Isomérisation de 2,3‐époxypinanes fonctionnels en présence d'acides de Lewis

Isomerization of Functionalized 2,3‐Epoxypinanes in the Presence of Lewis Acids The functionalized 2,3‐epoxypinanes 1b – i were submitted to isomerization in the presence of ZnBr₂ at 110° (Table 1) or of BF₃⋅Et₂O at different temperatures (Table 2), and their behavior was compared with that of the non‐functionalized parent 1a and with similar known transpositions. The produced campholenals 2 , pinocamphones 3 , and in some cases, fencholenals 4 were isolated and characterized. A mechanism involving a concerted oxirane ring opening is proposed (Scheme 4).     

Lewis Acids as Mild and Effective Catalysts for the Synthesis of 3,5‐Bis[(hetero)arylidene]piperidin‐4‐ones

The aldol‐crotonic condensation reactions of N‐alkyl‐ and NH‐piperidin‐4‐one derivatives with (hetero)aromatic aldehydes promoted by Lewis acids or bases were examined. This comparative study has revealed three effective catalytic systems based on Lewis acids, i.e., LiClO₄ and MgBr₂ (in the presence of tertiary amine), and BF₃⋅Et₂O, for the synthesis of N‐alkyl‐substituted 3,5‐bis(heteroarylidene)piperidin‐4‐ones, including those bearing acid‐ or base‐labile groups both in the (hetero)aromatic groups and in the alkyl substituent at the N‐atom. The highest reaction rate was observed for LiClO₄‐mediated synthesis. Both MgBr₂‐ and LiClO₄‐mediated syntheses were inefficient in the case of NH‐piperidin‐4‐one, while BF₃⋅Et₂O provided the final compounds in high yields. This catalyst is especially advantageous as it allows simultaneous condensation and deprotection in the case of O‐protected piperidin‐4‐one.     

A Direct Route to 3-(D-Ribofuranosyl)pyridine Nucleosides

A route for synthesizing C-nucleosides with 2,6-substituted pyridines as heterocyclic aglycones is described. Condensation of appropriately substituted lithiated pyridines with ribono-1,4-lactone derivatives yields hemiacetal 4a – g (Table 1), which can be reduced by Et₃SiH and BF₃·Et₂O to the corresponding C-nucleoside (see Scheme 1 for 4d → β-D - 5 ). Conditions are presented that optimize the amount of the 2,6-dichloropyridine-derived β-D -anomer β-D - 5 formed (Table 3). Aminolysis of β-D - 5 yields the diaminonucleoside 14 (Scheme 3).     

Regioselectivity of the 1,3‐Oxathiolane Formation in the Lewis Acid‐Catalyzed Reaction of Thioketones with Asymmetrically Substituted Oxiranes

The reactions of the aromatic thioketone 4,4′‐dimethoxythiobenzophenone ( 1 ) with three monosubstituted oxiranes 3a – c in the presence of BF₃⋅Et₂O or SnCl₄ in dry CH₂Cl₂ led to the corresponding 1 : 1 adducts, i.e., 1,3‐oxathiolanes 4a – b with R at C(5) and 8c with Ph at C(4). In addition, 1,3‐dioxolanes 7a and 7c , and the unexpected 1 : 2 adducts 6a – b were obtained (Scheme 2 and Table 1). In the case of the aliphatic, nonenolizable thioketone 1,1,3,3‐tetramethylindane‐2‐thione ( 2 ) and 3a – c with BF₃⋅Et₂O as catalyst, only 1 : 1 adducts, i.e. 1,3‐oxathiolanes 10a – b with R at C(5) and 11a – c with R or Ph at C(4), were formed (Scheme 6 and Table 2). In control experiments, the 1 : 1 adducts 4a and 4b were treated with 2‐methyloxirane ( 3a ) in the presence of BF₃⋅Et₂O to yield the 1 : 2 adduct 6a and 1 : 1 : 1 adduct 9 , respectively (Scheme 5). The structures of 6a , 8c , 10a , 11a , and 11c were confirmed by X‐ray crystallography (Figs. 1 – 5). The results described in the present paper show that alkyl and aryl substituents have significant influence upon the regioselectivity in the process of the ring opening of the complexed oxirane by the nucleophilic attack of the thiocarbonyl S‐atom: the preferred nucleophilic attack occurs at C(3) of alkyl‐substituted oxiranes (O−C(3) cleavage) but at C(2) of phenyloxirane (O−C(2) cleavage).     

