Light‐Induced Cycloaddition of 2,3‐Dihydro‐2,2‐dimethyl‐4H‐thiopyran‐4‐one (a 4‐Thiacyclohex‐2‐enone) to Alkenes and Dienes

The reactivity of (thiacyclic)‐2,3‐dihydro‐2,2‐dimethyl‐4H‐thiopyran‐4‐one ( 1a ) in light‐induced cycloadditions to furan ( F ), acrylonitrile ( AN ), or 2,3‐dimethylbut‐2‐ene ( TME ) is compared to that of (carbocyclic) 5,5‐dimethylcyclohex‐2‐enone ( 1b ). Whereas for the more‐flexible thiacycle, the efficiency of [2+2]‐photocycloadduct formation with AN or TME is generally much lower, the diastereoselectivity regarding the ring fusion in the bicyclo[4.2.0]octanes is quite similar for both enones. In contrast, 1a affords exclusively trans‐fused [4+2] cycloadducts with F , while 1b gives predominantly the corresponding cis‐fused products.     

Peri-selective photocycloadditions of methyl 2-pyrone-5-carboxylate with unsaturated cyclic ethers

Photosensitized cycloaddition reaction of methyl 2-pyrone-5-carboxylate ( 1 ) with 2,3-dihydrofuran gave cis- exo- and cis-endo-[2 + 2] cycloadducts across the C₃-C₄ double bond in 1 , and a [4 + 2] cycloadduct which was different in addition-orientation from the Diels-Alder adducts. Each [2 + 2] cycloadduct was obtained by the use of sensitizers having different triplet energies. Photosensitized reactions of 1 with 3,4-dihydro-2H-pyrans afforded cis-endo-[2 + 2] cycloadducts, respectively. The photocycloaddition mechanism was also explained from the excited state of 1 calculated by means of MNDO-Cl method.     

Photocycloaddition of Cyclohex‐2‐enones to 2‐Methylbut‐1‐en‐3‐yne

Irradiation (350 nm) of 2‐alkynylcyclohex‐2‐enones 1 in benzene in the presence of an excess of 2‐methylbut‐1‐en‐3‐yne ( 2 ) affords in each case a mixture of a cis‐fused 3,4,4a,5,6,8a‐hexahydronaphthalen‐1(2H)‐one 3 and a bicyclo[4.2.0]octan‐2‐one 4 (Scheme 2), the former being formed as main product via 1,6‐cyclization of the common biradical intermediate. The (parent) cyclohex‐2‐enone and other alkylcyclohex‐2‐enones 7 also give naphthalenones 8 , albeit in lower yields, the major products being bicyclo[4.2.0]octan‐2‐ones (Scheme 4). No product derived from such a 1,6‐cyclization is observed in the irradiation of 3‐alkynylcyclohex‐2‐enone 9 in the presence of 2 (Scheme 4). Irradiation of the 2‐cyano‐substituted cyclohexenone 12 under these conditions again affords only traces of naphthalenone 13 , the main product now being the substituted bicyclo[4.2.0]oct‐7‐ene 16 (Scheme 5), resulting from [2+2] cycloaddition of the acetylenic C−C bond of 2 to excited 12 .     

From 1-(Silyloxy)Butadiene to 4-Amino-4-Deoxy-DL-Erythrose and to 1-Amino-1-Deoxy-DL-Erythritol Derivatives via Hetero-Diels-Alder Reactions with Acylnitroso Dienophiles

Acylnitroso dienophiles 4 reacted instantly with 1-(silyloxy)butadiene 5α and led in good yield to the regioisomeric cycloadducts 6 (major) and 7 (minor; Scheme 2, Table 1). cis-Hydroxylation of these primary cycloadducts with OsO₄ (catalyst) occurred stereospecifically and in high yield (→ 8 and 9 , resp.; Scheme 2). It was followed by reductive ring cleavage to give either 1-amino-1-deoxy-DL -erythritol or 4-amino-4-deoxy-DL -erythrose derivatives 10 and 14 , respectively, depending on the nature of the reducing agent (Schemes 3 and 4).     

