuran‐2‐ones

A number of aryl 3‐arylprop‐2‐ynoates 3 has been prepared (cf. Table 1 and Schemes 3 – 5). In contrast to aryl prop‐2‐ynoates and but‐2‐ynoates, 3‐arylprop‐2‐ynoates 3 (with the exception of 3b ) do not undergo, by flash vacuum pyrolysis (FVP), rearrangement to corresponding cyclohepta[b]furan‐2(2H)‐ones 2 (cf. Schemes 1 and 2). On melting, however, or in solution at temperatures >150°, the compounds 3 are converted stereospecifically to the dimers 3‐[(Z)‐diarylmethylidene]‐2,3‐dihydrofuran‐2‐ones (Z)‐ 11 and the cyclic anhydrides 12 of 1,4‐diarylnaphthalene‐2,3‐dicarboxylic acids, which also represent dimers of 3 , formed by loss of one molecule of the corresponding phenol from the aryloxy part (cf. Scheme 6). Small amounts of diaryl naphthalene‐2,3‐dicarboxylates 13 accompanied the product types (Z)‐ 11 and 12 , when the thermal transformation of 3 was performed in the molten state or at high concentration of 3 in solution (cf. Tables 2 and 4). The structure of the dihydrofuranone (Z)‐ 11c was established by an X‐ray crystal‐structure analysis (Fig. 1). The structures of the dihydrofuranones 11 and the cyclic anhydrides 12 indicate that the 3‐arylprop‐2‐ynoates 3 , on heating, must undergo an aryl O→C(3) migration leading to a reactive intermediate, which attacks a second molecule of 3 , finally under formation of (Z)‐ 11 or 12 . Formation of the diaryl dicarboxylates 13 , on the other hand, are the result of the well‐known thermal Diels‐Alder‐type dimerization of 3 without rearrangement (cf. Scheme 7). At low concentration of 3 in decalin, the decrease of 3 follows up to ca. 20% conversion first‐order kinetics (cf. Table 5), which is in agreement with a monomolecular rearrangement of 3 . Moreover, heating the highly reactive 2,4,6‐trimethylphenyl 3‐(4‐nitrophenyl)prop‐2‐ynonate ( 3f ) in the presence of a twofold molar amount of the much less reactive phenyl 3‐(4‐nitrophenyl)prop‐2‐ynonate ( 3g ) led, beside (Z)‐ 11f , to the cross products (Z)‐ 11fg , and, due to subsequent thermal isomerization, (E)‐ 11fg (cf. Scheme 10), the structures of which indicated that they were composed, as expected, of rearranged 3f and structurally unaltered 3g . Finally, thermal transposition of [¹⁷O]‐ 3i with the ¹⁷O‐label at the aryloxy group gave (Z)‐ and (E)‐[¹⁷O₂]‐ 11i with the ¹⁷O‐label of rearranged [¹⁷O]‐ 3i specifically at the oxo group of the two isomeric dihydrofuranones (cf. Scheme 8), indicating a highly ordered cyclic transition state of the aryl O→C(3) migration (cf. Scheme 9).     

Synthesis of Substituted Pyrrolo[3,4‐a]carbazoles

The cycloaddition between N‐protected 3‐{1‐[(trimethylsilyl)oxy]ethenyl}‐1H‐indoles and substituted maleimides (= 1H‐pyrrole‐2,5‐diones) yielded substituted pyrrolo[3,4‐a]carbazole derivatives bearing an additional succinimide (= pyrrolidine‐2,5‐dione) moiety either at C(5a) or C(10b) depending on the type of the protection group at the indole N‐atom. Derivatives substituted at C(10b) were isolated when the protection group, Me₃Si or Boc (^tBuOCO), was eliminated during the reaction (Schemes 2 and 3), whereas a substitution at C(5a) was observed when an electron‐withdrawing group, Tos (4‐MeC₆H₄SO₂) or pivaloyl (Me₃CCO), was not eliminated (Scheme 1). Complex results were found in reactions between 1‐(trimethylsilyl)‐3‐{1‐[(trimethylsilyl)oxy]ethenyl}‐1H‐indole, in contrast to formerly reported results (Scheme 3). Some derivatives of 1H,5H‐[1,2,4]triazolo[1′,2 : 1,2]pyridazino[3,4‐b]indole‐1,3(2H)‐dione were obtained from reactions with 4‐phenyl‐3H‐1,2,4‐triazole‐3,5(4H)‐dione (Scheme 2). All structures were established by spectroscopic data, by calculations, and one representative structure was confirmed by an X‐ray crystallographic analysis (Fig.). Finally, the formation of the different structure types was discussed, and compared with similar reactions reported in the literature.     

