Akikazu Kakehi

Asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides catalyzed by chiral binaphthyldiimine-Ni(II) complexes.

Asymmetric cycloaddition reactions between several nitrile oxides and 3-(2-alkenoyl)-2-oxazolidinones and 2-(2-alkenoyl)-3-pyrazolidinone derivatives were carried out in the presence of chiral binaphthyldiimine (BINIM)-Ni(II) complexes as catalysts. Using (R)-BINIM-4(3,5-xylyl)-2QN-Ni(II) complex (30 mol %), good regioselectivity (4-Me/5-Me = 85:15) along with high enantioselectivity (96% ee) of the 4-Me adduct were obtained for the reaction between isolable 2,4,6-trimethylbenzonitrile oxide and 3-crotonoyl-5,5-dimethyl-2-oxazolidinone. Substituted and unsubstituted benzonitrile oxides and aliphatic nitrile oxides, which were generated from the corresponding hydroximoyl chloride in the presence of MS 4A, were reacted with 3-crotonoyl-5,5-dimethyl-2-oxazolidinone, 5,5-dimethyl-3-(2-pentenoyl)-2-oxazolidinone, 5,5-dimethy-3-[3-(ethoxycarbonyl)propenoyl]-2-oxazolidinone, 1-benzyl-2-crotonoyl-5,5-dimethyl-3-pyrazolidinone, and 1-benzyl-2-[3-(ethoxycarbonyl)propenoyl]-5,5-dimethy-3-pyrazolidinone in the presence of (R)-BINIM-4Ph-2QN-Ni(II) or (R)-BINIM-4(3,5-xylyl)-2QN-Ni(II) complexes (10-30 mol %) as catalysts to give the corresponding cycloadducts in high yields, with high regioselectively (4-R/5-R = 85:15-99:1) and with moderate to high enantioselectivities (42-95% ee) of the 4-R adducts. Higher enantioselectivities and regioselectivities were obtained for the reactions using pyrazolidinone derivatives as the dipolarophiles. For the cycloadditions of 2-(2-alkenoyl)-1-benzyl-5,5-dimethyl-3-pyrazolidinones catalyzed by (R)-BINIM-4(3,5-xylyl)-2QN-Ni(II) complex (30 mol %), the enantioselectivity varied from 75% to 95% ee. The reactions between several nitrile oxides and 2-acryloyl-1-benzyl-5,5-dimethyl-3-pyrazolidinone in the presence of (R)-BINIM-4(3,5-xylyl)-2QN-Ni(II) complex (10 mol %) resulted in enantioselectivities (79-91% ee) that exceed those of previously reported enantioselective cycloadditions of acrylic acid derivatives. Furthermore, studies using a molecular modeling program using PM3 calculations were carried out to gain insight into the mechanisms of the asymmetric induction.

THE FIRST TOTAL SYNTHESIS OF (-)-BENZOMALVIN A AND BENZOMALVIN B VIA THE INTRAMOLECULAR AZA-WITTIG REACTIONS

Abstract The first total synthesis of (−)-benzomalvin A, which possesses 4(3H)-quinazolinone and 1,4-benzodiazepin-5-one moieties, was described. Both of 6- and 7-membered ring skeletons were efficiently constructed by the intramolecular aza-Wittig reactions as the key reactions. The enantiomeric excess of synthetic (−)-benzomalvin A was more than 99.7% based on HPLC analysis using specially modified cellulose as a stationary phase. Furthermore, investigation on a specific conformational dynamic behavior of (−)-benzomalvin A was carried out by NMR studies and X-ray crystallographic analysis, and benzomalvin B was readily synthesized from (−)-benzomalvin A by only two steps.

