1,3‐Dipolar Cycloadditions of α‐Diazo Ketones Derived from N‐Protected (S)‐Proline with Aromatic and Cycloaliphatic Thioketones

Enantiomerically pure α‐oxo diazo compounds derived from (S)‐proline were used for 1,3‐dipolar cycloaddition with aryl and hetaryl thioketones, as well as with cycloalkanethiones. Whereas the reactions with hetaryl thioketones in boiling THF yield α,β‐unsaturated ketones via a cascade of cycloaddition, 1,3‐dipolar electrocyclization, and desulfurization, the analogous reactions with thiobenzophenone and cycloalkanethiones result in the formation of 1,3‐oxathiole derivatives. In the latter case, the 1,5‐dipolar electrocyclization of the intermediate thiocarbonyl ylide is the key step of the reaction sequence. In all cases, the isolated products are optically active, i.e., the multistep processes occur with retention of the stereogenic center incorporated via the use of (S)‐proline as the precursor of the diazo compounds.     

Thermal [2+3]‐Cycloadditions of trans‐1‐Methyl‐2,3‐diphenylaziridine with CS and CC Dipolarophiles: An Unexpected Course with Dimethyl Dicyanofumarate

The thermal reaction of trans‐1‐methyl‐2,3‐diphenylaziridine (trans‐ 1a ) with aromatic and cycloaliphatic thioketones 2 in boiling toluene yielded the corresponding cis‐2,4‐diphenyl‐1,3‐thiazolidines cis‐ 4 via conrotatory ring opening of trans‐ 1a and a concerted [2+3]‐cycloaddition of the intermediate (E,E)‐configured azomethine ylide 3a (Scheme 1). The analogous reaction of cis‐ 1a with dimethyl acetylenedicarboxylate ( 5 ) gave dimethyl trans‐2,5‐dihydro‐1‐methyl‐2,5‐diphenylpyrrole‐3,4‐dicarboxylate (trans‐ 6 ) in accord with orbital‐symmetry‐controlled reactions (Scheme 2). On the other hand, the reactions of cis‐ 1a and trans‐ 1a with dimethyl dicyanofumarate ( 7a ), as well as that of cis‐ 1a and dimethyl dicyanomaleate ( 7b ), led to mixtures of the same two stereoisomeric dimethyl 3,4‐dicyano‐1‐methyl‐2,5‐diphenylpyrrolidine‐3,4‐dicarboxylates 8a and 8b (Scheme 3). This result has to be explained via a stepwise reaction mechanism, in which the intermediate zwitterions 11a and 11b equilibrate (Scheme 6). In contrast, cis‐1,2,3‐triphenylaziridine (cis‐ 1b ) and 7a gave only one stereoisomeric pyrrolidine‐3,4‐dicarboxylate 10 , with the configuration expected on the basis of orbital‐symmetry control, i.e., via concerted reaction steps (Scheme 10). The configuration of 8a and 10 , as well as that of a derivative of 8b , were established by X‐ray crystallography.     

Reactions of Thioketones Possessing a Conjugated CC Bond with Diazo Compounds

The reactions of several thioketones containing a conjugated C?C bond with diazo compounds were investigated. All of the selected compounds reacted via a 1,3‐dipolar cycloaddition with the C?S group and subsequent N₂ elimination to yield thiocarbonyl ylides as intermediates, which underwent a 1,3‐dipolar electrocyclization to give the corresponding thiirane 25 , or, by a subsequent desulfurization, to give the olefins 33a and 33b . None of the intermediate thiocarbonyl ylides reacted via 1,5‐dipolar electrocyclization. If the α,β‐unsaturated thiocarbonyl compound bears an amino group in the β‐position, the reactions with diazo compounds led to the 2,5‐dihydrothiophenes 40a – 40d . In these cases, the proposed mechanism of the reactions led once more to the thiocarbonyl ylides 36 and thiiranes 38 , respectively. The thiiranes reacted via an S_Ni′‐like mechanism to give the corresponding thiolate/ammonium zwitterion 39 , which underwent a ring closure to yield the 2,5‐dihydrothiophenes 40 . Also in these cases, no 1,5‐dipolar electrocyclization could be observed. The structures of several key products were established by X‐ray crystallography.     

