Search

Treatment of acyclic conjugated (η4-4–7)-triene Fe(CO)3 complexes bearing an electron-withdrawing group at a terminal position with a base such as KN(SiMe3)2 and NaH promoted 1,3-migration of an Fe(CO)3 group on the triene, giving (η4-2–5)-triene Fe(CO)3 complexes. The 1,3-migration of an Fe(CO)3 group is revealed to proceed in a stereospecific manner.

Whereas treatment of the allylic mesylates of N -protected 2-alkyl-4-amino-( E )-2-alken-1-ols with sodium hydride in DMF yields exclusively the corresponding 2,3- trans -2-alkenyl-3-alkylaziridines, exposure of the methyl carbonates of N -protected 2-alkyl-4-amino-( E )-2-alken-1-ols to Pd(PPh 3 ) 4 (5–20 mol%) in THF or 1,4-dioxane affords predominantly the corresponding 2,3- cis -2-alkenyl-3-alkylaziridines. These reactions can be used to synthesize either of two diastereomers from single common intermediates.

Amino allenes have been synthesized from 2-ethynylaziridines via organocopper-mediated reactions. Whereas treatment of enantiomerically pure (2 R,3 S)-2,3-trans-3-alkyl-2-ethynylaziridines with RCu(CN)M (M = Li or MgX) yield exclusively (S,S)-allenylamines in high yields, isomeric (2 S,3S)-2,3-cis-3-alkyl-2-ethynylaziridines afford (S,R)-allenylamines in comparable high yields.

The title model compound represents a redox property of the active site of flavoenzymes and an environmental effect of the apoproteins. Its stereostructure was elucidated by 1 H-NMR spectra and energy minimum calculations. The stereoselectivity of this model was observed during the reaction with Me 2 PNPH, as NAD(P)H model.

Palladium(0)-catalyzed reactions of N-methanesulfonyl- or N-(arenesulfonyl)-3-alkyl-2-vinylaziridines reveal that 2,3-cis-isomers are more stable than the corresponding 2,3-trans-isomers in accord ...

Azamacrocycles 1–4 were synthesized from 9 in 20–30% overall yields. Compounds 1 and 2 extracted some metal ions into organic solvents, but 3 and 4 did not. The crystal structure of the Ag(I) complex of 1 has been determined.

The structures of nine new diterpenoids from the leaves and stems of Taxus chinensis were elucidated by means of NMR spectroscopy. Some of the new diterpenes were subjected to X-ray crystallographic analysis. Taxchinins D, G, E, H, I, J, K have an 11(15 → 1)-abeotaxane skeleton. Taxchins A and B possess an ordinary taxane skeleton, and belong to the first example of taxoids without an oxygen functionality or the sp 2 -hybridized carbon at C-4. Taxchinins D, E, I, and J exist as a mixture of conformational isomers in solution, whose behavior was investigated by variable-temperature NMR spectra and compared with the conformation obtained from X-ray analysis.

Polarized infrared spectra of well-oriented crystalline thin films of dodecylbutyldimethylammonum bromide monohydrate (DBDM·H 2 O) were recorded in the region 5400-250 cm −1 . From the results of the polychroism measurements and the X-ray analysis, water bands were divided into three symmetry species ( A 1 , B 1 , and B 2 ). With the aid of frequencies and isotopic frequency ratios, the water bands were assigned to the three fundamental vibrations, two librations, and their overtones and combinations. In the crystalline environment, we have observed the doublets for each of the vibrationally isolated OH and OD stretching bands of HOD in isotopically diluted monohydrates DBDM·HOD. This spectral feature suggests the presence of two distinct configurations of the water molecule, one forming a strong and linear O 1 H ⋯ Br 1 hydrogen bond and the other forming a weak and slightly bent O 1 H ⋯ Br 2 hydrogen bond. The results in this work are discussed in connection with previous work. The OH stretcing bands in DBDM·H 2 O appear at higher frequency than in dodecylpropyldimethylammonium bromide (DPDM)·H 2 O or in DPDM· 1 2 H 2 O, while the HOH scissoring, rocking, and wagging modes occur at lower frequency. These are consistent with the weaker hydrogen bond of the water molecule and the longer O ⋯ Br hydrogen bond distance in DBDM·H 2 O than in the DPDM hydrates. Correlation between ν(OD) and the O ⋯ Br distance is also argued.