A facile and regio- and stereo-selective preparation of bicyclic guanidines by iodocyclization of 3-(alk-2-enyl)-2-(substituted amino)-1-imidazolin-4-ones

Abstract The iodocyclization of 3-(alk-2-enyl)-2-(substituted amino)-1-imidazolin-4-ones proceeded in regio- and stereo-selective manners to give bicyclic guanidines, imidazo[1,2- a ]imidazole and/or imidazo[1,2- a ]pyrimidine, in good yields. The regiochemistry of the cyclization depended mainly on the kind of substituents of the alkenyl moieties and was interpretable by the PM3 MO calculations of the iodonium ion intermediates.

Asymmetric cycloaddition reactions of diazoesters with 2-alkenoic acid derivatives catalyzed by binaphthyldiimine-Ni(II) complexes.

The catalytic activity of chiral binaphthyldiimine (BINIM)-Ni(II) complexes for asymmetric enantioselective diazoalkane cycloadditions of ethyl diazoacetate with 3-acryloyl-2-oxazolidinone and 2-(2-alkenoyl)-3-pyrazolidinone derivatives was evaluated. The cycloadditions of 3-acryloyl-2-oxazolidinone and its 5,5-dimethyl derivative, in the presence of the BINIM-Ni(II) complex (10 mol %; prepared from (R)-BINIM-4Ph-2QN (ligand C) and Ni(ClO(4))(2)·6H(2)O) afforded 2-pyrazolines having a methine carbon substituted with an oxazolidinonyl group in moderate ratios (70:30 to 72:28), along with high enantioselectivities (90-92% ee) via 1,3-proton migration. On the basis of the investigations on the counteranions of the Ni(II) complex, the N-substituent of pyrazolidinone, and reaction temperatures, the optimal enantioselectivity (97% ee) and ratio (85:15) of 2-pyrazoline were obtained for the reaction of 2-acryloyl-1-benzyl-5,5-dimethyl-3-pyrazolidinone in the presence of (R)-BINIM-4Ph-2QN-Ni(II) ((R)-C/Ni(II)) complex prepared using Ni(BF(4))(2)·6H(2)O. In the cases of 1-benzyl-2-crotonoyl-5,5-dimethyl-3-pyrazolidinone, 1-benzyl-2-(2-butenoyl)-5,5-dimethyl-3-pyrazolidinone, and 1-benzyl-5,5-dimethyl-2-(3-ethoxycarbonyl)propenoyl-3-pyrazolidinone, the use of the (R)-BINIM-2QN-Ni(II) ((R)-A/Ni(II)) complex gave good to high enantioselectivities (85-93% ee) with the sole formation of the 2-pyrazoline having a methine carbon substituted with a pyrazolidinonyl group. Relatively good enantioselectivity (77% ee) was observed for the reaction between 2-acryloyl-5,5-dimethyl-1-naphthylmethyl-3-pyrazolidinone and an α-substituted diazo ester, ethyl 2-diazo-3-phenylpropanoate, which has yet to be employed as a diazo substrate in asymmetric cycloaddition reactions catalyzed by a chiral Lewis acid.

Enantioselective preparation of 3,4,5-trisubstituted 4,5-dihydroisoxazoles and their stereoselective elaboration of 5-side chain

Abstract The magnesium ion-induced nitrile oxide cycloaddition to chiral α-silylallyl alcohols was examined. Treatment of chiral allyl alcohol with nitrile oxide in the presence of magnesium cation resulted in the smooth progress of the 1,3-dipolar cycloaddition to give optically active 4,5-dihydroisoxazoles in good yields. The present procedure serves as a good method for the preparation of multi-substituted 4,5-dihydroisoxazoles with high enantiomeric excess. When exposed to tetrabutylammonium fluoride (TBAF), the cycloadducts were converted into 4-substituted-5,6-dihydro-4H-[1,2]-oxazines in good yields without loss of optical purity. Acylsilanes, obtained by the oxidation of the cycloadduct, underwent stereoselective allylation reaction induced by Lewis acid to accomplish the elongation of C5-side chain in a stereoselective manner.

Chiral Lewis acid catalyzed asymmetric cycloadditions of carbonyl ylides generated from diazoimide derivatives and their synthetic applications to indolizidine alkaloids.