Synthesis of Dispiro[indole‐3,2′‐pyrrolidine‐3′,2″‐pyrrolizine]‐1″,2(1H)‐diones via Azomethine Ylide 1,3‐Dipolar Cycloaddition

The 1,3‐dipolar cycloaddition of azomethine ylide generated in situ from isatin and sarcosine to 2‐arylmethylidene‐2,3‐dihydro‐1H‐pyrrolizin‐1‐ones afforded novel 1′‐methyl‐4′‐(aryl)‐1″H‐dispiro[indole‐3,2′‐pyrrolidine‐3′,2″‐pyrrolizine]‐1″,2(1H)‐diones in good yields. The structures of all the products were characterized thoroughly by NMR, infrared spectroscopy, mass spectrum, and elemental analysis.     

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.     

Reactions of 2‐Unsubstituted 1H‐Imidazole 3‐Oxides with 2,2‐Bis(trifluoromethyl)ethene‐1,1‐dicarbonitrile: A Stepwise 1,3‐Dipolar Cycloaddition

The reaction of 1,4,5‐trisubstituted 1H‐imidazole‐3‐oxides 1 with 2,2‐bis(trifluoromethyl)ethene‐1,1‐dicarbonitrile ( 7 , BTF) yielded the corresponding 1,3‐dihydro‐2H‐imidazol‐2‐ones 10 and 2‐(1,3‐dihydro‐2H‐imidazol‐2‐ylidene)malononitriles 11 , respectively, depending on the solvent used. In one example, a 1 : 1 complex, 12 , of the 1H‐imidazole 3‐oxide and hexafluoroacetone hydrate was isolated as a second product. The formation of the products is explained by a stepwise 1,3‐dipolar cycloaddition and subsequent fragmentation. The structures of 11d and 12 were established by X‐ray crystallography.     

Reactions of Methyl Diazoacetate with (E)‐ and (Z)‐1,2‐Bis(trifluoromethyl)ethene‐1,2‐dicarbonitrile: Novel and Unanticipated Pathways

The cycloadditions of methyl diazoacetate to 2,3‐bis(trifluoromethyl)fumaronitrile ((E)‐ BTE ) and 2,3‐bis(trifluoromethyl)maleonitrile ((Z)‐ BTE ) furnish the 4,5‐dihydro‐1H‐pyrazoles 13 . The retention of dipolarophile configuration proceeds for (E)‐ BTE with > 99.93% and for (Z)‐ BTE with > 99.8% (CDCl₃, 25°), suggesting concertedness. Base catalysis (1,4‐diazabicyclo[2.2.2]octane (DABCO), proton sponge) converts the cycloadducts, trans‐ 13 and cis‐ 13 , to a 94 : 6 equilibrium mixture (CDCl₃, r.t.); the first step is N‐deprotonation, since reaction with methyl fluorosulfonate affords the 4,5‐dihydro‐1‐methyl‐1H‐pyrazoles. Competing with the cis/trans isomerization of 13 is the formation of a bis(dehydrofluoro) dimer (two diastereoisomers), the structure of which was elucidated by IR, ¹⁹F‐NMR, and ¹³C‐NMR spectroscopy. The reaction slows when DABCO is bound by HF, but F^? as base keeps the conversion to 22 going and binds HF. The diazo group in 22 suggests a common intermediate for cis/trans isomerization of 13 and conversion to 22 : reversible ring opening of N‐deprotonated 13 provides 18 , a derivative of methyl diazoacetate with a carbanionic substituent. Mechanistic comparison with the reaction of diazomethane and dimethyl 2,3‐dicyanofumarate, a related tetra‐acceptor‐ethylene, brings to light unanticipated divergencies.     

1,3‐Dipolar Cycloadditions of Ethyl 2‐Diazo‐3,3,3‐trifluoropropanoate to Alkynes and [1,5] Sigmatropic Rearrangements of the Resulting 3H‐Pyrazoles: Synthesis of Mono‐, Bis‐ and Tris(trifluoromethyl)‐Substituted Pyrazoles

The 1,3‐dipolar cycloadditions of ethyl 2‐diazo‐3,3,3‐trifluoropropanoate with electron‐rich and electron‐deficient alkynes, as well as the van Alphen? Hüttel rearrangements of the resulting 3H‐pyrazoles were investigated. These reactions led to a series of CF₃‐substituted pyrazoles in good overall yields. Phenyl‐ and diphenylacetylene proved to be unreactive, but, at high temperature, the diazoalkane and phenylacetylene furnished a cyclopropene derivative. As expected, the 1,3‐dipolar cycloaddition to the ynamine occurred much faster than those to electron‐deficient alkynes. With one exception, all cycloadditions proceeded with excellent regioselectivities. The [1,5] sigmatropic rearrangement of the primary 3H‐pyrazoles provided products with shifted acyl groups; products resulting from the migration of a CF₃ group were not detected. In agreement with literature reports, this rearrangement occurs faster with 3H‐pyrazoles bearing electron‐withdrawing substituents.     