New Studies on [2+3] Cycloadditions of Thermally Generated N‐Isopropyl‐ and N‐(4‐Methoxyphenyl)‐Substituted Azomethine Ylides

The thermal reaction of 1‐substituted 2,3‐diphenylaziridines 2 with thiobenzophenone ( 6a ) and 9H‐fluorene‐9‐thione ( 6b ) led to the corresponding 1,3‐thiazolidines (Scheme 2). Whereas the cis‐disubstituted aziridines and 6a yielded only trans‐2,4,5,5‐tetraphenyl‐1,3‐thiazolidines of type 7 , the analogous reaction with 6b gave a mixture of trans‐ and cis‐2,4‐diphenyl‐1,3‐thiazolidines 7 and 8 . During chromatography on SiO₂, the trans‐configured spiro[9H‐fluorene‐9,5′‐[1,3]thiazolidines] 7c and 7d isomerized to the cis‐isomers. The substituent at N(1) of the aziridine influences the reaction rate significantly, i.e., the more sterically demanding the substituent the slower the reaction. The reaction of cis‐2,3‐diphenylaziridines 2 with dimethyl azodicarboxylate ( 9 ) and dimethyl acetylenedicarboxylate ( 11 ) gave the trans‐cycloadducts 10 and 12 , respectively (Schemes 3 and 4). In the latter case, a partial dehydrogenation led to the corresponding pyrroles. Two stereoisomeric cycloadducts, 15 and 16 , with a trans‐relationship of the Ph groups were obtained from the reaction with dimethyl fumarate ( 14 ; Scheme 5); with dimethyl maleate ( 17 ), the expected cycloadduct 18 together with the 2,3‐dihydropyrrole 19 was obtained (Scheme 6). The structures of the cycloadducts 7b, 8a, 15b , and 16b were established by X‐ray crystallography.     

1,3-Dipolare Cycloadditionen eines Carbonyl-ylids mit 1,3-Thiazol-5(4H)-thionen und Thioketonen

1,3-Dipolar Cycloadditions of a Carhonyl-ylide with 1,3-Thiazole-5(4H)-thiones and Thioketones Inp-xylene at 150°, 3-phenyloxirane-2,2-dicarbonitrile ( 4b ) and 2-phenyl-3-thia-1-azaspiro[4.4]non-1-ene-4-thione ( 1a ) gave the three 1:1 adduets trans- 3a , cis- 3a , and 13a in 61, 21, and 3% yield, respectively (Scheme 3). The stereoisomers trans- 3a and cis- 3a are the products of a regioselective 1,3-dipolar cycloaddition of carbonylylide 2b , generated thermally by an electrocyclic ring opening of 4b (Scheme 6), and the C?S group of 1a . Surprisingly, 13a proved not to be a regioisomeric cycloadduct of 1a and 2b , but an isomer formed via cleavage of the O? C(3) bond of the oxirane 4b . A reaction mechanism rationalizing the formation of 13a is proposed in Scheme 6. Analogous results were obtained from the reaction of 4b and 4,4-dimethyl-2-phenyl-1,3-thiazole-5 (4H)-thione ( 1b , Scheme 3). The thermolysis of 4b in p-xylene at 130° in the presence of adamantine–thione ( 10 ) led to two isomeric 1:1 adducts 15 and 16 in a ratio of ca. 2:1, however, in low yield (Scheme 4). Most likely the products are again formed viathe two competing reaction mechanisms depicted in Scheme 6. The analogous reactions of 4b with 2,2,4,4-tetramethylcyclobutane-1,3-thione ( 11 ) and 9H-xanthene-9-thione ( 12 ) yielded a single 1:1 adduct in each case (Schemes). In the former case, spirocyclic 1,3-oxathiolane 17 , the product of the 1,3-dipolar cycloaddition with 2a corresponding to 3a , was isolated in only 11 % yield. It is remarkable that no 2:1 adduct was formed even in the presence of an excess of 4b. In contrast, 4b and 12 reacted smoothly to give 18 in 81 % yield; no cycloadduct of the carbonylylide 2a could be detected. The structures of cis- 3a , 13a , 15 , and 18 , as well as the structure of 14 , which is a derivative of trans- 3a , have been established by X-ray crystallography (Figs. 1–3, Table).     