Several chiral 2-nitro-1-sulfinylalkenes were prepared and examined for their utility as dienophiles for asymmetric [4+2] cycloaddition. Trisu- bstituted dienophiles (3-6) undergo distereoselective cycloaddition with cyclopentadiene in the presence of Lewis acids. Even the tetrasubsti- tuted chiral sulfinyl dienophiles (1) and (2) could be coerced into un- dergoing [4+2] cycloaddition to afford the cycloadducts in high yield without concomitant elimination if the reaction was conducted under high pressure. Generally, the Z-sulfinyl dienophiles (1-4) showed higher diastereo- and exdo/exo-selectivity than the E-dienophiles (5) and (6)

Five new diterpenoids, taxchinins A {3}, B{4}, and C{5}, 19-hydroxy-7-epibaccatin III {6}, and 10-deacetyl-10-oxobaccatin V {7}, have been isolated from the needles and stems of Taxus chinensis together with twelve known compounds including taxol {1}. These structures were elucidated and identified by spectroscopic techniques. A single X-ray crystallographic analysis of taxchinin A {3} determined its unique 51716-membered ring system. Taxchinins B {4} and C {5} were shown to have the same ring system

The crystal structures of the isovaline (Iva) containing dipeptides, Boc-D-Iva-L-Pro-OBzl and Boc-L-Iva-L-Pro-OBzl, were determined by x-ray diffraction. The diastereomeric peptides were shown to adopt unturned conformations closely similar to each other (phi Iva 52 degrees, psi Iva 46 degrees, phi Pro -65 degrees, and psi Pro 143 degrees for D-Iva-L-Pro sequence and phi Iva 52 degrees, psi Iva 44 degrees, phi Pro -63 degrees, and psi Pro 148 degrees for L-Iva-L-Pro sequence). The Pro ring of each peptide was in C gamma-endo conformation. The unusually large angle CIva-NPro-C delta Pro values (131 degrees in both peptides) were observed, that was due to steric repulsion between the delta-methylene of Pro and the alkyl side chain of Iva residue. These conformations were essentially the same as that of the corresponding alpha-aminoisobutyric acid (Aib)-containing peptide Boc-Aib-L-Pro-OBzl. The result has demonstrated that replacement of either one of the two methyl groups of the Aib residue in Boc-Aib-L-Pro-OBzl with an ethyl group does not cause any significant change in the unturned conformation of the dipeptide.

A chiral phosphonoacetate 2 differentiates the enantiotopic carbonyl groups in an α-diketone 5 to afford the Z-olefin 6 in high yield and ee. The absolute stereochemistry of 6 was determined by X-ray analysis of 8 derived from 6.

The crystal and molecular structure of β- d -fructofuranosyl α- d -xylopyranoside (xylosucrose) hemihydrate, C 11 H 20 O 10 ·0.5H 2 O, is orthorhombic, P 2 1 2 1 2, with a = 20.919(5), b = 18.727(2), c = 7.071(1) A, V = 2770.1(2) A 3 , Z = 8, and D x = 1.541 g·cm −3 . The structure was solved by direct methods and refined to R = 0.040 for 2564 observed reflections. Two independent xylosucrose molecules exist in the unit cell, and their conformations about the 1 → 2′ glycosidic bond are similar to sucrose. The orientations of the primary hydroxyl groups in the two molecules differ. An O-1′ ··· O-2 intramolecular hydrogen bond was observed in the one molecule, while an O-6′ ··· O-5 intramolecular hydrogen bond was observed in the other involving disorder of O-6′.

The reactions of protected serine- and threonine-derived γ-(mesyloxy) α,β-unsaturated esters with various magnesio organocyanocopper Lewis acid complexes have been investigated. The formation of syn-S N 2' products in addition to the normally expected anti-S N 2'products is taken as an indication that the reaction proceeds by a mechanism involving coordination of the magnesiocuprate with the C(δ)-N:C(γ)-O syn-γ-(mesyloxy) α,β-enoates

The Diels-Alder reaction of chiral 2-alkylsulfinyl-1-nitroalkene 1 and Danishefsky's diene (2) afforded adducts 3 and 4 in good chemical yield with a high enantiomeric excess, while diastereomeric nitroalkene 7 gave 8 and 9 , enantiomeric to 3 and 4 , respectively

The previously proposed structure of an acid-induced dehydration product of physalin A, a 13,14-seco-16,24-cyclosteroid, is revised as having a new carbon skeletal structure named neophysalin. The reaction from physalins to neophysalins proceed via a kind of the benzilic acid rearrangement.

Using the high pressure technique, asymmetric Diels-Alder reaction utilizing chiral 2-nitro-1-sulfinylolefins and dienes proceeded smoothly to give highly functionalized cyclohexene derivatives with satisfactory chemical yield and high diastereomeric excess.