Selenophen‐2‐yl‐Substituted Thiocarbonyl Ylides – at the Borderline of Dipolar and Biradical Reactivity

The reactions of aryl (selenophen‐2‐yl) thioketones with CH₂N₂ occur with spontaneous elimination of N₂, even at low temperature (?65°), to give regioselectively sterically crowded 4,4,5,5‐tetrasubstituted 1,3‐dithiolanes and/or a novel type of twelve‐membered dithia‐diselena heterocycles as dimers of the transient thiocarbonyl S‐methanides. The ratio of these products depends on the type of substituent located at C(4) of the phenyl ring. Whereas the formation of the 1,3‐dithiolanes corresponds to a [3+2] cycloaddition of an intermediate thiocarbonyl ylide with the starting thioketone, the twelve‐memberd ring has to be formed via dimerization of the ‘thiocarbonyl ylide’ with an extended biradical structure.     

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).     

An Approach to the Synthesis of Spiro[indene‐pyridoisoquinoline] Derivatives via 1,4‐Dipolar Cycloaddition of Isoquinoline and Acetylene Esters,and (1,3‐Dihydro‐1,3‐dioxo‐2H‐inden‐2‐ylidene)malononitrile

A concise and efficient approach to the spiro‐tetrahydroisoquinoline derivatives has been developed by 1,4‐dipolar cycloaddition of zwitterions resulting from isoquinoline and acetylene esters and (1,3‐dihydro‐1,3‐dioxo‐2H‐inden‐2‐ylidene)malononitrile in MeCN at room temperature. The significance of this method lies in good yields and ease of product purification, and no inert atmosphere is required. The structures of the products were confirmed spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS) and by elemental analyses. A plausible mechanism for this reaction is proposed (Scheme).     

Analogues of α‐Campholenal (= (1R)‐2,2,3‐Trimethylcyclopent‐3‐ene‐1‐acetaldehyde) as Building Blocks for (+)‐β‐Necrodol (= (1S,3S)‐2,2,3‐Trimethyl‐4‐methylenecyclopentanemethanol) and Sandalwood‐like Alcohols

To complete our panorama in structure–activity relationships (SARs) of sandalwood‐like alcohols derived from analogues of α‐campholenal (= (1R)‐2,2,3‐trimethylcyclopent‐3‐ene‐1‐acetaldehyde), we isomerized the epoxy‐isopropyl‐apopinene (?)‐ 2d to the corresponding unreported α‐campholenal analogue (+)‐ 4d (Scheme 1). Derived from the known 3‐demethyl‐α‐campholenal (+)‐ 4a , we prepared the saturated analogue (+)‐ 5a by hydrogenation, while the heterocyclic aldehyde (+)‐ 5b was obtained via a Bayer‐Villiger reaction from the known methyl ketone (+)‐ 6 . Oxidative hydroboration of the known α‐campholenal acetal (?)‐ 8b allowed, after subsequent oxidation of alcohol (+)‐ 9b to ketone (+)‐ 10 , and appropriate alkyl Grignard reaction, access to the 3,4‐disubstituted analogues (+)‐ 4f,g following dehydration and deprotection. (Scheme 2). Epoxidation of either (+)‐ 4b or its methyl ketone (+)‐ 4h , afforded stereoselectively the trans‐epoxy derivatives 11a,b , while the minor cis‐stereoisomer (+)‐ 12a was isolated by chromatography (trans/cis of the epoxy moiety relative to the C₂ or C₃ side chain). Alternatively, the corresponding trans‐epoxy alcohol or acetate 13a,b was obtained either by reduction/esterification from trans‐epoxy aldehyde (+)‐ 11a or by stereoselective epoxidation of the α‐campholenol (+)‐ 15a or of its acetate (?)‐ 15b , respectively. Their cis‐analogues were prepared starting from (+)‐ 12a . Either (+)‐ 4h or (?)‐ 11b , was submitted to a Bayer‐Villiger oxidation to afford acetate (?)‐ 16a . Since isomerizations of (?)‐ 16 lead preferentially to β‐campholene isomers, we followed a known procedure for the isomerization of (?)‐epoxyverbenone (?)‐ 2e to the norcampholenal analogue (+)‐ 19a . Reduction and subsequent protection afforded the silyl ether (?)‐ 19c , which was stereoselectively hydroborated under oxidative condition to afford the secondary alcohol (+)‐ 20c . Further oxidation and epimerization furnished the trans‐ketone (?)‐ 17a , a known intermediate of either (+)‐β‐necrodol (= (+)‐(1S,3S)‐2,2,3‐trimethyl‐4‐methylenecyclopentanemethanol; 17c ) or (+)‐(Z)‐lancifolol (= (1S,3R,4Z)‐2,2,3‐trimethyl‐4‐(4‐methylpent‐3‐enylidene)cyclopentanemethanol). Finally, hydrogenation of (+)‐ 4b gave the saturated cis‐aldehyde (+)‐ 21 , readily reduced to its corresponding alcohol (+)‐ 22a . Similarly, hydrogenation of β‐campholenol (= 2,3,3‐trimethylcyclopent‐1‐ene‐1‐ethanol) gave access via the cis‐alcohol rac‐ 23a , to the cis‐aldehyde rac‐ 24 .     

