Stereoselective Epoxidation Research Articles

Unexpectedly selective formation and reactions of epoxycyclooctenones under microwave-mediated conditions.

Topologically mobile difluorinated cyclooctenones undergo rapid, high-yielding, and completely stereoselective epoxidations with methyl(trifluoromethyl)dioxirane. The epoxides resist conventional hydrolysis but react smoothly in basic media under microwave irradiation to afford unique hemiacetals and hemiaminals in good yield. [reaction: see text]

Enantioselective total synthesis of FR900482.

The development of two approaches for the enantioselective total synthesis of FR900482 is described. A precursor for the formation of the benzazocine ring was assembled effectively by a modification of the Sonogashira coupling of an aryl triflate with a chiral acetylene unit derived from tartaric acid and the subsequent novel ketone formation via conjugate addition of pyrrolidine to the o-nitrophenylacetylene derivative. The first-generation approach to the key pentacyclic intermediate of our racemic total synthesis utilizes an intramolecular Mitsunobu reaction of an omega-hydroxynitrobenzenesulfonamide to form the benzazocine ring and a stepwise sequence to construct the hydroxymethyl group at the C(7) position. The key intermediate could be synthesized in optically pure form via formation of the characteristic hydroxylamine hemiacetal and a stereoselective epoxide formation. In the second-generation approach, the N-hydroxybenzazocine ring could be constructed directly from an omega-formylnitrobenzene derivative by intramolecular reductive hydroxylamination. The crucial stereoselective hydroxymethylation and the formation of the hydroxylamine hemiacetal could be performed efficiently by a one-pot sequence. After leading to the pentacyclic key intermediate, the total synthesis of (+)-FR900482 was accomplished by a modification of our protocol established in the racemic total synthesis. Stereochemical issues involved in the hydroxymethylation at the C(7) position and formation of the hydroxylamine hemiacetal are also discussed in detail.

First total synthesis of 1-O-beta-D-glucopyranosyl-5-deoxyadenophorine and its aglycon congener: determination of the absolute configuration.

The first total synthesis of the potent glycosidase inhibitors 1-O-beta-D-glucopyranosyl-5-deoxyadenophorine and its aglycon congener is described in respectively 13 steps (9% overall yield) and 9 steps (29% overall yield) from (R)-Garner aldehyde. The synthesis takes advantage of several key reactions including a diastereoselective allylation of a chiral imine, a stereoselective epoxidation, and a glycoside coupling. In addition this study established unambiguously the absolute configuration of the natural products.

Chiral Hydroperoxides as Oxygen Source in the Catalytic Stereoselective Epoxidation of Allylic Alcohols by Sandwich‐Type Polyoxometalates: Control of Enantioselectivity Through a Metal‐Coordinated Template.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stereoselective Epoxide Generation with Cyclic Rhodium Carbenoids: A New Access to Spiro-indolooxiranes

The reaction of cyclic diazoamides with aryl aldehydes catalyzed by rhodium(II) acetate leads to intermolecular stereoselective epoxide ring formation. A series of spiro-indolooxiranes has been synthesized following the described method in a facile manner. The use of aryl dialdehydes in the course of reaction of cyclic diazoamide resulted the formation of bis-spiro-indolooxiranes

Synthesis of the 1,6,8-trioxadispiro[4.1.5.2]tetradec-11-ene ring system present in the spirolide family of shellfish toxins and its conversion into a 1,6,8-trioxadispiro[4.1.5.2]-tetradec-9-en-12-ol via base-induced rearrangement of an epoxide

The synthesis of the 1,6,8-trioxadispiro[4.1.5.2]tetradec-11-enes 12 present in the shellfish toxins spirolides B and D 2, is reported. The two spirocentres were constructed via iterative radical oxidative cyclization of hydroxyalkyl dihydropyran 14 and hydroxyalkyl spiroacetal 13 using iodobenzene diacetate and iodine. This procedure initially afforded a 1 : 1 : 1 : 1 mixture of bis-spiroacetals 12a : 12b : 12c : 12d, however subsequent acid catalysed equilibration afforded a 3 : 1 : 0.9 thermodynamic mixture of 12a : 12b : 12c. The major bis-spiroacetal 12a underwent stereoselective epoxidation using dimethyldioxirane to alpha-epoxide 33a. Subsequent base induced rearrangement of this epoxide 33a using lithium diethylamide in pentane afforded allylic alcohol 34a, that was converted to the more thermodynamically favoured homoallylic alcohol 11a upon treatment with lithium pyrrolidinylamide in tetrahydrofuran. Homoallylic alcohol 11a possesses a hydroxyl group at C-12 as required for introduction of the tertiary alcohol group present at this position in spirolides B 1 and D 2.

