Iridium‐Catalyzed Regio‐ and Enantioselective Allylic Substitution of Silyl Dienolates Derived from Dioxinones

Reported herein is the iridium‐catalyzed regio‐ and enantioselective allylic substitution reactions of unstabilized silyl dienolates derived from dioxinones. Asymmetric allylic substitution of a variety of allylic trichloroethyl carbonates with these silyl dienolates gave γ‐allylated products selectively in 60–84 % yield and 90–98 % ee.     

Synthesis of optically active 1,3-dioxin-4-one derivatives having a hydroxymethyl group at the 2-position and their use for regio-, diastereo-, and enantioselective synthesis of substituted cyclobutanols

A new method for preparing optically active 1,3-dioxin-4-one derivatives is presented. A series of prochiral 2,2-bis(hydroxymethyl)-1,3-dioxin-4-ones was synthesized by [4+2]cycloaddition of acylketene to protected 1,3-dihydroxy-2-propanone derivatives followed by deprotection of the hydroxyl groups. Desymmetrization of the prochiral dioxinones by lipase-catalyzed monoacetylation afforded optically active 2-(hydroxymethyl)dioxinones. Intramolecular photo[2+2]cycloaddition of ω-alkenyl esters of these alcohols provided an efficient method for regio-, diastereo-, and enantioselective synthesis of cyclobutanols.     

Regioselective synthesis of 2-iminooxazinones from dioxinones and carbodiimides

Iminooxazinones are readily formed via microwave heating of dioxinones in the presence of carbodiimides. Unsymmetrically substituted carbodiimides generally react with high or complete regioselectivity, allowing for assembly of the target ring systems with full control of substitution pattern under convenient conditions.     

Photolytic Studies on the Generation and Trapping of 6-Oxomethylidenecyclohexa-2,4-diene-1-one Derivatives with Various Nucleophiles

α-Oxoketenes and cyclohexadiene α-oxoketenes are reactive intermediates, particularly the latter due to their high re-aromatization potential. In this communication, we report photolytic studies on the generation of such species from 4-OMe and 4-OMOM protected resorcylate dioxinones and their trapping to give resorcylate esters and amides as well as the formation of adducts with enol-ethers as trapping reagents.     

Highly stereoselective Brønsted acid catalyzed synthesis of spirooxindole pyrans

A Br?nsted acid-catalyzed Prins-type cyclization sequence to construct spirooxindole pyrans in high yields and excellent diastereoselectivity has been developed. The combination of a β-hydroxy dioxinone fragment and isatin dimethyl acetal generate oxa-spirooxindoles efficiently. These compounds are diversifiable scaffolds that tap into the rich chemistry of dioxinones.     

Stereoselective synthesis of tetrahydropyran-4-ones from dioxinones catalyzed by scandium(III) triflate

[reaction: see text] A scandium triflate catalyzed, diastereoselective cyclization between aldehydes and beta-hydroxy dioxinones has been discovered. This process capitalizes on the untapped nucleophilicity of the embedded enol ether within the dioxinone core. The bicyclic compounds from the resulting cyclization can be isolated, or alternatively, alkoxide nucleophiles can be directly added. This in situ addition fragments the dioxinone rings and delivers the 3-carboxy-substituted tetrahydropyran-4-ones in good yields with high levels of diastereoselectivity.     

Iridium‐Catalyzed Regio‐ and Enantioselective Allylic Substitution of Trisubstituted Allylic Electrophiles

The first Ir‐catalyzed enantioselective allylation of trisubstituted allylic electrophiles has been developed. Through modification of the leaving group of allylic electrophiles, we found that trisubstituted allylic phosphates are suitable electrophiles for asymmetric allylation. The reaction of allylic phosphates with enol silanes derived from dioxinones gave allylated products in good yields with high enantioselectivities.     

Lewis acid catalysis in supercritical carbon dioxide. Use of poly(ethylene glycol) derivatives and perfluoroalkylbenzenes as surfactant molecules which enable efficient catalysis in ScCO2

Lewis acid catalysis in supercritical carbon dioxide (CO(2)) was investigated. While solubility of most organic materials is low in scCO(2), poly(ethylene glycol) derivatives or perfluoroalkylbenzenes were found to work as surfactants to dissolve organic materials in scCO(2). In the presence of these molecules, Lewis acid catalyzed organic reactions such as aldol-, Mannich-, and Friedel-Crafts-type reactions proceeded smoothly in scCO(2). Formation of emulsions was observed in these reactions, and the systems were studied in detail.     