Reactions of α,β‐Unsaturated Thioamides with Diazo Compounds

Several reactions of the α,β‐unsaturated thioamide 8 with diazo compounds 1a – 1d were investigated. The reactions with CH₂N₂ ( 1a ), diazocyclohexane ( 1b ), and phenyldiazomethane ( 1c ) proceeded via a 1,3‐dipolar cycloaddition of the diazo dipole at the C?C bond to give the corresponding 4,5‐dihydro‐1H‐pyrazole‐3‐carbothioamides 12a – 12c , i.e., the regioisomer which arose from the bond formation between the N‐terminus of the diazo compound and the C(α)‐atom of 8 . In the reaction of 1a with 8 , the initially formed cycloadduct, the 4,5‐dihydro‐3H‐pyrazole‐3‐carbothioamide 11a , was obtained after a short reaction time. In the case of 1c , two tautomers 12c and 12c ′ were formed, which, by derivatization with 2‐chlorobenzoyl chloride 14 , led to the crystalline products 15 and 15 ′. Their structures were established by X‐ray crystallography. From the reaction of 8 and ethyl diazoacetate ( 1d ), the opposite regioisomer 13 was formed. The monosubstituted thioamide 16 reacted with 1a to give the unstable 4,5‐dihydro‐1H‐pyrazole‐3‐carbothioamide 17 .     

Reactions of Sterically Hindered ‘Thiocarbonyl Ylides' with 1,2‐Bis(trifluoromethyl)ethene‐1,2‐dicarbonitrile: Isolation of a Cyclic Seven‐Membered Ketene Imine

When ‘thiocarbonyl ylide' 1A (=(2,2,4,4‐tetramethyl‐3‐oxocyclobutylidenesulfonio)methanide) is generated from the dihydrothiadiazole 5A by N₂ extrusion at 40° in the presence of 2,3‐bis(trifluoromethyl)fumaronitrile ((E)‐ 10 ), a cyclic seven‐membered ketene imine 11 and trans‐thiolane 12 are formed (81 : 19). The reaction of 1A with (Z)‐ 10 furnishes 11, 12 , and cis‐thiolane 25 in the ratio of 82 : 12 : 6. The strained ketene imine 11 is crystalline and storable as a consequence of the stabilizing ‘perfluoroalkyl effect'. The ketene imine group is stereogenic; 11 has a transoid structure with respect to the CF₃ groups, and there is no evidence for the cisoid diastereoisomer. Ketene imine 11 adds H₂O, MeOH, and PhNH₂. In solution at 60°, 11 undergoes an irreversible ring contraction, furnishing the thiolanes 12 / 25 98 : 2. The rate constant of this first‐order rearrangement increases 850‐fold, as the solvent polarity rises from cyclohexane to CD₃CN, in accordance with a zwitterionic intermediate. It is the same intermediate that is initially formed from 1A and 10 , and its intramolecular N‐ and C‐alkylation give rise to 11 and 12 + 25 , respectively. In contrast to 1A , thiocarbonyl ylide 27 , which harbors the sterically less‐demanding adamantylidene group, reacts with (E)‐ 10 to give trans‐thiolane 29 , but no ketene imine. The precursor 26 catalyzes the (Z)/(E) isomerization of 10 ((E)/(Z) ca. 95 : 5 at equilibrium), thus obviating conclusions on steric course and mechanism of this cycloaddition.     