1,3‐Thiazolidine‐dicarboxylates from Thioketones and Thermally Generated Azomethine Ylides

The reaction of 9H‐fluorene‐9‐thione ( 1 ) with the cis‐ and trans‐isomers of dimethyl 1‐(4‐methoxyphenyl)aziridine‐2,3‐dicarboxylate (cis‐ and trans‐ 2 , resp.) in xylene at 110° yielded exclusively the spirocyclic cycloadduct with trans‐ and cis‐configurations, respectively (trans‐ and cis‐ 3 , resp.; Scheme 1). Analogously, less‐reactive thioketones, e.g., thiobenzophenone ( 5 ), and cis‐ 2 reacted stereoselectively to give the corresponding trans‐1,3‐thiazolidine‐2,4‐dicarboxylate (e.g., trans‐ 8 ; Scheme 2). On the other hand, the reaction of 5 and trans‐ 2 proceeded in a nonstereoselective course to provide a mixture of trans‐ and cis‐substituted cycloadducts. This result can be explained by an isomerization of the intermediate azomethine ylide. Dimethyl 1,3‐thiazolidine‐2,2‐dicarboxylates 14 and 15 were formed in the thermal reaction of dimethyl aziridine‐2,2‐dicarboxylate 11 with aromatic thioketones (Scheme 3). On treatment of 14 and 15 with Raney‐Ni in refluxing EtOH, a desulfurization and ring‐contraction led to the formation of azetidine‐2,2‐dicarboxylates 17 and 18 , respectively (Scheme 4).     

Photochemistry of Isothiocoumarin (= 1H-[2]benzothiopyran-1-one)

Irradiation (λ = 350 nm) of 1H-[2]benzothiopyran-1-one ( 2 ) in the solid state affords selectively and in good yield 6aα, 6bα, 12bα, 12cα -tetrahydrocyclobuta[1, 2-c:4, 3-c′]bis([2]benzothiopyran)-5, 8-dione ( 3 ), the head-to-head (HH) cis-cisoid-cis-cyclodimer of 2 , X-Ray analysis of 2 confirms that this reaction proceeds according to the well-established topochemical principles. The same dimer 3 is obtained in low yields on irradiation of 10⁻¹ M solutions of 2 in either MeOH or MeCN, while no conversion at all is observed in benzene. On irradiation of 2 in MeCN in the presence of tetrachloroethene, the [2 + 2] photocycloadduct 4 is formed in good yield, the conversion 2 → 4 being efficiently quenched by naphthalene. In contrast, no reaction is observed -on irradiation of 2 in the presence of 2, 3-dimethylbut-2-ene, neither in polar nor in apolar solvents.     

Unexpected Formation of Dimethylthioketene Cycloadducts in the Reaction of 1,3‐Diphenylaziridine‐2,2‐dicarboxylate with Cyclobutanethione Derivatives

The reaction of 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanone ( 1 ) with cis‐1‐alkyl‐2,3‐diphenylaziridines 5 in boiling toluene yielded the expected trans‐configured spirocyclic 1,3‐thiazolidines 6 (Scheme 1). Analogously, dimethyl trans‐1‐(4‐methoxyphenyl)aziridine‐2,3‐dicarboxylate (trans‐ 7 ) reacted with 1 and the corresponding dithione 2 , respectively, to give spirocyclic 1,3‐thiazolidine‐2,4‐dicarboxylates 8 (Scheme 2). However, mixtures of cis‐ and trans‐derivatives were obtained in these cases. Unexpectedly, the reaction of 1 with dimethyl 1,3‐diphenylaziridine‐2,2‐dicarboxylate ( 11 ) led to a mixture of the cycloadduct 13 and 5‐(isopropylidene)‐4‐phenyl‐1,3‐thiazolidine‐2,2‐dicarboxylate ( 14 ), a formal cycloadduct of azomethine ylide 12 with dimethylthioketene (Scheme 3). The regioisomeric adduct 16 was obtained from the reaction between 2 and 11 . The structures of 6b , cis‐ 8a , cis‐ 8b, 10 , and 16 have been established by X‐ray crystallography.     