Several chiral 2-nitro-1-sulfinylalkenes were prepared and examined for their utility as dienophiles for asymmetric [4+2] cycloaddition. Trisu- bstituted dienophiles (3-6) undergo distereoselective cycloaddition with cyclopentadiene in the presence of Lewis acids. Even the tetrasubsti- tuted chiral sulfinyl dienophiles (1) and (2) could be coerced into un- dergoing [4+2] cycloaddition to afford the cycloadducts in high yield without concomitant elimination if the reaction was conducted under high pressure. Generally, the Z-sulfinyl dienophiles (1-4) showed higher diastereo- and exdo/exo-selectivity than the E-dienophiles (5) and (6)

The 70% EtOH extract of Scoparia dulcis showed inhibitory activity against beta-glucuronidase from bovine liver. Bioassay-directed fractionation of the active extract led to the isolation of three labdane-type diterpene acids, scoparic acid A [1] [6-benzoyl-12-hydroxy-labda-8(17), 13-dien-18-oic acid], scoparic acid B [2] [6-benzoyl-14,15-dinor-13-oxo-8(17)-labden-18-oic acid], and scoparic acid C [3] [6-benzoyl-15-nor-14-oxo-8(17)-labden-18-oic acid], the structures of which were established by spectral means, including X-ray analysis. Scoparic acid A was found to be a potent beta-glucuronidase inhibitor.

New 5-deazaflavinocyclophane with planar chirality, a novel flavoenzyme model, has been synthesized. This compound provides the first example for selective diastereoface-differentiation during its own reduction and oxidation.

A novel chromone derivative, chrysograyanon ( 1 ), has been isolated from fresh whole plant of Chrysosplenium grayanum MAXIM. (Saxifragaceae) and its structure was determined to be 3,7-dimethoxy-5-hydroxy-2-(4-methoxy-1β,5α-dihydroxy-2-oxo-5β-methoxycarbonylcyclopent-3-enyl)-chromone ( 1 ) on the basis of spectroscopic evidences and X-ray analysis.

Alkylation of rac 3-substituted 4-acetoxyazetidin-2-one 4 with chiral tin(II) enolate 8 derived from 3-acetyl-(4S)-ethyl-1,3-thiazolidin-2-thione (7) proceeded in a highly diastereoselective manner to give alkylated products 9, 10, and 11 in 42%, 20%, and 9% yields, respectively, after chromatographic purification. Similar alkylation of rac-5 with 8 gave 16, 17, and 18 in 45%, 29%, and 5% yields, respectively. Compound 9 was readily converted to 33, a chiral key intermediate for the synthesis of carbapenems

Using the high pressure technique, asymmetric Diels-Alder reaction utilizing chiral 2-nitro-1-sulfinylolefins and dienes proceeded smoothly to give highly functionalized cyclohexene derivatives with satisfactory chemical yield and high diastereomeric excess.

The Diels-Alder reaction of chiral 2-alkylsulfinyl-1-nitroalkene 4 with Danishefsky's diene afforded adducts 9a and 10a in good chemical yield with a high enantiomeric excess, while diastereomeric nitroalkene 5 gave 9b and 10b, enantiomeric to 9a and 10a, respectively. The synthesis of the chiral dienophiles and mechanism of the chiral induction in the cycloaddition are discussed.

The Diels-Alder reaction of chiral 2-alkylsulfinyl-1-nitroalkene 1 and Danishefsky's diene (2) afforded adducts 3 and 4 in good chemical yield with a high enantiomeric excess, while diastereomeric nitroalkene 7 gave 8 and 9 , enantiomeric to 3 and 4 , respectively

The previously proposed structure of an acid-induced dehydration product of physalin A, a 13,14-seco-16,24-cyclosteroid, is revised as having a new carbon skeletal structure named neophysalin. The reaction from physalins to neophysalins proceed via a kind of the benzilic acid rearrangement.

The x-ray diffraction study of the C-terminally unprotected tripeptide benzyloxy-carbonyl-L-alanyl-alpha-aminoisobutyl-alpha- aminoisobutylic acid (Z-L-Ala-Aib-Aib-OH) has shown that the molecule adopts a consecutive type III beta-turn, which characterizes a right-handed 3(10) helix. A very weak 4----1 intramolecular hydrogen bond with the long N...O distance of 3.32 A, and a unique "oxy analogue" of the 4----1 hydrogen bond wih the O...O distance of 2.77 A, were observed. The stability of the observed conformation with the asymmetric Aib residues was theoretically evaluated by a conformation-energy calculation. The stereochemical characteristics of Aib and Ala residues were made clear by a comparison of the conformations of the short peptides containing both Aib and Ala residues.