Reactions of Diazomethanes with 5‐Benzylidene‐3‐phenylrhodanine – A Computational Study

It has been shown previously that the reaction of diazomethane with 5‐benzylidene‐3‐phenylrhodanine ( 1 ) in THF at ?20° occurs at the exocyclic C?C bond via cyclopropanation to give 3a and methylation to yield 4 , respectively, whereas the corresponding reaction with phenyldiazomethane in toluene at 0° leads to the cyclopropane derivative 3b exclusively. Surprisingly, under similar conditions, no reaction was observed between 1 and diphenyldiazomethane, but the 2‐diphenylmethylidene derivative 5 was formed in boiling toluene. In the present study, these results have been rationalized by calculations at the DFT B3LYP/6‐31G(d) level using PCM solvent model. In the case of diazomethane, the formation of 3a occurs via initial Michael addition, whereas 4 is formed via [3+2] cycloaddition followed by N₂ elimination and H‐migration. The preferred pathway of the reaction of 1 with phenyldiazomethane is a [3+2] cycloaddition, subsequent N₂ elimination and ring closure of an intermediate zwitterion to give 3b . Finally, the calculations show that the energetically most favorable reaction of 1 with diphenyldiazomethane is the initial formation of diphenylcarbene, which adds to the S‐atom to give a thiocarbonyl ylide, followed by 1,3‐dipolar electrocyclization and S‐elimination.     

1,3-Oxathiole and Thiirane Derivatives from the Reactions of Azibenzil and α-diazo amides with thiocarbonyl compounds

The reactions of α-diazo ketones 1a,b with 9H-fluorene-9-thione ( 2f ) in THF at room temperature yielded the symmetrical 1,3-dithiolanes 7a,b , whereas 1b and 2,2,4,4-tetramethylcyclobutane-1,3-dithione ( 2d ) in THF at 60° led to a mixture of two stereoisomeric 1,3-oxathiole derivatives cis- and trans- 9a (Scheme 2). With 2-diazo-1,2-diphenylethanone ( 1c ), thio ketones 2a–d as well as 1,3-thiazole-5(4H)-thione 2g reacted to give 1,3-oxathiole derivatives exclusively (Schemes 3 and 4). As the reactions with 1c were more sluggish than those with 1a,b , they were catalyzed either by the addition of LiClO₄ or by Rh₂(OAc)₄. In the case of 2d in THF/LiClO₄ at room temperature, a mixture of the monoadduct 4d and the stereoisomeric bis-adducts cis- and trans- 9b was formed. Monoadduct 4d could be transformed to cis- and trans- 9b by treatment with 1c in the presence of Rh₂(OAc)₄ (Scheme 4). Xanthione ( 2e ) and 1c in THF at room temperature reacted only when catalyzed with Rh₂(OAc)₄, and, in contrast to the previous reactions, the benzoyl-substituted thiirane derivative 5a was the sole product (Scheme 4). Both types of reaction were observed with α-diazo amides 1d,e (Schemes 5–7). It is worth mentioning that formation of 1,3-oxathiole or thiirane is not only dependent on the type of the carbonyl compound 2 but also on the α-diazo amide. In the case of 1d and thioxocyclobutanone 2c in THF at room temperature, the primary cycloadduct 12 was the main product. Heating the mixture to 60°, 1,3-oxathiole 10d as well as the spirocyclic thiirane-carboxamide 11b were formed. Thiirane-carboxamides 11d–g were desulfurized with (Me₂N)₃P in THF at 60°, yielding the corresponding acrylamide derivatives (Scheme 7). All reactions are rationalized by a mechanism via initial formation of acyl-substituted thiocarbonyl ylides which undergo either a 1,5-dipolar electrocyclization to give 1,3-oxathiole derivatives or a 1,3-dipolar electrocyclization to yield thiiranes. Only in the case of the most reactive 9H-fluorene-9-thione ( 2f ) is the thiocarbonyl ylide trapped by a second molecule of 2f to give 1,3-dithiolane derivatives by a 1,3-dipolar cycloaddition.     