Umsetzung von Diazoessigsäure-ethylester mit 1,3-Thiazol-5(4H)-thionen

Reaction of Ethyl Diazoacetate with 1,3-Thiazole-5(4H)-thiones Reaction of ethyl diazoacetate ( 2a ) and 1,3-thiazole-5(4H)-thiones 1a,b in Et₂O at room temperature leads to a complex mixture of the products 5–9 (Scheme 2). Without solvent, 1a and 2a react to give 10a in addition to 5a–9a . In Et₂O in the presence of aniline, reaction of 1a,b with 2a affords the ethyl 1,3,4-thiadiazole-2-carboxylate 10a and 10b , respectively, as major products. The structures of the unexpected products 6a, 7a , and 10a have been established by X-ray crystallography. Ethyl 4H-1,3-thiazine-carboxylate 8b was transformed into ethyl 7H-thieno[2,3-e][1,3]thiazine-carboxylate 11 (Scheme 3) by treatment with aqueous NaOH or during chromatography. The structure of the latter has also been established by X-ray crystallography. In the presence of thiols and alcohols, the reaction of 1a and 2a yields mainly adducts of type 12 (Scheme 4), compounds 5a,7a , and 9a being by-products (Table 1). Reaction mechanisms for the formation of the isolated products are delineated in Schemes 4–7: the primary cycloadduct 3 of the diazo compound and the C?S bond of 1 undergoes a base-catalyzed ring opening of the 1,3-thiazole-ring to give 10 . In the absence of a base, elimination of N₂ yields the thiocarbonyl ylide A ′, which is trapped by nucleophiles to give 12 . Trapping of A ′, by H₂O yields 1,3-thiazole-5(4H)-one 9 and ethyl mercaptoacetate, which is also a trapping agent for A ′, yielding the diester 7 . The formation of products 6 and 8 can be explained again via trapping of thiocarbonyl ylide A ′, either by thiirane C (Scheme 6) or by 2a (Scheme 7). The latter adduct F yields 8 via a Demjanoff-Tiffeneau-type ring expansion of a 1,3-thiazole to give the 1,3-thiazine.     

Ring-Transformation von Imidazolidin–2,4-dionen ( = Hydantoinen) zu 4H-Imidazolen bei der Umsetzung mit 3-(Dimethylamino)-2,2-dimethyl-2H -azirin

Ring Transformation of Imidazolidine-2,4-diones ( = Hydantoins) to 4H-Imidazoles in the Reaction with 3-(Dimethylamino)-2,2-dimethyl-2H-azirines At ca. 70°, 3-(dimethylamino)-2,2-dimethyl-2H -azirine ( 1 ) and 5,5-disubstituted hydantoins 4 in MeCN or i-PrOH give 2-(1-aminoalkyl)-5-(dimethylamino)-4,4-dimethyl-4H -imidazoles 5 in good yield (Scheme 2). These products are decarboxylated 1:1 adducts of 1 and 4 . A reaction mechanism is suggested in analogy to the previously reported reactions of 1 and NH-acidic heterocycles containing the CO? NH? CO? NH moiety (Scheme 5). The formation of ureas 6 and 7 can be rationalized by trapping the intermediate isocyanate F by an amine. No reaction is observed between 1 and 1,5,5- or 3,5,5-trisubstituted hydantoins in refluxing MeCN or i-PrOH, but an N-isopropylation of 1,5,5-trimethylhydantoin ( 8b ) occurs in the presence of morpholine (Scheme 3). The reaction of the bis(azirine)dibromozink complex 11 and hydantoines 4 in refluxing MeCN yields zink complexes 12 of the corresponding 2-(1-aminoalkyl)-4H -imidazoles 5 (Scheme 4).     

4-Amino-1,5-dihydro-2H-pyrrol-2-one aus Bortrifluorid-katalysierten Umsetzungen von 3-Amino-2H-azirinen mit Carbonsäure-Derivaten