(2R, 3S)-2-Acetoxymethyl-3-hydroxymethylazirizines of high enantiomeric purity were prepared through enzymatic transesterification of meso-bis(acetoxymethyl)aziridines.

A novel approach to ellipticine derivatives is described. The palladium catalysed cross-coupling reaction of indolylborate 1 with vinylbromides 4 gives hexatrienes 5, which are subsequently converted to pyrido[4,3-b]carbazoles. In addition, the acid promoted spiroannelation reaction of hexatrienes 5 is observed to give spiroindoles 9.

Rules relating the stereochemistry of N-Dnp (Dnp: 2,4-dinitrophenyl) derivatives of α-amino acids and peptides and the sign of the Cotton effects at the longest wavelength band (ca. 400 nm) are surveyed. Some new data and insights concerning the CD spectra of Dnp–α-amino acids are included: i.e., the spectra of Dnp derivatives as the composite of the corresponding o-nitrophenyl and p-nitrophenyl derivatives; the crystal structure of Dnp–I-phenylalanine and its solid-state CD spectra; the CD spectra of Dnp–α-amino acids containing sulfur atom on their side chains; and the theoretical approach to the CD spectra using molecular orbital method-based calculation. Conformational analyses of cyclic and linear peptides by the CD spectra of their Dnp derivatives are also discussed. © 2005 Wiley Periodicals, Inc. Biopolymers (Pept Sci), 2005

Enantiomerically pure (1 R ,2 R ,4 R ,5 R )- and (1 S ,2 R ,4 S ,5 R )-2,5-diphenyl-1,4-diazabicyclo[2.2.2]octane (2,5-diphenyl-DABCO) have been prepared and their crystal structures studied.

The asymmetric 1,4-addition reaction of arylboronic acids with cycloalkenones was catalyzed by 1 mol % of an amidomonophosphine-rhodium(I) catalyst in a 10:1 mixture of 1,4-dioxane and water at 100 degrees C, affording 3-arylcycloalkanones in reasonably high enantioselectivity and high yields. It was revealed by NMR, IR, and X-ray spectroscopies that this bidentate amidomonophosphine behaves as a hemilabile ligand that contains a hard donor site in addition to the soft donor in a molecule. Phosphorus atom strongly bonds to rhodium(I), and the amide carbonyl oxygen is coordinatively labile. The reaction efficacy of phenylboronic acid with cyclopent-2-en-1-one was significantly dependent on the possibility of coordination of the amide carbonyl oxygen to rhodium(I).

The stereochemical outcome of the 1,3- and 1,5-migration of an Fe(CO)3 group on (acyclic polyene)Fe(CO)3 complexes and their application to stereoselective construction of remote and contiguous stereogenic centers are described. Treatment of the [(eta(4)-4-7)triene]Fe(CO)3 complexes 1a-d bearing an electron-withdrawing group on the terminal position of an uncomplexed olefin with a base such as KN(SiMe3)2 (KHMDS) and LiCH2CN induced the 1,3-migration reaction of the Fe(CO)3 group, giving the [(eta4-2-5)triene]Fe(CO)3 complexes 2a-d in moderate to good yields, depending on the electron-withdrawing groups. From an experiment using the chiral (trienenitril)Fe(CO)3 complex 5, it is revealed that the 1,3-migration proceeds with inversion of configuration. Similarly, the 1,5-migration reaction of the[(eta4-6-9)tetraenone]Fe(CO)3 complexes 9 occurred with a catalytic amount of KHMDS, giving the [(eta4-2-5)tetraenone]Fe(CO)3 complexes 10 with retention of configuration. Furthermore, we have succeeded in the first regio- and stereoselective nucleophilic substitution of the (3,5-diene-1,2-diol) Fe(CO)3 complexes (15 --24a-h) with various nucleophiles via the ortho esters 21. By using iterative manipulation of the above two reactions, remote stereocontrol of the terminal substituents on acyclic polyene (9 --12) and construction of contiguous stereogenic centers (19, 28) have been achieved.

Seven novel cycloartane-type triterpenes were isolated from Combretum quadrangulare , and their structures were elucidated on the basis of spectral analysis. All these compounds were tested for their cytotoxicity against murine colon 26-L5 carcinoma cells. Methyl quadrangularate B ( 2 ) and methyl quadrangularate D ( 4 ) exhibited potent cytotoxicity having ED 50 values 9.54 and 5.42 μM, respectively.