Präparative chromatographische Enantiomerentrennung von synthetisch nützlichen cyclischen Acetalen

Preparative Chromatographic Resolution of Synthetically Useful Cyclic Acetals Racemic cyclic acetals derived from aldehydes and glycine, glycolic acid, thioglycolic acid, formylacetic acid, and acetoacetic acid (oxazolidinones 4 – 13 , dioxolanones 14 , 15 , oxathiolanone 16 , dioxinones 17 – 23 ) are resolved by preparative high-pressure liquid chromatography on silica gel coated with the polymer from N-acryloylphenylalanine ethyl ester (Chiraspher®). The separation factors α range from1,1 to 2,4. Use of a Prepbar®-chromatography system allows injection of several grams at a time. The enantiomeric acetals thus obtained are fully characterized. First application to amino-acid synthesis are mentioned.     

Heterocyclization reactions of 6-aryl-2,2-dimethyl-1,3-dioxin-4-ones with α-oxoketeneaminals

During thermolysis 6-aryl-2,2-dimethyl-1,3-dioxin-4-ones react with -mono and ,-dioxoketene aminals to yield 3-acetyl-6-aryl-2-benzoylamino-4-pyridones or 6-aryl-2-methylene-4-pyrimidones, respectively. A scheme of the formation of pyrido[2,3-d]pyrimidines from dioxinones and 1-amino-1-benzoylamino-1-bytene-3-one has been suggested.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1475–1478, August, 1994.     

Pyrolysis of aminonitriles,cyanohydrazones, and cyanoacetamides. Part II. Elimination reactions of arylacetylhydrazone,arylcyanoacetylhydrazone, and substituted cyanoacetamides

Rates of pyrolytic reactions of arylacetylhydrazone and arylcyanoacetylhydrazone (1–4) of the general formula GCH₂CONHNCHAr (G = H,CN) have been measured. The increase in the acidity of the hydrogen atom involved in the six-centered elimination process suggested for these reactions causes a significant increase in rates and thus appears to be the limiting factor in these pyrolytic reactions. The implication of this conclusion for the pyrolytic reactions of substituted cyanoacetamides (5–8), NCCH₂CONHAr are considered. The mechanism of pyrolytic reactions of compounds (5–8) appears to proceed through a 4-membered cyclic transition state. © 1996 John Wiley & Sons, Inc.     

Réactions d'addition-élimination à partir d'hétérocycles germaniés du type R2Ge. II. Les germa-2-diazolidines-1,3

Addition-elimination reactions from germanium heterocycles . II. 2-Germa-1,3-diazolidines (X ? Y ? NMe). The reactivity of 2-Germa-1,3-diazolidines with unsaturated compounds such as heterocumulenes (CS₂, PhNCO, PhNCS) and with carbonyl compounds (aldehydes and ketones) has been investigated. Generally the formation of mono- and diinsertion derivatives is observed. The elimination reactions of (Et₂GeO)_n and (Et₂GeS)₃ from these addition derivatives lead to corresponding carbon diazolidines. The mechanism of these addition-elimination reactions is precised. The interest of these reactions in organic synthesis is underlined.     

Flash vacuum pyrolysis over solid catalysts. 2. pyrazoles over hydrotalcites

Flash vacuum pyrolysis (fvp) reactions of NH-pyrazole (1) and 3,5-diphenylpyrazole (2) were investigated in the presence of anionic clays having hydrotalcite structure (HT). Solid catalysts with Mg:Al ratio equal to 2:1 containing carbonate (HT-1), nitrate (HT-2), and silicate (HT-3) as interlayer anions were employed. Between 400 and 600 degrees C, compound 1 remained almost unchanged and only unidentified volatile products were detected in small amounts. In contrast, 2 afforded benzonitrile (3) and phenylacetonitrile (4) by a ring fragmentation reaction at 450 degrees C. At a higher temperature (660 degrees C), the same products obtained in homogeneous fvp reactions, i.e., 2-phenylindene (5) and 3-phenylindene (6), were obtained showing no catalysis by HT under these conditions. Results showed that the yield is strongly dependent on the nature of the interlayer anion in the hydrotalcite structure. In comparison with reactions of 2 over zeolites, HTs exhibit selectivity for ring fragmentation reaction.     

