1,5‐Dipolar Electrocyclizations of Thiocarbonyl Ylides Bearing CN Groups: Reactions of N‐[(Dimethylamino)methylene]thiobenzamide and 2‐(Dimethylhydrazono)‐1‐phenylethane‐1‐thione with Diazo Compounds

The reactions of thiobenzamide 8 with diazo compounds proceeded via reactive thiocarbonyl ylides as intermediates, which underwent either a 1,5‐dipolar electrocyclization to give the corresponding five membered heterocycles, i.e., 4‐amino‐4,5‐dihydro‐1,3‐thiazole derivatives (i.e., 10a, 10b, 10c , cis‐ 10d , and trans‐ 10d ) or a 1,3‐dipolar electrocyclization to give the corresponding thiiranes as intermediates, which underwent a S_Ni′‐like ring opening and subsequent 5‐exo‐trig cyclization to yield the isomeric 2‐amino‐2,5‐dihydro‐1,3‐thiazole derivatives (i.e., 11a, 11b, 11c , cis‐ 11d , and trans‐ 11d ). In general, isomer 10 was formed in higher yield than isomer 11 . In the case of the reaction of 8 with diazo(phenyl)methane ( 3d ), a mixture of two pairs of diastereoisomers was formed, of which two, namely cis‐ 10d and trans‐ 10d , could be isolated as pure compounds. The isomers cis‐ 11d and trans‐ 11d remained as a mixture. In the reactions of the thioxohydrazone 9 with diazo compounds 3b and 3d , the main products were the alkenes 18 and 23 , respectively. Their formation was rationalized by a 1,3‐dipolar electrocyclization of the corresponding thiocarbonyl ylide and subsequent desulfurization of the intermediate thiiran. As minor products, 2,5‐dihydro‐1,3‐thiazol‐5‐amines 21 and 24 were obtained, which have been formed by 1,5‐dipolar electrocyclization of the thiocarbonyl ylide, followed by a 1,3‐shift of the dimethylamino group.     

Novel 1:1 adducts from the reaction of hexafluoroacetone azine with various olefins and 1,3-dienes; Diels-Alder adducts as intermediates in the formation of criss-cross 2:1 adducts

Hexafluoroacetone azine¹ (I) has been reported^2,3 to react thermally (160–180°) with electron-rich olefins of type H₂CCHR (R  H, Me, or Et) to give “criss-cross” (1,3-:4,2-) adducts (II). In contrast, cis- or trans-but-2-ene and cyclohexene react under comparable conditions to give nitrogen and products derived formally from addition and insertion reactions of bis(trifluoromethyl)-carbene, although it is doubtful if the free carbene is involved^2,4. The reaction of azine (I) with alkanes is reported⁵ to involve a radical mechanism in which the diazo compound (CF₃)₂CN₂, and thence the carbene (CF₃)₂C:, is formed by a secondary process.     

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 .     

A Conjugate Addition Approach to Diazo‐Containing Scaffolds with β Quaternary Centers

Structurally complex diazo‐containing scaffolds are formed by conjugate addition to vinyl diazonium salts. The electrophile, a little studied α‐diazonium‐α,β‐unsaturated carbonyl compound, is formed at low temperature under mild conditions by treating β‐hydroxy‐α‐diazo carbonyls with Sc(OTf)₃. Conjugate addition occurs selectively at the 3‐position of indole to give α‐diazo‐β‐indole carbonyls, and enoxy silanes react to give 2‐diazo‐1,4‐dicarbonyl products. These reactions result in the formation of tertiary and quaternary centers, and give products that would be otherwise difficult to form. Importantly, the diazo functional group is retained within the molecule for future manipulation. Treating an α‐diazo ester indole addition product with Rh₂(OAc)₄ caused a rearrangement to occur to give a 2‐(1H‐indol‐3‐yl)‐2‐enoate. In the case of diazo ketone compounds, this shift occurred spontaneously on prolonged exposure to the Lewis acidic reaction conditions.     