4-Amino-1,5-dihydro-2H-pyrrol-2-ones from Boron Trifluoride Catalyzed Reactions of 3-Amino-2H-azirines with Carboxylic Acid Derivatives Reaction of 3-amino-2H-azirines 1 with ethyl 2-nitroacetate ( 6a ) in refluxing MeCN affords 4-amino-1,5-dihydro-2H-pyrrol-2-ones 7 and 3,6-diamino-2,5-dihydropyrazines 8 , the dimerization product of 1 (Scheme 2). Thus, 6a reacts with 1 as a CH-acidic compound by C? C bond formation via C-nucleophilic attack of deprotonated 6a onto the amidinium-C-atom of protonated 1 (Scheme 5). The scope of this reaction seems to be rather limited as 1 and 2-substituted 2-nitroacetates do not give any products besides the azirine dimer 8 (see Table 1). Sodium enolates of carboxylic esters and carboxamides 11 react with 1 under BF₃ catalysis to give 4-amino-1,5-dihydro-2H-pyrrol-2-ones 12 in 50–80% yield (Scheme 3, Table 2). In an analogous reaction, 3-amino-2H-pyrrole 13 is formed from 1c and the Li-enolate of acetophenone (Scheme 4). A reaction mechanism for the ring enlargement of 1 involving BF₃ catalysis is proposed in Scheme 6.     

Reaktion von Phenyldiazomethan mit 1,3-Thiazol-5(4H)-thionen: Basenkatalysierte Ring-Öffnung des Primäradduktes

Reaction of Phenyldiazomethane with 1,3-Thiazole-5(4H)-thiones: Base-Catalyzed Ring Opening of the Primary Adduct Reaction of 1,3-thiazole-5(4H)-thiones 1 and phenyldiazomethane ( 2a ) in toluene at room temperature yields the thiiranes trans- and cis-1,4-dithia-6-azaspiro[2.4]hept-5-enes (trans- and cis- 4 ; Scheme 2). With Ph₃P in THF at 70°, these thiiranes are transformed stereospecifically into (E)- and (Z)-5-benzylidene-4,5-dihydro-1,3-thiazoles 5 , respectively. In the presence of DBU, 1 and 2a react to give 1,3,4-thiadiazole derivatives 6 or 7 via base-catalyzed ring opening of the primary cycloadduct (Scheme 3). In the case of 2-(alkylthio)-substituted 1,3-thiazole-5(4H)-thiones 1c and 1d , this ring opening proceeds by elimination of the corresponding alkylthiolate, yielding isothiocyanate 7 . The structures of (Z)- 5c and 6b have been established by X-ray crystallography.     

Solid-State Photocyclodimerization of 1-Thiocoumarin (= 2H-1-Benzothiopyran-2-one)

Irradiation (λ > 390 nm) of 2H-1-benzothiopyran-2-one ( 1 ) in the solid state affords selectively 6aα,6bα,12bα,12cα -tetrahydrocyclobuta[1,2-c:4,3-c′]bis[1]benzothiopyran -6,7-dione ( 2 ), the head-to-head (HH) cis-cisoid-cis-dimer, while irradiation of 1 in the solid state using shorter wavelengths (λ > 340 nm) affords a mixture of all four cis-fused tricyclic dimers 2 – 5 . These results represent a novel wavelength effect in solid-state photochemistry.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit kumulierten Doppelbindungen. Vorläufige Mitteilung

Irradiation of 2-methyl- ( 1c ) and 2,2-dimethyl-3-phenyl-2H-azirine ( 1d ) in benzene solution in the presence of carbon dioxide yields 2-methyl-4-phenyl- ( 3c ) and 2,2-dimethyl-4-phenyl-3-oxazolin-5-one ( 3d ), respectively. Similar cycloadducts are observed (see table) when 2,3-diphenyl-2H-azirine ( 1b ) and 1d are irradiated in the presence of phenylisocyanate, o-tolylisocyanate, phenylisothiocyanate or di-o-tolyl-carbodiimide.     

A Green and Novel Method of Synthesizing 2,2'-Arylmethylene Bis(3-hydroxy-5,5-dimethylcyclohex-2-enone) Catalyzed by L-Histidine in Ionic Liquid

An efficient and green approach to the synthesis of 2,2′‐arylmethylene bis(3‐hydroxy‐5,5‐dimethylcyclohex‐2‐enone) using L‐histidine as the catalyst is described. In addition, room temperature ionic liquid 1‐butyl‐3‐methylimidazonium tetrafluoroborate [bmim]BF₄ was used as green recyclable alternatives to volatile organic solvents for this condensation reaction. This green catalytic system can be recycled several times with no decreases in yields and reaction rates.     