1,3‐Oxathiolanes from the Reaction of Aromatic and Enolized Thioketones with Monosubstituted Oxiranes

The reactions of 4,4′‐dimethoxythiobenzophenone ( 1 ) with (S)‐2‐methyloxirane ((S)‐ 2 ) and (R)‐2‐phenyloxirane ((R)‐ 6 ) in the presence of a Lewis acid such as BF₃?Et₂O, ZnCl₂, or SiO₂ in dry CH₂Cl₂ led to the corresponding 1 : 1 adducts, i.e., 1,3‐oxathiolanes (S)‐ 3 with Me at C(5), and (S)‐ 7 and (R)‐ 8 with Ph at C(4) and C(5), respectively. A 1 : 2 adduct, 1,3,6‐dioxathiocane (4S,8S)‐ 4 and 1,3‐dioxolane (S)‐ 9 , respectively, were formed as minor products (Schemes 3 and 5, Tables 1 and 2). Treatment of the 1 : 1 adduct (S)‐ 3 with (S)‐ 2 and BF₃?Et₂O gave the 1 : 2 adduct (4S,8S)‐ 4 (Scheme 4). In the case of the enolized thioketone 1,3‐diphenylprop‐1‐ene‐2‐thiol ( 10 ) with (S)‐ 2 and (R)‐ 6 in the presence of SiO₂, the enesulfanyl alcohols (1′Z,2S)‐ 11 and (1′E,2S)‐ 11 , and (1′Z,2S)‐ 13 , (1′E,2S)‐ 13 , (1′Z,1R)‐ 15 , and (1′E,1R)‐ 15 , respectively, as well as a 1,3‐oxathiolane (S)‐ 14 were formed (Schemes 6 and 8). In the presence of HCl, the enesulfanyl alcohols (1′Z,2S)‐ 11 , (1′Z,2S)‐ 13 , (1′E,2S)‐ 13 , (1′Z,1R)‐ 15 , and (1′E,1R)‐ 15 cyclize to give the corresponding 1,3‐oxathiolanes (S)‐ 12 , (S)‐ 14 , and (R)‐ 16 , respectively (Schemes 7, 9, and 10). The structures of (1′E,2S)‐ 11 , (S)‐ 12 , and (S)‐ 14 were confirmed by X‐ray crystallography (Figs. 1–3). These results show that 1,3‐oxathiolanes can be prepared directly via the Lewis acid‐catalyzed reactions of oxiranes with non‐enolizable thioketones, and also in two steps with enolized thioketones. The nucleophilic attack of the thiocarbonyl or enesulfanyl S‐atom at the Lewis acid‐complexed oxirane ring proceeds with high regio‐ and stereoselectivity via an S_n 2‐type mechanism.     

Synthesis of Indolones via Radical Cyclization of N‐(2‐Halogenoalkanoyl)‐Substituted Anilines

The radical reactions of N‐(2‐halogenoalkanoyl)‐substituted anilines (anilides) of type 1 have been investigated under various conditions. Treatment of compounds 1a – 1o with Bu₃SnH in the presence of (2,2′‐azobis(isobutyronitrile) (AIBN) afforded a mixture of the indolones (oxindoles) 2a – 2o and the reduction products 5a – 5o (Table 1). In contrast, the N‐unsubstituted anilides 1p – 1s, 1u , and 1v gave the corresponding reduction products exclusively (Table 1). Similar results were obtained by treatment of 1 with Ni powder (Table 2) or wth Et₃B (Table 3). Anilides with longer N‐(phenylalkyl) chains such as 6 and 7 were inert towards radical cyclization, with the exception of N‐benzyl‐2‐bromo‐N,2‐dimethylpropanamide ( 6b ), which, upon treatment with Ni powder in i‐PrOH, afforded the cyclized product 9b in low yield (Table 4). Upon irradiation, the extended anilides 6, 7, 10 , and 11 yielded the corresponding dehydrobromination products exclusively (Table 5).     