A Conjugate Addition Approach to Diazo-Containing Scaffolds with β Quaternary Centers

Structurally complex diazo-containing scaffolds are formed by conjugate addition to vinyl diazonium salts. The electrophile, a little studied α-diazonium-α,β-unsaturated carbonyl compound, is formed at low temperature under mild conditions by treating β-hydroxy-α-diazo carbonyls with Sc(OTf)₃. Conjugate addition occurs selectively at the 3-position of indole to give α-diazo-β-indole carbonyls, and enoxy silanes react to give 2-diazo-1,4-dicarbonyl products. These reactions result in the formation of tertiary and quaternary centers, and give products that would be otherwise difficult to form. Importantly, the diazo functional group is retained within the molecule for future manipulation. Treating an α-diazo ester indole addition product with Rh₂(OAc)₄ caused a rearrangement to occur to give a 2-(1H-indol-3-yl)-2-enoate. In the case of diazo ketone compounds, this shift occurred spontaneously on prolonged exposure to the Lewis acidic reaction conditions.     

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.     

Reactions of diazo esters with electron-deficient alkenes in the presence of Lewis acids

1,3-Dipolar cycloaddition reaction of diazo esters to electron-deficient dipolarophiles to yield the corresponding 1- or 2-pyrazolines was found to be significantly accelerated with Lewis acids (Yb(OTf)₃, Sc(OTf)₃, GaCl₃, EtAlCl₂). The use of GaCl₃ as the catalyst leads to the acceleration not only of the 1,3-dipolar cycloaddition reaction, but also subsequent insertion of the CHCO₂Me electrophilic fragment of methyl diazoacetate into the N-H bond of 2-pyrazolines formed. Such Lewis acids as SnCl₄, BF₃, TiCl₄, and In(OTf)₃ are not efficient in the described processes, since they rapidly decompose starting diazo compounds.     

Quantification of the Electrophilicities of Diazoalkanes: Kinetics and Mechanism of Azo Couplings with Enamines and Sulfonium Ylides

Kinetics and mechanism of the reactions of methyl diazoacetate, dimethyl diazomalonate, 4-nitrophenyldiazomethane, and diphenyldiazomethane with sulfonium ylides and enamines were investigated by UV-Vis and NMR spectroscopy. Ordinary alkenes undergo 1,3-dipolar cycloadditions with these diazo compounds. In contrast, sulfonium ylides and enamines attack at the terminal nitrogen of the diazo alkanes to give zwitterions, which undergo various subsequent reactions. As only one new bond is formed in the rate-determining step of these reactions, the correlation lg k₂(20 °C)=s_N(N+E) could be used to determine the one-bond electrophilicities E of the diazo compounds from the measured second-order rate constants and the known reactivity indices N and s_N of the sulfonium ylides and enamines. The resulting electrophilicity parameters (−21<E<−18), which are 11–14 orders of magnitude smaller than that of the benzenediazonium ion, are used to define the scope of one-bond nucleophiles which may react with these diazoalkanes.     