Do Antarafacial Cycloadditions Occur? Cycloaddition of Heptafulvalene with Tetracyanoethylene

The cycloaddition of heptafulvalene ( 1 ) with tetracyanoethylene (TCNE) was previously described as an example of an antarafacial cycloaddition, a [π¹⁴_a+π²_s] process that afforded only the trans cycloadduct by virtue of the edge-to-face approach of TCNE, facilitated by the S shape of 1 . The reaction has been investigated in depth and found not to be a concerted antarafacial process. At low temperature, the reaction is observed to give a mixture of cis and trans cycloadducts as well as a [4+2] cycloadduct. The mixture of products is converted to the trans cycloadduct by equilibration upon warming to room temperature. Studies with diethyl 2,3-dicyanofumarate and -maleate confirmed the formation of cis cycloadducts. DFT studies at the M06-2X/6-311+G(2d,p) SCRF=acetone level of theory show that the originally proposed edge-to-face approach of TCNE to 1 is highly disfavored, whereas a stepwise mechanism involving the addition of TCNE at C2 to form a zwitterion followed by collapse at either C2′ or C7′ is energetically accessible. The Diels-Alder adduct is also formed in a stepwise reaction by competitive addition of TCNE at C4 of 1 . These studies suggest that edge-to-face interactions are prohibitive in even the most favorable cases.     

Cycloadditions of `Thiocarbonyl Ylides' with Tetracyanoethylene (=Ethenetetracarbonitrile): Interception of Intermediates

Thiocarbonyl ylides (=sulfonium ylides) belong to the most nucleophilic 1,3‐dipoles (high HO energy). In their reactions with tetracyanoethylene (TCNE=ethenetetracarbonitrile; low LU energy), a borderline crossing from the concerted mechanism to a two‐step pathway via a 1,5‐zwitterion was observed. Steric hindrance at one or both termini of the 1,3‐dipole is an additional requirement. The ylides 3 and 13 , set free by N₂ elimination of dihydro‐1,3,4‐thiadiazoles, underwent electrocyclization or 1,4‐H shift. Ylides 3 and 13 are bases and afforded MeOH adducts of different regiochemistry. Whereas 3 and TCNE in abs. THF at 45° furnished the (3+2) cycloadduct 20 , a MeOH content of 0.5 – 5 vol‐% in THF gave rise to a seven‐membered lactim ether 22 and thiolane 20 in a 65: 35 ratio (Scheme 4). Water (0.5 – 1 vol‐%) in THF led to lactam 24 and adduct 20 in the same ratio. The zwitterion 26 , assumed to be the first intermediate, enters competing reactions: the irreversible ring closure to thiolane 20 and the reversible formation of a strained, cyclic seven‐membered `ketene imine' 28 , which is intercepted by MeOH or H<sub>2</sub>O. The gauche‐conformation 32 of an analogous zwitterion, produced from the tetrasubstituted `thiocarbonyl ylide' 13 with TCNE (Scheme 5), led to the thiolane derivative 35 , while the anti‐conformation 33 afforded the thioxo compound 5 and cyclopropane derivative 36 by intramolecular nucleophilic substitution.     

Exploring the Border between Concerted and Two‐Step Pathways of 1,3‐Dipolar Cycloadditions of Organic Azides to Cyclic Ketene N,X‐Acetals. – Synthesis and 15N‐NMR Spectra of Zwitterions and Spirocyclic Cycloadducts

