Asymmetric cyclopropane synthesis via phosphine oxide mediated cascade reactions

A silyloxy-THF has been converted into a cyclopropane containing three stereocentres as mixture of diastereoisomers. The mechanism of the reaction has been established and the source of stereochemical leakage proposed. An alternative stereospecific cascade reaction has been discovered.     

Common-intermediate strategy for synthesis of conduritols and inositols via beta-hydroxy cyclohexenylsilanes

[reaction: see text] Syntheses of conduritols B-D and F and d-(+)-chiro- and neo-inositols from cyclohexenylsilane intermediates are described. The key cyclohexylsilane intermediates 5 and 14 were synthesized by stereoselective olefin dihydroxylation of the corresponding cyclohexenylsilanes. Selective Peterson elimination reactions and Fleming-Tamao oxidations of 5 and 14 then delivered the targeted cyclitol derivatives.     

Asymmetric induction in domino Heck-aza-Michael reactions

Domino Heck-aza-Michael reactions are an efficient method for the rapid synthesis of functionalised N-heterocycles. An asymmetric version of this domino process has been developed to access chiral 1,3-disubstituted N-heterocycles from amino acid precursors in excellent yields (68–81%) with moderate to high diastereoselectivity (up to 92% de).     

Asymmetric synthesis of 4-deoxyverrucarol via two types of ring expansion reactions

Asymmetric synthesis of a trichothecane analogue, 4-deoxyverrucarol (2), was carried out through two types of ring expansion reactions. First, synthesis of the racemate of 2 was investigated. Thus, 1-[1-(tert-butyldimethylsiloxy)-ethyl]-1-methoxycarbonyl-2-hexen-4-on e (10), prepared by Diels-Alder reaction, was converted into the cyclopropylidene 15. The cyclobutanone (+/-)-18 was obtained from 15 via dihydroxylation, followed by successive treatments with SO(2)Cl(2) in the presence of imidazole and Florisil. After transformation of (+/-)-18 into the vinylcyclobutanol (+/-)-19, the second ring expansion reaction was performed with Pd(OAc)(2) to provide the cyclopentanone (+/-)-20. The product was converted into the racemate of 4-deoxyverrucarol (2) through the cyclohexenone (+/-)-22, but the diastereoselectivity during the introduction of the double bond was unsatisfactory. The selectivity was improved in the case of the asymmetric synthesis. The optically active cyclobutanone (+)-18 was prepared via AD reaction of 15 with 73% ee. After the transformation of (+)-18 into the cyclohexanone (-)-30 through the palladium-mediated ring expansion reaction, (-)-30 was subjected to the diastereoselective deprotonation reaction using the chiral amide. The key synthetic intermediate (-)-25 of 4-deoxyverrucarol (2) was synthesized in an optically pure form by taking advantage of a kind of kinetic resolution that occurred during the deprotonation step.     

Asymmetric polymerization via cycloaddition reactions

The reaction of N‐phthaloyl‐L ‐leucine acid chloride (1) with isoeugenol (2) was carried out in chloroform, and novel optically active isoeugenol ester derivative 3 as a chiral monomer was obtained in high yield. Compound 3 was characterized by ¹H‐NMR, IR, and mass and elemental analysis and then was used for the preparation of model compound 5 and polymerization reactions. 4‐Phenyl‐1,2,4‐triazoline‐3,5‐dione, PhTD (4), was allowed to react with compound 3. The reaction is very fast and gives only one diastereomer of 5 via Diels–Alder and ene pathways in quantitative yield. In order to explain this diastereoselectivity, a nonconcerted two‐step mechanism involving benzylic cation (BC) and aziridinium (AI) have been proposed for the Diels–Alder and ene reactions, respectively. The polymerization reactions of novel monomer 3 with bis(triazolinedione)s [bis(p‐3,5‐dioxo‐1,2,4‐triazolin‐4‐ylphenyl)methane (8) and 1,6‐bis(3,5‐dioxo‐1,2,4‐triazolin‐4‐yl)hexane] (9)] were performed in N,N‐dimethylacetamide (DMAc) at room temperature. The reactions are exothermic, fast, and gave novel optically active polymers 10 and 11 via repetitive Diels–Alder–ene polyaddition reactions. These polymers have inherent viscosities in a range about 0.18–0.22 dL/g. Some physical properties and structural characterizations of these new polymers have been studied and are reported. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1211–1219, 1999     

Asymmetric induction in Diels-Alder reactions of o-quinodimethanes

Asymmetric inductive effects have been measured on the Diels-Alder reaction of dimethyl fumarate with o-quinodimethane bearing a chiral α-alkoxy group. The chiral substituents used were 1-phenylethoxy, 2-(1-phenyl)propoxy, 1-(2-phenyl)propoxy, 2-(4-phenyl)-butoxy and 1-cyclohexylethoxy. The greatest asymmetric induction was found with the first of these chiral substituents (47% ee). A π-stacking effect, previously suggested as the rationale for asymmetric induction in a similar system, is shown to be inconsistent with the results from this study.     

