Novel synthesis of (Z)-difluoroacrylates via a highly stereoselective addition-elimination reaction

[STRUCTURE: SEE TEXT] On treating readily prepared benzyl 2,3,3-trifluoroacrylate with various Grignard reagents, e.g., aryl-, alkyl-, or alkenylmagnesium halide, in the presence of a catalytic amount of copper(I) salt in THF at -78 degrees C for 1 h, the corresponding alpha,beta-difluoroacrylates were obtained in 54-98% yields with high Z-selectivity.     

Stereoselective synthesis of (Z)-enethiols and their derivatives: vinylic S(N)2 reaction of (E)-alkenyl(phenyl)-lambda3-iodanes with thioamides

[reaction: see text]. Exposure of (E)-beta-alkylvinyl(phenyl)-lambda3-iodanes to thioamides in dichloromethane at room temperature was found to result in a bimolecular nucleophilic substitution (S(N)2) at the vinylic carbon atom to give inverted (Z)-enethiols and/or (Z)-S-vinylthioimidonium salts. Vinylic S(N)2 reactions with thioureas are also discussed.     

Nitrolaldol reaction of (R)-2,3-cyclohexylideneglyceraldehyde: a simple and stereoselective synthesis of the cytotoxic Pachastrissamine (Jaspine B)

(R)-2,3-Cyclohexylideneglyceraldehyde 1 has been found to be a good substrate for nitroaldol reaction both in anhydrous and aqueous conditions. In both cases, the reaction took place with good substrate-controlled anti-selectivity. The major nitroaldol product 2b has been exploited to develop a simple and stereoselective synthesis of jaspine B X. Also, beginning with nitroaldol reaction of 1, this route presents a simple approach for the stereoselective preparation of (anti–anti-5,7)- and (syn–syn-9, 11, 12)-1,2,3,4-alkanetetrols, each possessing three contiguous oxygenated stereocenters.     

(9Z)- and (11Z)-8-methylretinals for artificial visual pigment studies: stereoselective synthesis, structure, and binding models

Artificial visual pigment formation was studied by using 8-methyl-substituted retinals in an effort to understand the effect that alkyl substitution of the chromophore side chain has on the visual cycle. The stereoselective synthesis of the 9-cis and 11-cis isomers of 8-methylretinal, as well as the 5-demethylated analogues is also described. The key bond formations consist of a thallium-accelerated Suzuki cross-coupling reaction between cyclohexenylboronic acids and dienyliodides (C6-C7), and a highly stereocontrolled Horner-Wadsworth-Emmons or Wittig condensation (C11-C12). The cyclohexenylboronic acid was prepared by trapping the precursor cyclohexenyllithium species with B(OiPr)(3) or B(OMe)(3). The cyclohexenyllithium species is itself obtained by nBuLi-induced elimination of a trisylhydrazone (Shapiro reaction), or depending upon the steric hindrance of the ring, by iodine-metal exchange. In binding experiments with the apoprotein opsin, only 9-cis-5-demethyl-8-methylretinal yielded an artificial pigment; 9-cis-8-methylretinal simply provided residual binding, while evidence of artificial pigment formation was not found for the 11-cis analogues. Molecular-mechanics-based docking simulations with the crystal structure of rhodopsin have allowed us to rationalize the lack of binding displayed by the 11-cis analogues. Our results indicate that these isomers are highly strained, especially when bound, due to steric clashes with the receptor, and that these interactions are undoubtedly alleviated when 9-cis-5-demethyl-8-methylretinal binds opsin.     

Facile and highly stereoselective one-pot synthesis of either (E)- or (Z)-nitro alkenes

Aliphatic aldehydes were reacted with nitro alkanes in the presence of catalytic amounts of piperidine over 4 A molecular sieves. Simply by changing reaction conditions (solvent and temperature) it is possible to control the stereochemical outcome of the reactions, obtaining pure (E)- and (Z)-nitro alkenes in high to excellent yields. The role of molecular sieves on the stereochemical control seems crucial in addition to that of piperidine, especially for the synthesis of the Z isomer.     