Diazo-Transfer Reaction with Diphenyl Phosphorazidate

Diphenyl phosphorazidate (DPPA) was used as the azide source in a one-pot synthesis of 2,2-disubstituted 3-amino-2H-azirines 1 (Scheme 1). The reaction with lithium enolates of amides of type 2 , bearing two substituents at C(2), proceeded smoothly in THF at 0°; keteniminium azides C and azidoenamines D are likely intermediates. Under analogous reaction conditions, DPPA and amides of type 3 with only one substituent at C(2) gave 2-diazoamides 5 in fair-to-good yield (Scheme 2). The corresponding 2-diazo derivatives 6–8 were formed in low yield by treatment of the lithium enolates of N,N-dimethyl-2-phenylacetamide, methyl 2-phenylacetate, and benzyl phenyl ketone, respectively, with DPPA. Thermolysis of 2-diazo-N-methyl-N-phenylcarboxamides 5a and 5b yielded 3-substituted 1,3-dihydro-N-methyl-2H-indol-2-ones 9a and 9b , respectively (Scheme 3). The diazo compounds 5–8 reacted with 1,3-thiazole-5 (4H)-thiones 10 and thiobenzophenone ( 13 ) to give 6-oxa-1,9-dithia-3-azaspiro[4.4]nona-2,7-dienes 11 (Scheme 4) and thiirane-2-carboxylic acid derivatives 14 (Scheme 5), respectively. In analogy to previously described reactions, a mechanism via 1,3-dipolar cycloaddition, leading to 2,5-dihydro-1,3,4-thiadiazoles, and elimination of N₂ to give the ‘thiocarbonyl ylides’ of type H or K is proposed. These dipolar intermediates with a conjugated C?O group then undergo either a 1,5-dipolar electrocyclization to give spirohetrocycles 11 or a 1,3-dipolar electrocyclization to thiiranes 14 .     

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.     

Intramolecular Reactions of Metal Carbenoids with Allylic Ethers: Is a Free Ylide Involved in Every Case?

Rhodium‐, copper‐ and iridium‐catalyzed reactions of the ¹³C‐labelled diazo carbonyl substrates 18* and 19* were performed. Results obtained from copper‐ and iridium‐catalyzed reactions of the ¹³C‐labelled α‐diazo β‐keto ester 19* indicate that either or both of these reactions do not proceed via a free oxonium ylide but instead follow a competing non‐ylide route that delivers apparent [2,3]‐sigmatropic rearrangement products. In the case of the iridium‐catalyzed reaction of α‐diazo β‐keto ester 19* , results obtained from crossover experiments indicate that the initially formed metal‐bound ylide dissociates to give an iridium enolate and an allyl cation, which recombine to form the C?C bond.     

Generation of cyclopropanediazonium and its chemical transformations in the presence of phenol

The reaction of phenol with cyclopropanediazonium ion generated in situ from N-cyclopropyl-N-nitrosourea by the action of K₂CO₃ or Cs₂CO₃ was studied. The main reaction pathway is diazo coupling of cyclopropanediazonium with phenol to give 4-(cyclopropyldiazenyl)phenol, and only traces of isomeric 2-(cyclopropyldiazenyl)phenol were formed. The reaction was accompanied by partial denitrogenation of the diazonium ion with formation of cyclopropyl and allyl cations which gave rise to a number of by-products. All transformation products were characterized by the ¹H and ¹³C NMR spectra with detailed signal assignment.     

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.     

Selective carbometallation of α,β-unsaturated carbonyl compounds and substituted oxacyclopropanes with zirconium-diene complexes

The reactions of zirconium-dience complexes, ZrCp₂(s-cis-diene), with bifunctional electrophiles, i.e. α,β-unsaturated ketones, unsaturated esters and substituted oxacyclopropanes, were investigated. Reaction of ZrCp₂(s-cis-isoprene) with an equivalent of 3-buten-2-one or alkyl acrylates, selectively gives 1,2-addition products. CC bond formation occured at the C(1) atom of the isoprene moiety whereas 1,3-pentadiene-, 2-methyl-1,3-pentadiene- and 2,4-dimethyl-1,3-pentadiene complexes induced the regioselective 1,2-addition at the C(4) position of the diene moiety. Phenyloxacyclopropane and 2-methyl-3-phenyl-oxacyclopropane also react with ZrCp₂(isoprene) leading to CC bond formation from the C(1) atom of isoprene to the oxirane carbon bearing the phenyl group. The corresponding reactions of 2-methyl-2-butene-1,4-diylmagnesium with α,β-unsaturated carbonyl compounds were also studied and found to give quite different products.     

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.     