Chiral hydroperoxides as oxygen source in the catalytic stereoselective epoxidation of allylic alcohols by sandwich-type polyoxometalates: control of enantioselectivity through a metal-coordinated template.

The epoxidation of allylic alcohols is shown to be efficiently and selectively catalyzed by the oxidatively resistant sandwich-type polyoxometalates, POMs, namely [WZnM(2)(ZnW(9)O(34))(2)](q)(-) [M = OV(IV), Mn(II), Ru(III), Fe(III), Pd(II), Pt(II), Zn(II); q = 10-12], with organic hydroperoxides as oxygen source. Conspicuous is the fact that the nature of the transition metal M in the central ring of polyoxometalate affects significantly the reactivity, chemoselectivity, regioselectivity, and stereoselectivity of the allylic alcohol epoxidation. For the first time, it is demonstrated that the oxovanadium(IV)-substituted POM, namely [ZnW(VO)(2)(ZnW(9)O(34))(2)](12-), is a highly chemoselective, regioselective, and also stereoselective catalyst for the clean epoxidation of allylic alcohols. A high enantioselectivity (er values up to 95:5) has been achieved with [ZnW(VO)(2)(ZnW(9)O(34))(2)](12)(-) and the sterically demanding TADOOL-derived hydroperoxide TADOOH as regenerative chiral oxygen source. Thus, a POM-catalyzed asymmetric epoxidation of excellent catalytic efficiency (up to 42 000 TON) has been made available for the development of sustainable oxidation processes. The high reactivity and selectivity of this unprecedented oxygen-transfer process are mechanistically rationalized in terms of a peroxy-type vanadium(V) template.

Towards the synthesis of [15]-membered stevastelins through the 2,3-epoxy analogues

A synthetic approach to the [15]-membered stevastelins, a novel class of immunosuppressant agents, is reported based on a macrolactamization route to the 2,3-epoxy derivative 6. The synthesis of this compound was achieved via a stereoselective epoxidation of the allylic alcohol 13, followed by a coupling reaction with a variety of peptide derivatives to give the epoxy peptides 7–10. After an extensive study of cyclizations with these precursors, the best result was achieved with the macrolactamization of 8 in the presence of DEPC, to obtain the epoxy cyclic depsipeptide 6 in 42% yield. From this product, an epoxy analogue of stevastelin B, compound 27, was prepared. Finally, the synthesis of the natural product was attempted through the opening of the oxirane ring contained in 6 and 26, with a variety of methyl cuprate reagents, but without success.

α‐Fluorotropinone Immobilized on Silica: A New Stereoselective Heterogeneous Catalyst for Epoxidation of Alkenes with Oxone.

AbstractFor Abstract see ChemInform Abstract in Full Text.

SiMe3-based homologation-epoxidation-cyclization strategy for ladder THP synthesis.

A trimethylsilyl (SiMe(3)) group is the basis of a strategy that emulates the three fundamental proposed processes in ladder polyether biosynthesis: chain homologation, stereoselective epoxidation (>95% ee or >95:5 dr), and endo-selective, stereospecific (inversion) hydroxyepoxide cyclization (>95:5 endo:exo, >95% dr). A tris-THP was synthesized in 18 total operations from commercial materials using this approach. [reaction: see text]

Stereoselective Epoxidation and Bromoalkoxylation with 3‐Ylidenepyrazine‐2,5‐diones.