Lewis Acid‐Mediated Addition of Amino Acid‐Substituted α‐Allylsilanes to Aromatic Acetals

Unnatural amino acids extend the pharmacological formulator's toolkit. Strategies to prepare unnatural amino acid derivatives using Lewis acid‐activated allylsilane reactions are few. In this regard, we examined the utility of allylsilanes bearing an amino acid substituent in the reaction. Diastereoselective addition of methyl 2‐(N‐PG‐amino)‐3‐(trimethylsilyl)pent‐4‐enoate and methyl (E)‐2‐(N‐PG‐amino)‐3‐(trimethylsilyl)hex‐4‐enoate (PG=protecting group), 2 and 13 , respectively, to aromatic acetals in the presence of Lewis acids is described. Of those examined, TiCl₄ was found to be the most effective Lewis acid for promoting the addition. At least 1 equiv. of TiCl₄ was required to achieve high yields, whereas 2 equiv. of BF₃?OEt₂ were required for comparable outcomes. Excellent selectivity (>99% syn/anti) and high yield (up to 89%) were obtained with halo‐substituted aromatic acetals, while more electron‐rich electrophiles led to both lower yields and diastereoselectivities.     

Alcohol Solvates of Lanthanoid(III) Trifluoromethanesulfonates

After vacuum dehydration, a number of hydrated trivalent lanthanoid trifluoromethanesulfonates (“triflate”, “OTf” = F₃CSO₃), when recrystallized from various alcohol (ROH) solutions, yield solvates Ln(OTf)₃ · xROH, x = 3, 5 or 6. The following have been defined crystallographically (R/Ln/x): Me/La/3;Me/Gd/6; Et/Sm/3; Et/Gd/5 iPr/Nd,Sm/3. The Me/Gd/6complex, Gd(OTf)₃ · 6MeOH is a mononuclear/ionic form [(MeOH)₆Gd(O–OTf)₂](OTf), the gadolinium environment being octacoordinate, square‐antiprismatic with the O–OTf donors quasi‐trans on different faces of the coordination polyhedron; the Et/Gd/5 complex is neutral, molecular, mononuclear [(EtOH)₅Gd(O–OTf)₃], also with an octacoordinate, square‐antiprismatic coordination sphere, derivative of that of the methanol solvate. The remainder form one‐dimensional polymeric arrays, successive lanthanoid atoms linked by (μ‐O–OTf–O′)₃ triads, at either end of a tricapped trigonal prismatic array, the ROH molecules contributing the capping atoms. A (“baseline”) (re‐)determination of the “parent” Sm(OTf)₃ · 9H₂O is also recorded.     

Cationic (Azulene)(diene)rhodium(I) Complexes: Spectroscopic,Dynamic, and Catalytic Properties

New [Rh^I(η⁵‐azulene)(η⁴‐diene)][BF₄] complex salts 3 – 5 (diene=8,9,10‐trinorborna‐2,5‐diene (nbd) and (1Z,5Z)‐cycloocta‐1,5‐diene (cod)) were synthesized according to a known procedure (Scheme 1). All of these complexes show dynamic behavior of the diene ligand at room temperature. In the case of the [Rh^I(η⁵‐azulene)(cod)]⁺ complex salts 3 and [Rh^I(η⁵‐guaiazulene)(nbd)]⁺ complex salt 4a (guaiazulene=7‐isopropyl‐1,4‐dimethylazulene), the coalescence temperature of the ¹H‐NMR signals of the olefinic H‐atoms was determined. The free energy of activation (ΔG; Table 1) for the intramolecular movement of the diene ligands exhibits a distinct dependency on the HOMO/LUMO properties of the coordinated azulene ligand. The DFT (density‐functional theory) calculated ΔG values for the internal diene rotation are in good to excellent agreement with the observed ones in CD₂Cl₂ as solvent (Table 2). Moreover, the ΔG values can also be estimated in good approximation from the position of the longest‐wavelength, azulene‐centered UV/VIS absorption band of the complex salts (Table 2). These cationic Rh^I complexes are stable and air‐resistant and can be used, e.g., as precursor complexes in situ in the presence of (M)‐6,7‐bis[(diphenylphosphino)methyl]‐8,12‐diphenylbenzo[a]heptalene for asymmetric hydrogenation of (Z)‐α‐(acetamido)cinnamic acid with ee values of up to 68% (Table 4).     