Formation and reactions of substituted diazocyclopropanes and cyclopropyldiazonium ions

Decomposition of N-nitroso-N-cyclopropylureas at 5—7 °C on treatment with K₂CO₃ containing 15—20% H₂O allows simultaneous generation of both substituted diazocyclopropanes and cyclopropyldiazonium ions, which can react according to 1,3-dipolar cycloaddition or azo-coupling pattern with appropriate substrates. The nature of substituents in the cyclopropyl ring have a pronounced influence on the product ratio (and, probably, on the equilibrium between the diazo compound and the diazonium ion). Thus, on treatment with a base in the presence of equimolar amounts of methyl metacrylate as a trap for the diazo compound and 2-naphthol as a trap for the diazonium ion, N-cyclopropyl- and N-(2,2-dimethylcyclopropyl)-N-nitrosourea azo coupling products predominate. Conversely, N-(2,2-dichlorocyclopropyl)-N-nitrosourea is transformed predominantly into 1,3-cycloaddition products. A rationalization for the experimental data is proposed.     

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.     

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.     

Unprecedented and Effective Synthesis of Thiazolines from Perfluoro-3-isothiocyanato-2-methyl-2-pentene and Certain P-Nucleofuges

The reactions of perfluoro-3-isothiocyanato-2-methyl-2-pentene with PPh₃ and P(NEt₂)₃ in the presence of NaBF₄, KI, and NaBPh4 form phosphonium salts with the heterocyclic substituent (4E)-5,5-bis(trifluoromethyl)-4-(tetrafluoroethylidene)-4,5-dihydro-1,3-thiazol-2-yl, instead of involving desulfurization and formation of P-F-containing products. The reaction with tris(pentafluorophenyl)phosphine fails. The reactions with P(OEt)₃ in the presence of ClSiMe₃ or (CH₃O)₂POSiMe₃ yield diethyl or dimethyl [(4E)-5,5-bis(trifluoromethyl)-4-(tetrafluoroethylidene)-4,5-dihydro-1,3-thiazol-2-yl]phosphonates and no intramolecular alkylation products. The ¹H, ¹³C, ¹⁹F, and ³¹P spectra are presented, and the reaction pathways are discussed. Potential mechanisms of the biological and catalytic activity of the reaction products are considered.     

Stereochemical Course of the Reaction between Thiocarbonyl Compounds and Oxiranes: Reaction with cis‐ and trans‐2,3‐Dimethyloxirane

The reactions of thiocarbonyl compounds with cis‐2,3‐dimethyloxirane ( 1a ) in CH₂Cl₂ in the presence of BF₃⋅Et₂O or SnCl₄ led to trans‐4,5‐dimethyl‐1,3‐oxathiolanes, whereas with trans‐2,3‐dimethyloxirane ( 1b ) cis‐4,5‐dimethyl‐1,3‐oxathiolanes were formed. With the stronger Lewis acid SnCl₄, the formation of side‐products was also observed. In the case of 1,3‐thiazole‐5(4H)‐thione 2 , these side‐products are the corresponding 1,3‐thiazol‐5(4H)‐one 5 and the 1 : 2 adduct 8 (Schemes 2 – 4). Their formation can be rationalized by the decomposition of the initially formed spirocyclic 1,3‐oxathiolane and by a second addition onto the C=N bond of the 1 : 1 adduct, respectively. The secondary epimerization by inversion of the configuration of the spiro‐C‐atom (Schemes 5 – 7) can be explained by a Lewis‐acid‐catalyzed ring opening of the 1,3‐oxathiolane ring and subsequent ring closure to the thermodynamically more stable isomer (Scheme 12). In the case of 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanone ( 20 ), apart from the expected spirocyclic 1,3‐oxathiolanes 21 and 23 , dispirocyclic 1 : 2 adducts were formed by a secondary addition onto the C=O group of the four‐membered ring (Schemes 9 and 10).