3-Ylidenepyrazine-2,5-diones 1 were stereoselectively epoxidsed by dimethyldioxirane giving access to spirooxiranes 2 and diols 3. Bromohydroxylation and bromoalkoxylation of 3-ylidenepyrazine-2,5-diones 1 produced high yields of optically active 3-(1-bromoalkyl)pyrazine-2,5-diones 4 with a 3-hydroxy or 3-alkoxy function, respectively. Whereas direct hydrogenation of epoxides 2 afforded epimeric mixtures of 3-( 1-hydroxyalkyl)pyrazine-2,5-diones 5 and 6, the highly stereoselective transformation into 5 was possible by primary acid cleavage of the oxirane ring followed by hydrogenation of the resulting keto-enol mixtures 7/8.

Alpha-fluorotropinone immobilized on silica: a new stereoselective heterogeneous catalyst for epoxidation of alkenes with oxone.

The dioxirane-mediated epoxidation of alkenes in the presence of supported alpha-fluorotropinones 5 and 9 has been evaluated. The catalysts anchored onto silica supports 5 have shown comparable activity with respect to the homogeneous counterpart 10 and good stability on recycling. In the second part of this paper the enantiomerically enriched alpha-fluorotropinone 4 was anchored onto both mesoporous MCM-41 and amorphous KG-60 silicas. The chiral-supported catalysts promoted the stereoselective epoxidation of several trans-substituted and trisubstituted alkenes with ee values up to 80% and were perfectly reusable with the same performance for at least three catalytic cycles.

Mild, fast, and stereoselective epoxide opening by ketone enolate anions. Application to synthesis of the norlignan curculigine.

We report here that (1) enolate anions of five- to seven-membered cycloalkanones nucleophilically open cyclopentene and cyclohexene oxides in 57-76% yields and with 4-8:1 diastereoselectivity; (2) enolate anions formed regiospecifically via kinetic deprotonation of 2-cyclohexenone and 2-cycloheptenone open cyclohexene oxide in 60-62% yields and with 32-95:1 diastereoselectivity; and (3) an aryl methyl ketone enolate anion opens a monosubstituted epoxide as the key step in a short synthesis of the gamma-hydroxyketone (GHK) aglycon of the natural product curculigine.

Alternating Copolymerization Reaction of Propylene Oxide and CO: Variation of Polymer Stereoregularity and Investigation into Chain Termination

AbstractThe cobalt‐catalyzed alternating copolymerization reaction of racemic propylene oxide and CO affords regioregular but atactic poly(hydroxybutyrates). The application of enantiomerically pure (R)‐ or (S)‐propylene oxide results in the formation of isotactic, optically active and crystalline materials. In contrast to the perfect isotactic microstructures that are available via the microbial pathway, our approach allows to tailor the stereoregularity of the polymer products, and hence their melting points by application of mixtures of the epoxide enantiomers. Investigations into the polymer stereochemistry show that the epoxide units undergo a clean regioregular incorporation into the polymer chain with retention of configuration at the stereogenic carbon centers. MALDI‐TOF and online ATR‐IR experiments give insight into the mechanism of chain termination reactions.Sketch of regio‐ and stereoselective epoxide incorporation.imageSketch of regio‐ and stereoselective epoxide incorporation.

Highly efficient catalytic asymmetric epoxidation of allylic alcohols by an oxovanadium-substituted polyoxometalate with a regenerative TADDOL-derived hydroperoxide.

The oxovanadium(IV) sandwich-type POM catalyzes the chemo-, regio-, and stereoselective epoxidation of allylic alcohols by chiral hydroperoxides with very high catalytic efficiency (up to 42 000 TON), a potentially valuable oxidation for the development of sustainable processes. By using the sterically demanding, TADDOL-derived hydroperoxide TADOOH as the chiral oxygen source, enantiomeric ratios (er) of up to 95:5 have been achieved.

Stereoselective Epoxidation and Bromoalkoxylation with 3-Ylidenepyrazine-2,5-diones

3-Ylidenepyrazine-2,5-diones 1 were stereoselectively epoxidsed by dimethyldioxirane giving access to spirooxiranes 2 and diols 3. Bromohydroxylation and bromoalkoxylation of 3-ylidenepyrazine-2,5-diones 1 produced high yields of optically active 3-(1-bromoalkyl)pyrazine-2,5-diones 4 with a 3-hydroxy or 3-alkoxy function, respectively. Whereas direct hydrogenation of epoxides 2 afforded epimeric mixtures of 3-( 1-hydroxyalkyl)pyrazine-2,5-diones 5 and 6, the highly stereoselective transformation into 5 was possible by primary acid cleavage of the oxirane ring followed by hydrogenation of the resulting keto-enol mixtures 7/8.