A Catalytic and Stereoselective Glycosylation with β‐Glycosyl Fluorides

A catalytic and stereoselective glycosylation of several glycosyl acceptors with β‐D ‐glycosyl fluoride was successfully performed in the presence of a catalytic amount of trityl tetrakis(pentafluorophenyl)borate (TrB(C₆F₅)₄) or trifluoromethanesulfonic acid (TfOH). When TrB(C₆F₅)₄ was used as a catalyst in the solvent pivalonitrile/(trifluoromethyl)benzene 1 : 5, the glycosylation proceeded smoothly to afford the glycosides in high yields with high β‐D ‐stereoselectivities (see Table 3). Further, the glycosylation by the armed‐disarmed strategy in the presence of this catalyst was established (see Table 4). Similarly, glycosylation catalyzed by the strong protic acid TfOH afforded the corresponding β‐D ‐glycosides in good‐to‐excellent yields on treating β‐D ‐ glycosyl fluorides having a 2‐O‐benzoyl group with various glycosyl acceptors including thioglycosides (see Tables 6 and 7).     

Practical Stereo‐ and Regioselective,Copper(I)‐Promoted Strecker Synthesis of Sugar‐Modified α,β‐Unsaturated Imines

The regio‐ and stereoselective, Lewis acid catalyzed Strecker reaction between Me₃SiCN and different aldimines incorporating a 2,3,4,6‐tetrakis‐O‐pivaloyl‐D ‐glucopyranosyl (Piv₄Glc) chiral auxiliary has been worked out. Depending on the conditions used, high yields (up to 95%) and good diastereoselectivities (de > 86%) were achieved under mild conditions (Table 1), especially with CuBr ? Me₂S as catalyst. Our protocol allows the ready preparation of asymmetric β,γ‐unsaturated α‐amino acids such as (R)‐2‐amino‐4‐phenylbut‐3‐enoic acid ( 13 ; Scheme 2) and congeners thereof.     

Rhodium(II)‐Catalyzed Inter‐ and Intramolecular Enantioselective Cyclopropanations with Alkyl‐Diazo(triorganylsilyl)acetates

The intermolecular cyclopropanation of styrene with ethyl diazo(triethylsilyl)acetate ( 1a ) proceeds at room temperature in the presence of chiral Rh^II carboxylate catalysts derived from imide‐protected amino acids and affords mixtures of trans‐ and cis‐cyclopropane derivatives 2a in up to 72% yield but with modest enantioselectivities (<54%) (Scheme 1 and Table 1). Protiodesilylation of a diastereoisomer mixture 2a with Bu₄NF is accompanied by epimerization at C(1) (→ 3 ). The intramolecular cyclopropanation of allyl diazo(triethylsilyl)acetate ( 8a ), in turn, affords optically active 3‐oxabicyclo[3.1.0]hexan‐2‐one ( 9a ) with yields of up to 85% and 56% ee (Scheme 3 and Table 2). Similarly, the (2Z)‐pent‐2‐enyl derivative 8d reacts to 9d in up to 77% yield and 38% ee (Scheme 3 and Table 3). In contrast, the diazo decomposition of (2E)‐3‐phenylprop‐2‐enyl and 2‐methylprop‐2‐en‐1‐yl diazo(triethyl‐silyl)acetates ( 8b and 8c , resp.) is unsatisfactory and gives very poor yields of substituted 3‐oxabicyclo[3.1.0]hexan‐2‐ones 9b and 9c , respectively (Table 3).     

Ruthenium Complexes of an Abnormally Bound,Anionic N‐Heterocyclic Carbene

The abnormally bound, anionic NHC–borane complex [Ru(IDipp‐BF₃)(p‐cymene)Cl]₂ ( 4 ; IDipp‐BF₃=1,3‐(2,6‐iPr₂C₆H₃)₂‐2‐BF₃(C₃HN₂)‐4‐yl) was synthesized by transmetalation from Li[(IDipp‐BF₃)₂Ag]. Addition of donors gave species of the form [Ru(IDipp‐BF₃)(p‐cymene)(L)Cl], whereas halide abstraction with Ag(Et₂O)[B(C₆F₅)₄] gave C?H activation of the methine position of the IDipp?BF₃ ligand.     