`Segmented' crystals solved using synchrotron radiation: (2S,3R,4S,5R)‐4‐(10,10‐dimethyl‐3,3‐dioxo‐3λ6‐thia‐4‐azatricyclo[5.2.1.01,5]decan‐4‐ylcarbonyl)‐2,6‐diphenylperhydropyrrolo[3,4‐c]pyrrole‐1,3‐dione

The title compound, C₂₉H₃₁N₃O₅S, forms needle‐shaped `segmented' crystals, thereby inhibiting successful single‐crystal data collection using conventional laboratory facilities. One crystallite of dimensions 0.15 × 0.03 × 0.01 mm yielded sufficent single‐crystal diffraction data on the Australian Synchrotron PX1 beamline. The two independent molecules in the asymmetric unit are nearly superimposable and show only minor conformational deviations from closely related compounds. The molecules pack using one N—H...O hydrogen bond and several phenyl C—H...O(=S), phenyl C—H...O(=C) and methylene C—H...O(=C) hydrogen bonds and weak C—H...π interactions.     

Efficient Synthesis of Novel Coumarin‐3‐carboxamides (=2‐Oxo‐2H‐1‐benzopyran‐3‐carboxamides) Containing Lipophilic Spacers

The novel coumarin‐3‐carboxamides (=2‐oxo‐2H‐1‐benzopyran‐3‐carboxamides) 5a – 5g containing lipophilic spacers were synthesized through the Ugi‐four‐component reaction (Scheme 1). The reactions of aromatic aldehydes 1 , 4,4′‐oxybis[benzenamine] or 4,4′‐methylenebis[benzenamine] as diamine 2 , coumarin‐3‐carboxylic acid (=2‐oxo‐2H‐benzopyran‐3‐carboxylic acid; 3 ), and alkyl isocyanides 4 lead to the desired substituted coumarin‐3‐carboxamides 5a – 5g at room temperature with high bond‐forming efficiency. These novel coumarin derivatives exhibit brilliant fluorescence at 544 nm in CHCl₃.     

Ultrasound‐Promoted Catalyst‐Free Synthesis of 2,2′‐(1,4‐Phenylene)bis[1‐acetyl‐1,2‐dihydro‐4H‐3,1‐benzoxazin‐4‐one] Derivatives

A convenient approach to 2,2′‐(1,4‐phenylene)bis[1‐acetyl‐1,2‐dihydro‐4H‐3,1‐benzoxazin‐4‐one] derivatives 4 was explored employing the one‐pot condensation of anthranilic acids (=2‐aminobenzoic acids) 1 with terephthalaldehyde (=benzene‐1,4‐dicarboxaldehyde; 2 ) under ultrasound‐irradiation conditions (Scheme 1). The reactions proceeded smoothly in the presence of excess Ac₂O in the absence of any other catalyst and solvent to afford the respective products in high yields.     

Synthesis of Macrocyclic Lactones via Ring Transformation of 4‐(ω‐Hydroxyalkyl)‐1,3‐oxazol‐5(4H)‐ones

The synthesis of α‐benzamido‐α‐benzyl lactones 23 of various ring size was achieved either via ‘direct amide cyclization’ by treatment of 2‐benzamido‐2‐benzyl‐ω‐hydroxy‐N,N‐dimethylalkanamides 21 in toluene at 90 – 110° with HCl gas or by ‘ring transformation’ of 4‐benzyl‐4‐(ω‐hydroxyalkyl)‐2‐phenyl‐1,3‐oxazol‐5(4H)‐ones under the same conditions. The precursors were obtained by C‐alkylations of 4‐benzyl‐2‐phenyl‐1,3‐oxazol‐5(4H)‐one ( 15 ) with THP‐ or TBDMS‐protected ω‐hydroxyalkyl iodides. Ring opening of the THP‐protected oxazolones by treatment with Me₂NH followed by deprotection of the OH group gave the diamides 21 , whereas deprotection of the TBDMS series of oxazolones 25 with TBAF followed by treatment with HCl gas led to the corresponding lactones 23 in a one‐pot reaction.     

One‐Pot Synthesis of 4‐(Alkylamino)‐1‐(arylsulfonyl)‐3‐benzoyl‐1,5‐ dihydro‐5‐hydroxy‐5‐phenyl‐2H‐pyrrol‐2‐ones via a Multicomponent Reaction

An effective route to novel 4‐(alkylamino)‐1‐(arylsulfonyl)‐3‐benzoyl‐1,5‐dihydro‐5‐hydroxy‐5‐phenyl‐2H‐pyrrol‐2‐ones 10 is described (Scheme 2). This involves the reaction of an enamine, derived from the addition of a primary amine 5 to 1,4‐diphenylbut‐2‐yne‐1,4‐dione, with an arenesulfonyl isocyanate 7 . Some of these pyrrolones 10 exhibit a dynamic NMR behavior in solution because of restricted rotation around the C? N bond resulting from conjugation of the side‐chain N‐atom with the adjacent α,β‐unsaturated ketone group, and two rotamers are in equilibrium with each other in solution ( 10 ? 11 ; Scheme 3). The structures of the highly functionalized compounds 10 were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS), by elemental analyses, and, in the case of 10a , by X‐ray crystallography. A plausible mechanism for the reaction is proposed (Scheme 4).