Highly enantioselective 1,3-dipolar cycloaddition reactions, catalyzed by chiral Lewis acids, between several 3-(2-alkenoyl)-2-oxazolidinones and carbonyl ylides that were generated from N-diazoacetyl lactams are described. Reactions of N-diazoacetyl lactams that possess 5-, 6-, and 7-membered rings were transformed to the corresponding epoxy-bridged indolizidines, quinolizidines, and 1-azabicyclo[5.4.0]undecanes with good to high enantioselectivities. Regio- and stereoselective ring-opening of the epoxy-bridged indolizidine cycloadduct gave the corresponding alcohol as a single diastereomer. The sequence of asymmetric cycloaddition followed by ring-opening was applied to the syntheses of several chiral indolizidine derivatives, including (+)-tashiromine.

Reaction of 2-substituted-4-oxo-4H-pyrido[1,2-a]pyrimidine-3-carbaldehyde oximes with electron-deficient olefins and acetylenes

Abstract A facile oxime–nitrone isomerization through the 1,2-hydrogen shift in 4-oxo-4H-pyrido[1,2-a]pyrimidine-3-carbaldehyde oximes is discussed. The resultant NH-nitrones are trapped by maleimides to afford intermolecular cycloadducts. The reaction of the oximes with electron-deficient acetylenes undergoes via another path initiated by a nucleophilic attack of the oxime to acetylene moiety.

Stereoselective formation of optically active 2-oxy-1,3-oxazolidin-4-ones from chiral O-acylmandelamides or lactamides

Abstract (−)- O -Acyllactamides or mandelamides in the presence of TBSOTf underwent cyclization reaction to give optically active 2-oxy-1,3-oxazolidin-4-ones, a novel nitrogen analog of orthoesters, in good yields. An X-ray analysis and NOE studies indicated that the absolute configuration at the newly formed chiral carbon was S . For their synthetic application, the 1,3-dipolar cycloaddition of nitrile oxide was examined. The cycloadducts were obtained in a stereoselective manner. Subsequent treatment of the adduct with TBAF resulted in the one-step removal of mandelamide, giving optically active 4,5-dihydroisoxazole and mandelamide in good yields.

A facile and stereoselective azepine-ring formation at the periphery of pyridone and pyrido[1,2-a]pyrimidone systems via intramolecular imine and carbonyl ene reactions

Abstract Thermal reaction of the N-alkyl and N-aryl imines of 2-(alk-2-enylamino)-4-oxo-4H-pyrido[1,2-a]pyrimidine 3-carboxaldehyde (9) gave pyrido[1′,2′:1,2]pyrimido[4,5-b]azepines11–14 in good to excellent yields. Similarly, the imines of 4-(alk-2-enylamino)-1,6-dimethyl-2-oxo-1H-pyridine 3-carboxaldehyde (15) afforded pyrido[4,3-b]azepines 16 and 2,4-ethanopyrido[4,3-d]pyrimidines17 depending on the reaction conditions, the latter of which were secondary products from azepines 16. Heating aldehyde 15 in appropriate solvents also gave 2,4-ethanopyrido[4,3-d][1,3]oxazine 18. On the other hand, the thermal reaction of aldehyde 9 gave pyridopyrimidoazepine 20 and 12,13-ethanopyrido[1′,2′:1,2]pyrimido[5,4-b]azepine 21 in good total yields. These azepine-ring formation could be regarded as the intramolecular imine and carbonyl ene reactions at the periphery of the heterocyclic systems. Therein, the reactions proceeded in a highly stereoselective manner to give 4,5-cis azepines. The details and limitations of these ene reactions will be described.

Reaction of a-Bromoacetophenone Phenylsulfonylhydrazones. A New Synthetic Route to 2-Arylimidazoisoquinolines and -quinolines

2-Arylimidazo[2,1-a]isoquinolines and -[1,2-a]-quinolines were obtained in good to moderate yields by the reaction of the title hydrazones with isoquinoline and quinoline, respectively.