Asymmetric synthesis of (-)-dihydroxanthatin by the stereoselective Oshima-Utimoto reaction

[reaction: see text]. The catalytic stereoselective Oshima-Utimoto reaction is useful for the construction of five-membered oxacycles from simple starting materials and was employed for the preparation of the lactone group in the asymmetric synthesis of (-)-11alpha,13-dihydroxanthatin. Completion of the synthesis is facilitated by ring-closing enyne metathesis and alkene cross metathesis reactions.     

MgSiO3 NPs catalyzed intramolecular cycloaddition reaction: A simple and stereoselective synthesis of unprecedented julolidine analogs

A new julolidine hybrid analogs have been accomplished in a regio and stereo controlled manner using heterogenous MgSiO₃ nanoparticles (NPs) catalyzed intramolecular dipolar cycloaddition reaction as a key step. The synthesized catalyst was used in the preparation of various new julolidine analogs with a good yield ranging from 65 to 85%. Further, catalyst could be recycled in several times without loss of significant activity which was an added advantage to the principles of green chemistry.     

Serine-cis-proline and serine-trans-proline isosteres: stereoselective synthesis of (Z)- and (E)-alkene mimics by Still-Wittig and Ireland-Claisen rearrangements

Two new amide isosteres of Ser-cis-Pro and Ser-trans-Pro dipeptides were designed and stereoselectively synthesized to be incorporated into potential inhibitors of the phosphorylation-dependent peptidylprolyl isomerase Pin1, an essential regulator of the cell cycle. The cis mimic, the (Z)-alkene isomer, was formed through the use of a Still-Wittig [2,3]-sigmatropic rearrangement, while the trans mimic, the (E)-alkene, was synthesized through the use of an Ireland-Claisen [3,3]-sigmatropic rearrangement. Starting from N-Boc-Ser(OBn)-N(OMe)Me, both mimics were synthesized in Boc-protected form suitable for peptide synthesis with an overall yield of 20% in 10 steps for the cis mimic and 13% in eight steps for the trans mimic.     

General, stereoselective synthesis of (Z)-beta,gamma-unsaturated nitriles promoted by samarium diiodide

A method to obtain (Z)-beta,gamma-unsaturated nitriles in high or good yields and with moderate or high stereoselectivity is described. The products were achieved through the photoinduced metalation of 3-acetoxy-4-chloronitriles with SmI2. The starting compounds were readily prepared, and a mechanism is proposed to explain this stereoselective beta-elimination reaction.     

Efficient and stereoselective synthesis of threo-β-hydroxy-l-glutamic acid via a tandem (Z)-olefination-conjugate addition

threo-β-Hydroxy-l-glutamic acid 1 is an attractive target as a biologically active compound and as a chiral synthon. The required β-hydroxyl group in 1 was efficiently and stereoselectively introduced via an intramolecular conjugate addition of the N-hydroxymethyl group of γ-amino-α,β-unsaturated (Z)-ester 4. While the corresponding (E)-ester 3 gave a lower selectivity of ca. 5:1 in the intramolecular conjugate addition, a selectivity of up to 70:1 was shown with (Z)-ester 4. The tandem (Z)-olefination-conjugate addition could be achieved by simply changing the reaction conditions to give a selectivity of >20:1. Thus, the target compound 1 was obtained as its hydrochloride salt in 70% overall yield over four steps from lactol 2.     

Decarbonylative approach to the synthesis of enamides from amino acids: stereoselective synthesis of the (Z)-aminovinyl-D-cysteine unit of mersacidin

The Pd- and Ni-promoted decarbonylation of amino acid thioesters proceeds smoothly to yield enamides. The synthesis of the (S)-(Z)-AviMeCys subunit of mersacidin, an MRSA-active lantibiotic, via this approach, is described.     

Highly stereoselective synthesis of (Z)-1,2-dihaloalkenes by a Pd-catalyzed hydrohalogenation of alkynyl halides

An unprecedented Pd-catalyzed hydrohalogenation of alkynyl halides for the regio- and stereoselective synthesis of (Z)-1,2-dihaloalkenes has been realized using [(allyl)PdCl](2) as the catalyst and cis,cis-1,5-cyclooctadiene as the ligand. The advantages of this protocol are well illustrated by the assembly of trisubstituted (Z)-enynes and multifunctional benzenes via iterative cross-coupling reactions or tandem Diels-Alder-aromatization reactions, respectively.     