Studies on the Reactions of Thiocarbonyl S‐Methanides with Hetaryl Thioketones

Dihetaryl thioketones react with thiocarbonyl ylides to give 1,3‐dithiolanes in high yields. No competitive side reactions of the thiocarbonyl ylides were observed, evidencing the ‘superdipolarophilic’ character of this less‐known group of thioketones. Depending on the type of substituents present in both the thiocarbonyl ylide and the thioketone, formal [3+2] cycloadditions occur with complete regioselectivity or with formation of a mixture of both regioisomers. Regioselective formation of the sterically more crowded 1,3‐dithiolanes is explained via a mechanism involving stabilized 1,5‐biradicals. In systems with less‐efficient radical stabilization, e.g., in the case of adamantanethione S‐methanide, substantial violation of the regioselectivity was observed as a result of steric hindrance.     

Asymmetric 1,3‐Dipolar Cycloadditions of 2‐Diazocyclohexane‐1,3‐diones and Alkyl Diazopyruvates

The 1,3‐dipolar cycloaddition reactions of 2‐diazocyclohexane‐1,3‐dione ( 7a ; Table 1) and of alkyl diazopyruvates ( 11a – e ; Table 3) to 2,3‐dihydrofuran and other enol ethers have been investigated in the presence of chiral transition metal catalysts. With Rh^II catalysts, the cycloadditions were not enantioselective, but those catalyzed by [Ru^IICl₂( 1a )] and [Ru^IICl₂( 1b )] proceeded with enantioselectivities of up to 58% and 74% ee, respectively, when diazopyruvates 11 were used as substrates. The phenyliodonium ylide 7c yielded the adduct 8a in lower yield and poorer selectivity than the corresponding diazo precursor 7a (Table 2) upon decomposition with [Ru(pybox)] catalysts. This suggests that ylide decomposition by Ru^II catalysts, contrary to that of the corresponding diazo precursors, does not lead to Ru‐carbene complexes as reactive intermediates. Our method represents the first reproducible, enantioselective 1,3‐cycloaddition of these types of substrates.     

Chemoselectivity of the Reactions of Diazomethanes with 5‐Benzylidene‐3‐phenylrhodanine

The reactions of 5‐benzylidene‐3‐phenylrhodanine ( 2 ; rhodanine=2‐thioxo‐1,3‐thiazolidin‐4‐one) with diazomethane ( 7a ) and phenyldiazomethane ( 7b ) occurred chemoselectively at the exocyclic C?C bond to give the spirocyclopropane derivatives 9 and, in the case of 7a , also the C‐methylated products 8 (Scheme 1). In contrast, diphenyldiazomethane ( 7c ) reacted exclusively with the C?S group leading to the 2‐(diphenylmethylidene)‐1,3‐thiazolidine 11 via [2+3] cycloaddition and a ‘two‐fold extrusion reaction’. Treatment of 8 or 9b with an excess of 7a in refluxing CH₂Cl₂ and in THF at room temperature in the presence of [Rh₂(OAc)₄], respectively, led to the 1,3‐thiazolidine‐2,4‐diones 15 and 20 , respectively, i.e., the products of the hydrolysis of the intermediate thiocarbonyl ylide. On the other hand, the reactions with 7b and 7c in boiling toluene yielded the corresponding 2‐methylidene derivatives 16, 21a , and 21b . Finally, the reaction of 11 with 7a occurred exclusively at the electron‐poor C?C bond, which is conjugated with the C?O group. In addition to the spirocyclopropane 23 , the C‐methylated 22 was formed as a minor product. The structures of the products (Z)‐ 8, 9a, 9b, 11 , and 23 were established by X‐ray crystallography.     

A new insight into the mechanism of 1,3‐dienes cationic polymerization. II. Structure of poly(1,3‐pentadiene) synthesized with tBuCl/TiCl4 initiating system

The microstructure of poly(1,3‐pentadiene) synthesized by cationic polymerization of 1,3‐pentadiene with ^tBuCl/TiCl₄ initiating system is analyzed using one‐dimensional‐ and two‐dimensional‐NMR spectroscopy. It is shown that unsaturated part of chain contains only homo and mixed dyads with trans?1,4‐, trans?1,2‐, and cis?1,2‐structures with regular and inverse (head‐to‐head or tail‐to‐tail) enchainment, whereas cis?1,4‐ and 3,4‐units are totally absent. The new quantitative method for the calculation of content of different structural units in poly(1,3‐pentadiene)s based on the comparison of methyl region of ¹³C NMR spectra of original and hydrogenated polymer is proposed. The signals of tert‐butyl head and chloromethyl end groups are identified in a structure of poly(1,3‐pentadiene) chain and the new approaches for the quantitative calculation of number‐average functionality at the α‐ and ω‐end are proposed. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3297–3307