Lewis Acid Catalyzed Reactions of Thioketones with 1,2-Epoxycyclohexane and 1,2-Epoxycyclopentane

Non-enolizable thioketones and 1,2-epoxycycloalkanes undergo a Lewis acid catalyzed addition reaction to give 1,3-oxathiolanes. Appropriate reaction conditions are CH₂Cl₂ as the solvent, BF₃⋅Et₂O as the Lewis acid, and a temperature between −78° and r.t. Under the reaction conditions, the 1,3-oxathiolanes are only moderately stable. They decompose to yield the corresponding epithiocycloalkane and ketone. In general, 1,3-dithiolanes are isolated as minor products or, after prolonged reaction, as the main product. These secondary products are formed via the Lewis acid catalyzed reaction of the intermediate epithiocycloalkane and a second molecule of the thioketone. In the reaction of thiobenzophenone and 1,2-epoxycyclohexane, trans-8,8-diphenyl-7,9-dioxabicyclo[4.3.0]nonane is formed in small amounts as an additional side product (Scheme 12). In all cases, the newly formed heterocycle and the carbocycle are trans-fused. This result is consistent with a nucleophilic ring-opening of the complexed oxirane by the thioketone via inversion of the configuration and subsequent formation of the O(1)−C(2) bond of the 1,3-oxathiolane (Scheme 13). The surprising formation of the fused 1,4-oxathiepan derivative 23 (Scheme 9) is in accordance with an ionic reaction mechanism (cf. Scheme 15).     

Titanium(IV) Bromide and Boron(III) Tribromide Promoted Baylis‐Hillman Reactions of Arylaldehydes with But‐3‐yn‐2‐one

The reaction of arylaldehydes with but‐3‐yn‐2‐one in the presence of the Lewis acids titanium(IV) bromide (TiBr₄) or boron(III) tribromide (BBr₃) (1.4 equiv.) can be drastically affected by the reaction temperature. When the reaction was carried out at ≤−20°, the brominated compound 1 was obtained as the major product. However, when the reaction was carried out at room temperature (20°), both the brominated compound 1 and dibrominated compound 2 were formed as major products. The substituent on the phenyl ring can affect the (E)/(Z) ratio. Moreover, with 2 as the substrate, the Pd‐catalyzed allylic substitution and Suzuki‐type coupling reaction have been examined.     

Polymerization of phenylacetylene catalysed by RhTp(cod) and RhBp(cod) in ionic liquids: effect of alcohols and of tetraammonium halides

RhTp(cod) ( 1 ) and RhBp(cod) ( 2 ), almost inactive in CH₂Cl₂, became good catalysts of phenylacetylene polymerization in ionic liquids ([bmim]Cl, [bmim]BF₄: bmim = 1‐butyl‐3‐methylimidazolium, [mokt]BF₄: mokt = 1‐methyl‐3‐oktylimidazolium, [bumepy]BF₄: 1‐butyl‐4‐methylpyridinium) and in CH₂Cl₂ in the presence of tetraammonium halides ([R₄N]X, R = Bu, Et; X = Cl, Br). The highest yields of polyphenylacetylene with catalyst 1 were obtained in [bmim]Cl at 65°C (64% after 2 h) and in [mokt]BF₄ at 20°C (56% after 24 h). In alcohols (CH₃OH, (CH₃)₂CHOH, (CH₃)₃COH) as solvents, up to 100% of the polymer was produced. When a mixture of an ionic liquid and CH₃OH was used as the reaction medium, the polymer yield was similar to the yield achieved in an ionic liquid only, but the molecular weight increased remarkably. Tetraammonium salts, [R₄N]X, are co‐catalysts for 1 , and the yield of the polymer increased in the order [Et₄N]Br < [Bu₄N]Br < [Et₄N]Cl < [Bu₄N]Cl. Polymers with molecular weights from 6900 to 38 800 Da were obtained with catalyst 2 in [R₄N]Br or [R₄N]Cl, whereas in ionic liquids ([bmim]Cl, [bmim]BF₄) the corresponding molecular weights were higher, from 51 300 to 60 300 Da. Copyright © 2004 John Wiley & Sons, Ltd.     