Intramolecular Nicholas reaction: stereoselective synthesis of 5-alkynylproline derivatives

The intramolecular Nicholas reaction of propargylic alcohols derived from N, N-acyl-diprotected omega-semialdehydes obtained from glutamic acid provided stereoselectively 5-alkynylproline derivatives. The suitable choice of the N-protecting group (tosyl or benzoyl derivative) permitted control of the stereochemistry during the ring formation. Semiempirical calculations of the species involved in the cyclization support the observed stereochemistry.     

A novel diastereoselective synthesis of (Z)-fluoroalkenes via a Nozaki-Hiyama-Kishi-type reaction

A highly diastereoselective and straightforward synthesis for (Z)-2-fluoroallylic alcohols via a Nozaki-Hiyama-Kishi-type reaction with the corresponding bromofluoroolefins was developed, providing an easy access to highly interesting fluorinated synthons.     

New stereoselective preparation of (Z)-3-perfluoroalkyl-3-magnesiated enoates by an iodine-magnesium exchange reaction

(Z)-Ethyl-3-perfluoroalkyl-3-magnesiated crotonates were prepared from (Z)-ethyl-3-perfluoroalkyl-3-iodo enoates by iodine-magnesium exchange reaction with isopropylmagnesium bromide in THF at -78 degrees C. These new reagents reacted with a range of electrophiles, leading to polyfunctional products bearing a perfluoroalkyl group.     

Divergent synthesis of (+)-tanikolide and its analogues employing stereoselective rhodium(II)-catalyzed reaction

In this study, we described the divergent synthesis of (+)-tanikolide and its analogues, such as (4S)- and (4R)-hydroxytanikolides, and nortanikolide, employing a stereoselective dirhodium(II)-catalyzed reaction to construct the quaternary chiral center of tanokolides. The key steps involve (a) a dirhodium(II)-catalyzed oxonium ylide formation–[2,3]-sigmatropic rearrangement, (b) an N-heterocyclic carbene-catalyzed ring-expansion lactonization of tetrahydrofurfural, or (c) an oxidative cleavage of tetrahydrofuran-5-methanol to γ-lactone using a 2-iodobenzamide catalyst. This route would provide high flexibility for analogue synthesis because the long side chain can be introduced at a later stage in the synthesis.     

syn-Selective dihydroxylation of γ-amino-α,β-unsaturated (Z)-esters from d-serine: stereoselective synthesis of d-iminolyxitol

An efficient and stereoselective synthesis of d-iminolyxitol, a potent α-galactosidase inhibitor, is achieved in 11 steps over 45% overall yield from N-Boc-O-Bn-d-serine. The key step involves the OsO₄-catalyzed syn-selective dihydroxylation reaction of the acyclic γ-amino-α,β-unsaturated (Z)-ester controlled by an N-diphenylmethylene group.     

Organic reactions in water: an efficient zinc-mediated stereoselective synthesis of (E)- and (Z)-trisubstituted alkenes using unactivated alkyl halides

[reaction: see text] Treatment of the acetyl derivatives of the Baylis-Hillman adducts 3-hydroxy-2-methylene-alkanoates and 3-hydroxy-2-methylene-alkanenitriles with unactivated alkyl halides in the presence of Zn in saturated aqueous NH(4)Cl solution at room temperature afforded (2E)-2-substituted-alk-2-enoates in the first case and (2Z)-2-substituted-alk-2-enenitriles with high (Z)-selectivity in the second case.     

Regio- and stereoselective selenium dioxide allylic oxidation of (E)-dialkyl alkylidenesuccinates to (Z)-allylic alcohols: synthesis of natural and unnatural butenolides

The first SeO(2) induced (Z)-selective allylic alcohol formation of dialkyl alkylidenesuccinates has been demonstrated to accomplish one-step syntheses of several essential butenolides and fused butenolides via an unusual E- to Z- carbon-carbon double bond isomerisation followed by the lactonization pathway. The observed regio- and stereoselective SeO(2) allylic oxidation protocol has also been extended to the diastereoselective total synthesis of bioactive natural product isomintlactone, its direct conversion to mintlactone and an example of the base-catalyzed intramolecular rearrangement of γ-lactone to δ-lactone.