Functional Polymers. III. Endcapping and Substitution on Polymers with Compounds Containing Ultraviolet-Absorbing Groups

Endcapping of oligo(oxyethylene) glycols was carried out by transesterification of methyl N,N-dimethylaminobenzoate with sodium methoxide as the catalyst or by the reaction of sodium salicylate with the ditosyl ester of the oligo(oxyethyl-ene) glycols. Several other common reactions were tried for the endcapping of the oligo(oxyethylene) glycols but were found to be either more cumbersome or unsuccessful. All products were obtained in high yield and high purity. The reactions of tetraalkylammonium carboxylates with aliphatic halides were found to be very general and mild reactions for the preparation of esters in high yield and high purity. It was found that these reactions could be utilized for the preparation of esters on polymers even if the carboxylate group was directly attached to the polymer chain. It was also demonstrated that the aliphatic halide group could be on the polymer, as in the case of polyepichlorohydrin. Copolymers of epichlorohydrin and glycidyl N,N-dimethylaminobenzoate with up to 90% glycidyl benzoate as the comonomer were prepared, and poly(tetra-butylammonium methacrylate) was effectively transformed with 4-(2-bromoethoxy)-2-hydroxybenzophenone into the corresponding ester. The products were characterized by the usual spectral means.     

Acylaminoacetyl derivatives of active methylene compounds. 3 . C-Acylation Reactions via the Hippuric Acid Azlactone

The C-acylation reactions of the active methylene compounds, methyl cyanoacetate and Meldrum's acid, with hippuric acid, using the DCC activation and the mixed anhydride conditions, are shown to proceed through initial formation of 2-phenyl-5(4H)-oxazolone, the hippuric acid azlactone. The conditions of these reactions using the azlactone as the acylating agent were investigated and discussed.     

A mechanistic study of reactions of stable disilenes with haloalkanes.

Mechanisms of the reactions of three tetrakis(trialkylsilyl)disilenes and a tetraaryldisilene with various haloalkanes such as carbon tetrachloride, chloroform, dichloromethane, which gave the corresponding 1-alkyl-2-chlorodisilanes and/or 1,2-dichlorodisilanes, were investigated in detail. As evidenced by an ESR observation of an intermediate radical, these reactions were quite unusual, forming neutral radical pairs from two closed shell molecules at the first step; no similar reactions have been observed between alkenes and haloalkanes. Low oxidation potentials of these disilenes, large negative activation entropies, and solvent effects for the rates are in good accord with the direct halogen abstraction of disilenes from haloalkanes instead of single-electron transfer at the rate-determining first step. The structure--reactivity relationship of the reactions and the Hammond postulate suggest that the transition state structures for the first step are similar to those for the halogen abstraction by silyl radicals, but more product-like.     

On the synthesis of siloxanes. XXIV. Nucleophilic substitution reactions of 2-functional 1,3-dioxa-2-4,7-trisilacylcloheptanes and 1,3-dioxa-2,4,8-trisilacyclooctanes

2-Hydrogen-1,3-dioxa-2,4,7-trisilacycloheptanes and 2-hydrogen-1,3-dioxa-2,4,8-trisiacyclooctanes, each as a mixture of three configurational isomers, were synthesized and halogenated with chlorine and bromine in the presence of pyridine. The stereochemical course of the halogenation reactions was studied by gas chromatography. 2-Chloro-2,4,7-trimethyl-4,7-bis(trimethyl-siloxy)-1,3-dioxa-2,4,7-trisiacycloheptanes and 2-chloro-2,4,7-trimethyl-4,7-diphenyl-1,3-dioxa-2,4,7-trisilacycloheptanes reacted with alcohols in the presence of pyridine, triethylamine, or 2,6-dimethylpyridine. Gas chromatography, and¹H NMR and²⁹Si NMR spectroscopy were used to investigate the stereochemistry of these substitution reactions. It has been found that all reactions proceed with retention of configuration and that the differences of the relative reactivities of the configurational isomers were distinctly smaller than those observed for reactions of the configurations isomers of functional cyclotrisiloxanes.     

Photochemistry of imidazolides. I. The photo-Fries-type rearrangement of N-substituted imidazoles.

A number of N-substituted imidazoles 1a–1i have been found to photo-isomerize to give the corresponding 2-substituted- and 4(or 5)-substituted imidazoles ( 2a–2i and 3a–3i ). The role of a dissociative path in these reactions has been demonstrated.     

Computational study of the reactions of methanol with the hydroperoxyl and methyl radicals. 1. Accurate thermochemistry and barrier heights

The reactions of CH(3)OH with the HO(2) and CH(3) radicals are important in the combustion of methanol and are prototypes for reactions of heavier alcohols in biofuels. The reaction energies and barrier heights for these reaction systems are computed with CCSD(T) theory extrapolated to the complete basis set limit using correlation-consistent basis sets, both augmented and unaugmented, and further refined by including a fully coupled treatment of the connected triple excitations, a second-order perturbative treatment of quadruple excitations (by CCSDT(2)(Q)), core-valence corrections, and scalar relativistic effects. It is shown that the M08-HX and M08-SO hybrid meta-GGA density functionals can achieve sub-kcal mol(-1) agreement with the high-level ab initio results, identifying these functionals as important potential candidates for direct dynamics studies on the rates of these and homologous reaction systems.