Stereoselective oxidation of linalool with tert-butyl hydroperoxide, catalyzed by a vanadium(V) complex with a chiral terpenoid ligand

Stereoselective epoxidation of (−)-( R)-linalool by tert-butylhydroperoxide (TBHP) catalyzed by a vanadium(V) complex formed by interaction of [VO(acac) 2] or [VO(O n-Bu) 3] with a new chiral terpenoid ligand (L) is reported. Moderate diastereomeric excess (de 56%) was attained in toluene at 20 °C. 51 V , 13 C and 17 O NMR spectroscopic monitoring of the studied catalytic system showed that a new complex 1 forms quantitatively upon interaction of [VO(O n-Bu) 3] with L in CH 2Cl 2. Compound 1 is a mononuclear complex of vanadium(V), which incorporates the tridentate chiral ligand L, as well as VO moieties. It has been shown that TBHP binds to 1 giving a 1·TBHP adduct with the association constant K=1.36±0.11 l mol −1 at room temperature and enthalpy Δ r H 298 0=−6.5±2.0 kJ mol −1 and entropy Δ r S 298 0=−20±6 J mol −1 K −1. These thermodynamic parameters are typical for outer sphere complexes (OSCs). Adduct 1·TBHP is suggested to be a precursor of the active intermediate of the stereoselective epoxidation process.

Synthesis of L‐Sugars from 4‐Deoxypentenosides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Formal Synthesis of (+)‐Altholactone by Stereoselective Epoxidation Using Magnesium Monoperoxyphthalate (MMPP)

AbstractSynthesis of (+)‐altholactone has been achieved starting from L‐tartaric acid using (2S, 3R,4R,5R)‐2‐hydroxymethyl‐3,4‐bis(memoxymethoxy)‐5‐phenyltetrahydrofuran as key intermediate. The key epoxy ring was introduced by monoperoxyphthalate (MMPP) in high stereoselectivity (8.4:1).

The Total Synthesis and Biological Assessment of trans-Epothilone A

The total synthesis of (12S,13S)-trans-epothilone A (1a) was achieved based on two different convergent strategies. In a first-generation approach, construction of the C(11)C(12) bond by Pd0-catalyzed Negishi-type coupling between the C(12)-to-C(15) trans-vinyl iodide 5 and the C(7)-to-C(11) alkyl iodide 4 preceded the (nonselective) formation of the C(6)C(7) bond by aldol reaction between the C(7)-to-C(15) aldehyde 25 and the dianion derived from the C(1)-to-C(6) acid 3. The lack of selectivity in the aldol step was addressed in a second-generation approach, which involved construction of the C(6)C(7) bond in a highly diastereoselective fashion through reaction between the acetonide-protected C(1)-to-C(6) diol 31 (‘Schinzer's ketone') and the C(7)-to-C(11) aldehyde 30. As part of this strategy, the C(11)C(12) bond was established subsequent to the critical aldol step and was based on B-alkyl Suzuki coupling between the C(1)-to-C(11) fragment 40 and C(12)-to-C(15) trans-vinyl iodide 5. Both approaches converged at the stage of the 3-O, 7-O-bis-TBS-protected seco acid 27, which was converted to trans-deoxyepothilone A (2) via Yamaguchi macrolactonization and subsequent deprotection. Stereoselective epoxidation of the trans C(12)C(13) bond could be achieved by epoxidation with Oxone ® in the presence of the catalyst 1,2 : 4,5-di-O-isopropylidene-L-erythro-2,3-hexodiuro-2,6-pyranose (42a), which provided a 8 : 1 mixture of 1a and its (12R,13R)-epoxide isomer 1b in 27% yield (54% based on recovered starting material). The absolute configuration of 1a was established by X-ray crystallography. Compound 1a is at least equipotent with natural epothilone A in its ability to induce tubulin polymerization and to inhibit the growth of human cancer cell lines in vitro. In contrast, the biological activity of 1b is at least two orders of magnitude lower than that of epothilone A or 1a.