Asymmetric induction in the reactions of azinones with C-nucleophiles

Effect of acylating agents on the course of addition of C-nucleophiles to 1,2,4- and 1,3,5-triazinones, as well as to quinoxalin-2(1H)-one, was studied. A series of new azinone derivatives was obtained. A method for the preparation of diastereomerically pure addition products of indoles to 1,2,4-triazinones and quinoxalin-2(1H)-one in the presence of N-Ts-L-amino acid acyl chlorides was suggested.     

Asymmetric induction in thia-Diels-Alder reactions of chiral polyfluoroalkylthionocarboxylates

A series of chiral S- or O-alkyl thionoesters have been synthesized by treatment of trifluorothioacetyl- or 2,2,3,3-tetrafluorothiopropionyl chloride with corresponding thiols or alcohols. The thia-Diels-Alder reaction of the thionoesters with symmetrical 1,3-dienes proceeds with diastereoselectivity up to 60%. Structures of cycloaddition products and corresponding transition states have been studied at the DFT level of approximation. The experimentally observed diastereomeric excess has been referred to differences in activation energies of transition states, preceding formation of the diastereomeric cycloaducts.     

Asymmetric synthesis of beta-hydroxy amino acids via aldol reactions catalyzed by chiral ammonium salts

The Cinchona alkaloid derived chiral ammonium salt developed by Park and Jew functions as an effective catalyst for the synthesis of beta-hydroxy alpha-amino acids via asymmetric aldol reactions under homogeneous conditions. The syn diastereomers are obtained in good ee, and aryl-substituted aliphatic aldehydes are the best substrates for the reaction. These results represent the highest ee's obtained to date in direct aldol reactions of glycine equivalents catalyzed by inexpensive, readily prepared chiral ammonium salts.     

Stereospecific synthesis of novel methyl-substituted mono- and di-methoxy conduritols

Methyl-monomethoxy conduritol-B and methyl-dimethoxy conduritol-B were synthesized starting from 2-methylbenzo-1,4-quinone. Bromination of 2-methylbenzo-1,4-quinone was followed by the reduction of the carbonyl groups with NaBH₄ to give a dioldibromo compound. Methyl-dimethoxy conduritol-B was synthesized from the reaction of the dioldibromo compound with CH₃ONa, followed by acetylation with Ac₂O-pyridine to obtain methyl-dimethoxy diacetate. On the other hand, acetylation of the methyl-dioldibromo compound followed by reaction with LiOH gave a monoepoxide compound stereoselectively. The reaction of the epoxide with H⁺/Ac₂O afforded the monobromo triacetate. Controlled reaction of monobromo-triacetate with CH₃ONa in MeOH furnished the desired new methyl-monomethoxy conduritol-B. The structures of all synthesized compounds were characterized by spectroscopic methods.     

Asymmetric induction in intramolecular ene reactions of 1,7-dienes

Chiral, double-activated enophiles in 1,7-dienes 6a - d undergo thermal and Lewis-acid catalyzed intramolecular ene reactions yielding trans-cyclohexanes 7a-d and 8a-d. The extent of diastereoselectivity depends on the reaction temperature. The 1,7-dienes 6 are obtained via Knoevenagel condensation of citronellal 5 with acyclic 1,3-dicarbonyls and analogous compounds 4. Starting with the diene 6e, however, an intramolecular hetero-Diels-Alder reaction takes place.     

Amine synthesis via carbonylation reactions: aminomethylation

The preparation of aliphatic amines and alicyclic diamines by reaction of olefins, synthesis gas and dimethylamine, known as aminomethylation, was investigated. Synthesis involves homogeneous rhodium and ruthenium catalysts or mixtures thereof at very low concentrations. Employing 3–12 ppm Rh and 50–100 ppm Ru results in up to 97% selectivity towards the amines at conversions of up to 98% if aliphatic mono-olefins are used as starting materials. At high catalyst concentrations (173 ppm Rh, 2660 ppm Ru) the corresponding diamine is obtained from dicyclopentadiene in 89% yield. The influence of reaction parameters and catalyst ratios on the n/i-selectivity of the product indicates the scope as well as the limits of such a multi-step synthesis for a commercial process.     