Cyclic ketene N,X‐acetals 1 are electron‐rich dipolarophiles that undergo 1,3‐dipolar cycloaddition reactions with organic azides 2 ranging from alkyl to strongly electron‐deficient azides, e.g., picryl azide ( 2L ; R¹=2,4,6‐(NO₂)₃C₆H₂) and sulfonyl azides 2M – O (R¹=XSO₂; cf. Scheme 1). Reactions of the latter with the most‐nucleophilic ketene N,N‐acetals 1A provided the first examples for two‐step HOMO(dipolarophile)–LUMO(1,3‐dipole)‐controlled 1,3‐dipolar cycloadditions via intermediate zwitterions 3 . To set the stage for an exploration of the frontier between concerted and two‐step 1,3‐dipolar cycloadditions of this type, we first describe the scope and limitations of concerted cycloadditions of 2 to 1 and delineate a number of zwitterions 3 . Alkyl azides 2A – C add exclusively to ketene N,N‐acetals that are derived from 1H‐tetrazole (see 1A ) and 1H‐imidazole (see 1B , C ), while almost all aryl azides yield cycloadducts 4 with the ketene N,X‐acetals (X=NR, O, S) employed, except for the case of extreme steric hindrance of the 1,3‐dipole (see 2E ; R¹=2,4,6‐(^tBu)₃C₆H₂). The most electron‐deficient paradigm, 2L , affords zwitterions 16D , E in the reactions with 1A , while ketene N,O‐ and N,S‐acetals furnish products of unstable intermediate cycloadducts. By tuning the electronic and steric demands of aryl azides to those of ketene N,N‐acetals 1A , we discovered new borderlines between concerted and two‐step 1,3‐dipolar cycloadditions that involve similar pairs of dipoles and dipolarophiles: 4‐Nitrophenyl azide ( 2G ) and the 2,2‐dimethylpropylidene dipolarophile 1A (R, R=H, ^tBu) gave a cycloadduct 13 H , while 2‐nitrophenyl azide ( 2 H ) and the same dipolarophile afforded a zwitterion 16A . Isopropylidene dipolarophile 1A (R=Me) reacted with both 2G and 2 H to afford cycloadducts 13G , J ) but furnished a zwitterion 16B with 2,4‐dinitrophenyl azide ( 2I) . Likewise, 1A (R=Me) reacted with the isomeric encumbered nitrophenyl azides 2J and 2K to yield a cycloadduct 13L and a zwitterion 16C , respectively. These examples suggest that, in principle, a host of such borderlines exist which can be crossed by means of small structural variations of the reactants. Eventually, we use ¹⁵N‐NMR spectroscopy for the first time to characterize spirocyclic cycloadducts 10 – 14 and 17 (Table 6), and zwitterions 16 (Table 7).     

Photochemie von in 4-Stellung substituierten 5-Methyl-3-phenyl-isoxazolen.44. Mitteilung über Photoreaktionen

Photochemistry of 4-substituted 5-Methyl-3-phenyl-isoxazoles. 4-Trideuterioacetyl-5-methyl-3-phenyl-isoxazole ([CD₃CO]- 27 ), upon irradiation with 254 nm light, was converted into a 1:1 mixture of oxazoles [CD₃CO]- 35 and [CD₃]- 35 (Scheme 13). This isomerization is accompagnied by a slower transformation of ([CD₃CO]- 27 ) into [CD₃]- 27 . Irradiation of the isoxazole derivatives 28, 29, 30 and (E)- 31 yielded only oxazoles 36, 37, 38 and (E), (Z)- 39 ; no 4-acetyl-5-alkoxy-2-phenyl-oxazole, 2-acetyl-3-methyl-5-phenyl-pyrrole or 2-acetyl-4-methoxycarbonyl-3-methyl-5-phenyl-pyrrole, respectively, were formed (Scheme 9 and 10). Similarly (E)- 32 gave a mixture of (E), (Z)- 40 only (Scheme 11). Upon shorter irradiation, the intermediate 2H-azirines (E), (Z)- 41 could be isolated (Scheme 11). Photochemical (E)/(Z)-isomerization of the 2-(trifluoro-ethoxycarbonyl)-1-methyl-vinyl side chain in all the compounds 32, 40 and 41 is fast. At 230° the isoxazoles (E)- and (Z)- 32 are converted into oxazoles (E), (Z)- 40 . The same compounds are also obtained by thermal isomerization of the 2H-azirines (E), (Z)- 41 . The most probable mechanism for the photochemical transformations of the isoxazoles, as exemplified in the case of the isoxazole 27 , is shown in Scheme 13. A benzonitrile-methylide intermediate is postulated for the photochemical conversion of the 2H-azirines into oxazoles. 2H-Azirines are also intermediates in the thermal isoxazole-oxazole rearrangement. It is however not yet clear, if the thermal 2H-azirine-oxazole transformation involves the same transient species as the photochemical reaction. A mechanism for the photochemical isomerization of the 2H-azirine 11 to the oxazole 15 is proposed (Scheme 3).