Preparation of Polystyrene Beads with Dendritically Embedded TADDOL and Use in Enantioselective Lewis Acid Catalysis

A full account is given of the preparation and use of TADDOLates, which are dendritically incorporated in polystyrene beads (Scheme 1). A series of styryl‐substituted TADDOLs with flexible, rigid, or dendritically branching spacers between the TADDOL core and the styryl groups (2–16 in number) has been prepared ( 5 – 7, 20, 21, 26 in Schemes 2–4 and Fig. 1–3). These were used as cross‐linkers in styrene‐suspension polymerization, leading to beads of ca. 400‐μm diameter (Schemes 5 and 6, b). These, in turn, were loaded with titanate and used for the Lewis acid catalyzed addition of Et₂Zn to PhCHO as a test reaction (Scheme 6). A comparison of the enantioselectivities and degrees of conversion (both up to 99%), obtained under standard conditions, shows that these polymer‐incorporated Ti‐TADDOLates are highly efficient catalysts for this process (Table 1). In view of the effort necessary to prepare the novel, immobilized catalysts, emphasis was laid upon their multiple use. The performance over 20 cycles of the test reaction was best with the polymer obtained from the TADDOL bearing four first‐generation Fréchet branches with eight peripheral styryl groups ( 6 , p‐ 6 , p‐ 6 ⋅Ti(OⁱPr)₂): the enantioselectivity (Fig. 4), the rate of reaction (Fig. 5), and the swelling factor (Fig. 6) were essentially unchanged after numerous operations carried out with the corresponding beads of 400‐μm diameter and a degree of loading of 0.1 mmol TADDOLate/g polymer, with or without stirring (Fig. 7). The rate with the dendritically polymer‐embedded Ti‐TADDOLate (p‐ 6 ⋅Ti(OⁱPr)₂) was greater than that measured with the corresponding monomer, i.e., 6 ⋅Ti(OⁱPr)₂ (Fig. 8). Possible interpretations of this phenomenon are proposed. A polymer‐bound TADDOL, generated on a solid support (by Grignard addition to an immobilized tartrate ester ketal) did not perform well (Scheme 4 and Table 2). Also, when we prepared polystyrene beads by copolymerization of styrene, a zero‐, first‐, or second‐generation dendritic cross‐linker, and a mono‐styryl‐substituted TADDOL derivative, the performance in the test reaction did not rival that of the dendritically incorporated Ti‐TADDOLate ((p‐ 6 ⋅Ti(OⁱPr)₂) (Scheme 7 and Fig. 10). Finally, we have applied the dendritically immobilized Cl₂ and (TsO)₂Ti‐TADDOLate as chiral Lewis acid to preferentially prepare one enantiomer of the exo and the endo (3+2) cycloadduct, respectively, of diphenyl nitrone to 3‐crotonoyl‐1,3‐oxazolidinone; in one of these reaction modes, we have observed an interesting conditioning of the catalyst: with an increasing number of application cycles, the amount of polymer‐incorporated Lewis acid required to induce the same degree of enantioselectivity, decreased; the degrees of diastereo‐ and enantioselectivity were, again, comparable to those reported for homogeneous conditions (Fig. 9).     

Synthese von 4-Benzylthio- und 4-(Arylthio)-1,3-oxazol-5(2H)-onen

Synthesis of 4-(Benzylthio)-and 4-(Arylthio)-1,3-oxazole-5(2H)-ones Following a known procedure, 4-(benzylthio)-1,3-oxazol-5(2H)-one ( 4a ) was synthesized starting from sodium cyanodithioformate ( 1 ) and cyclohexanone (Scheme 1). The structure of the intermediate 4-(benzylthio)-1,3-thiazol-5(2H)-one ( 3a ) was established by X-ray crystallography. An alternative route was developed for the synthesis of 4-(arylthio)-1,3-oxazol-5(2H)-ones which are not accessible by the former reaction. Treatment of ethyl cyanoformate ( 5 ) with a thiophenol in the presence of catalytic amounts of Et₂NH and TiCl₄, followed by addition of a ketone and BF₃.Et₂O in a one-pot-reaction, gave 4f–i in low-to-fair yields (Scheme 3). Both synthetic pathways-complementary as for benzyl–S and aryl-S derivatives–seem to be limited with respect to variation of substituents of the ketone.     

Polymer- and Dendrimer-Bound Ti-TADDOLates in Catalytic (and Stoichiometric) Enantioselective Reactions: Are pentacoordinate cationic Ti complexes the catalytically active species?