Concise and enantioselective synthesis of the aminocyclitol core of hygromycin A

[reaction: see text] Stereoselective aminohydroxylation and dihydroxylation using osmium(VIII) oxidants enabled the short and efficient synthesis of the aminocyclitol core of hygromycin A. In addition to allowing the selective introduction of the heteroatoms N and O, the use of osmium (via an osmate ester) as a protecting group for a 1,2-glycol is also reported. This tactic allowed efficient differentiation of otherwise equivalent hydroxyl groups and allowed us to complete the synthesis in short order (14 steps) and excellent overall yield (12%).     

Asymmetric multicomponent reactions: diastereoselective synthesis of substituted pyrrolidines and prolines

A novel diastereoselective synthesis of substituted pyrrolidines has been developed. Asymmetric multicomponent reactions of optically active phenyldihydrofuran, N-tosyl imino ester, and silane reagents in a one-pot operation afforded highly substituted pyrrolidine derivatives diastereoselectively. The reaction is quite efficient and constructed up to three stereogenic centers in a single operation.     

Asymmetric synthesis of (-)-indolizidine 209D via B-alkyl Suzuki coupling and amination reactions

(-)-Indolizidine 209D has been synthesized using consecutive amination reactions of compound 11. The precursor was prepared concisely using B-alkyl Suzuki cross coupling of chiral homoallyl amine and vinyl iodide compounds. Reaction: see text.     

Asymmetric crotylation reactions in synthesis of polypropionate-derived macrolides: application to total synthesis of oleandolide

Complete details of a convergent asymmetric synthesis of oleandolide (1), the aglycon of the macrolide antibiotic oleandomycin, is described. The synthesis has been achieved through the assembly and coupling of the left- and right-hand subunits 12 and 38, respectively. These subunits were prepared from chiral silane-based asymmetric crotylation reactions to control the stereochemical relationships. The left- and right-hand subunits (C1-C7 and C8-C14) were brought together through a Pd(0)-catalyzed sp3-sp2 cross-coupling reaction between the zinc intermediate 40 and vinyl triflate 38 to give 27. This product was converted to seco acid 42a and cyclized to lactone 35 under Yamaguchi conditions. This material was then epoxidized with m-chloroperbenzoic acid (m-CPBA) to install the correct C8 epoxide as a single diastereomer, which after a short deprotection sequence completed the synthesis of oleandolide.     

Synthesis of aminocyclohexitol via carbon-carbon bond-forming radical cyclization of oxime ether

The stannyl radical mediated-cyclization of oxime ether, derived from D-glucose, gave the aminocyclohexitol derivative. Stereoselective C-C bond forming cyclization proceeded via favorable conformers minimizing A(1,3)-strain between the oxime ether group and alpha-substituents.     

Efficient synthesis of enantiopure conduritols by ring-closing metathesis.

Two short synthetic approaches to enantiopure conduritols are described starting from the chiral pool. In both cases, the cyclohexene ring is assembled via ring-closing olefin metathesis. The terminal diene precursers for the metathesis reaction are prepared either from octitols or from tartaric acids. The former route involves a new method for selective bromination of the primary positions in long-chain carbohydrate polyols. Subsequent reductive elimination with zinc then generates the diene. The latter route uses a highly diastereoselective addition of divinylzinc to tartaric dialdehydes for preparation of the dienes.     

Asymmetric synthesis of phosphonotrifluoroalanine derivatives via proline-catalyzed direct enantioselective CC bond formation reactions of NH trifluoroacetimidoyl phosphonate

Based on enantioselective proline-catalyzed reactions between NH-iminotrifluoroethylphosphonate and acetone, an effective synthetic approach to both enantiomers of α-amino-α-trifluoromethyl-γ-oxobutylphosphonate was developed. The synthetic potential of these novel chiral synthons was illustrated by their cyclocondensation reactions with 4-chlorophenylisocyanate or 2,5-dimethoxytetrahydrofuran to afford the first representatives of phosphorylated 3,4-dihydropyrimidin-2-ones or 3H-pyrrolizines, incorporating pharmacophoric optically active fragments of phosphonotrifluoroalanine.