α,α,α′,α′-Tetraaryl-1,3-dioxolane-4,5-dimethanols (TADDOLs), containing styryl groups either at C(2) of the heterocyclic ring or in the α-position, were prepared in the usual way ( 18–22, 24, 25 ). These compounds were copolymerized with styrene and divinylbenzene in a suspension, yielding polymers ( 33–40 , Scheme 3) as beads with a rather uniform particle-size distribution (150–45 μm), swellable in common organic solvents. HOCH₂- and BrCH₂-substituted TADDOLs were also prepared and used for attachement to Merrifield resin or to dendritic molecules ( 23, 26–32 ). The TADDOL moieties in these materials are accessible to form Ti (and Al) complexes (Scheme 4) which can be used as polymer- or dendrimer-bound reagents (stoichiometric) or Lewis acids (catalytic). The reactions studied with these new chiral auxiliaries are: enantioselective nucleophilic additions to aldehydes (of R₂Zn and RTi(OCHMe₂)₃; Scheme 5, Table 1) and to ketones (of LiAlH₄, Table 2); enantioselective ring opening of meso-anhydrides (Scheme 6); [4+2] and [3+2] cycloadditions of 3-crotonyl-1,3-oxazolidin-2-one to cyclopentadiene and to (Z)-N-benzylidenephenylamine N-oxide ( → 48, 49 , Scheme 7, Tables 3, 4, and Fig. 5). The enantioselectivities reached with most of the polymer-bound or dendritic TADDOL ligands were comparable or identical to those observed with the soluble analogs. The activity of the polymer-bound Lewis acids was only slightly reduced as compared with that encountered under homogeneous conditions. Multiple use of the beads (up to 10 times), without decreased performance, has been demonstrated (Figs. 3 and 4). The poorer selectivity in the Diels-Alder reaction (Scheme 7a), induced by the polymer-bound Cl₂Ti-TADDOLate as compared to the soluble one, is taken as an opportunity to discuss the mechanism of this Lewis-acid catalysis, and to propose a cationic, trigonal-bipyramidal complex as the catalytically active species (Fig. 6). It is suggested that similar cations may be involved in other Ti-TADDOLate-mediated reactions as well.     

Regio‐ and Stereoselectivity in the Lewis Acid‐ and NaH‐Induced Reactions of Thiocamphor with (R)‐2‐Vinyloxirane

The reaction of the enolizable thioketone (1R,4R)‐thiocamphor (= (1R,4R)‐1,7,7‐trimethylbicyclo[2.2.1]heptane‐2‐thione; 1 ) with (R)‐2‐vinyloxirane ( 2 ) in the presence of a Lewis acid such as SnCl₄ or SiO₂ in anhydrous CH₂Cl₂ gave the spirocyclic 1,3‐oxathiolane 3 with the vinyl group at C(4′), as well as the isomeric enesulfanyl alcohol 4 . In the case of SnCl₄, an allylic alcohol 5 was obtained in low yield in addition to 3 and 4 (Scheme 2). Repetition of the reaction in the presence of ZnCl₂ yielded two diastereoisomeric 4‐vinyl‐1,3‐oxathiolanes 3 and 7 together with an alcohol 4 , and a ‘1 : 2 adduct’ 8 (Scheme 3). The reaction of 1 and 2 in the presence of NaH afforded regioselectively two enesulfanyl alcohols 4 and 9 , which, in CDCl₃, cyclized smoothly to give the corresponding spirocyclic 1,3‐oxathiolanes 3, 10 , and 11 , respectively (Scheme 4). In the presence of HCl, epimerization of 3 and 10 occurred to yield the corresponding epimers 7 and 11 , respectively (Scheme 5). The thio‐Claisen rearrangement of 4 in boiling mesitylene led to the allylic alcohol 12 , and the analogous [3,3]‐sigmatropic rearrangement of the intermediate xanthate 13 , which was formed by treatment of the allylic alcohol 9 with CS₂ and MeI under basic conditions, occurred already at room temperature to give the dithiocarbonate 14 (Schemes 6 and 7). The presented results show that the Lewis acid‐catalyzed as well as the NaH‐induced addition of (R)‐vinyloxirane ( 2 ) to the enolizable thiocamphor ( 1 ) proceeds stereoselectively via an S_N2‐type mechanism, but with different regioselectivity.