Palladium(0)-catalyzed reactions of five sets of four stereoisomeric 4,5-epimino-N-(methanesulfonyl) or -N-(arylsulfonyl) 2-enoates reveal that 4,5-cis-(2E)-isomers are thermodynamically more stable than other isomers, in accord with calculations. A highly stereoselective synthesis of (E)-alkene dipeptide isosteres having the desired stereochemistries from unwanted stereoisomeric 4,5-epimino-N-(arylsulfonyl) 2-enoates is also presented.

Erlose [beta-D-fructofuranosyl O-alpha-D-glucopyranosyl-(1--4)-D-glucopyranoside] trihydrate, C18H32 O16.3H2O, M(r) = 558.48, is orthorhombic, P2(1)2(1)2(1) with a = 31.164(7), b = 13.111(5), c = 11.636(5) A, and Z = 8. The structure was solved by direct methods, and refined to R = 0.035 for 3926 observed reflections. The unit cell contains two independent molecules having a similar conformation. The conformation of the alpha-(1--2) glycosidic linkage is similar to that observed in erlose monohydrate, whereas the conformation of the alpha-(1--4) glycosidic linkage differs significantly. The molecule has no intramolecular hydrogen-bonds except for the minor components of three-center bonds, but indirect intramolecular hydrogen-bonds through the water molecules are formed. The hydrogen-bond system in the crystal structure consists of infinite and finite chains crosslinked by water molecules.

Erlose [ O -β- d -fructofuranosyl-(1 → 2)- O -α- d -glucopyranosyl-(1 → 4)-α- d -glucopyranoside] monohydrate, C 18 H 32 O 16 ·H 2 O, M r = 522.45, is orthorhombic, P 2 1 2 1 2 1 with a = 30.748 (3), b = 8.757 (1), c = 8.270 (1) A, and Z = 4. The structure was solved by direct methods, and refined to R = 0.048 for 1909 observed reflections. The torsion angles about the (1 → 2) and (1 → 4) glycosidic bonds are similar to those observed in other sucrose- and maltose-like oligosaccharides. The maltose moiety has an O-3′H ··· O-2 intramolecualr hydrogen-bond, but the sucrose moiety has no intramolecular hydrogen bonds. The hydrogen bonding in the crystal includes infinite and finite chains crosslinked by the water molecule.

A diterpenoid with a novel basic skeleton has been isolated from Taxus chinensis , the structure of which was elucidated by spectral means and unambiguously determined by an X-ray analysis.

Structures of spiramines E (1), F (2) and G (3) were elucidated by chemical and spectroscopic means

New atisine-type alkaloids, spiramines A, B, C, and D were isolated from Spiraea japonica and their structures were elucidated by chemical and spectroscopic means

Title compound has been found to form a 2:2 crystalline inclusion complex with ethanol, and X-ray crystal structure of the complex was determined. In the structure of the complex, four intermolecular hydrogen bonds are involved between the two host and two guest molecules. By using the complexation, extraction of ethanol from aqueous solution is demonstrated.

The protected dipeptide Boc-Aib-Pro-OBzl, C21H30N2O5, crystallizes in the orthorhombic space group P212121, with a = 12.820, b = 10.529, c = 16.548A, and Z = 4. The crystal structure has been solved by direct methods and refined to an R value of 0.074 for 1352 reflections. The Boc-Aib-Pro-OBzl molecule has been shown to adopt an unfolded conformation in the solid state with ϕAib = 50.5°, ΨAib = 45.3°, ϕPro = −64.6°, and ΨPro = 148.1°. The result is in marked contrast with the reported crystal structure of Cbz-Aib-Pro-NHMe, which adopts an intramolecularly hydrogen-bonded β-turn conformation. Comparison with 13 reported conformations of Aib-Pro sequences in the crystalline state revealed that the Aib-Pro sequence adopts an unfolded conformation if the residue that immediately follows the dipeptide sequence possesses no hydrogen available for hydrogen bonding, while a β-turn conformation is preferred if the Pro residue is followed by an NH group. Correlation between pyrrolidine ring puckering of the Pro residue and main-chain conformation in Aib-Pro sequences is discussed.

Hydrolysis of the flavin (5) with Triton B gave the 4a-spirohydantoin (6) as the major product together with, among other products, the imidazo[4,5-b]quinoxaline (7). Formation of such products indicates that nucleophilic attack by hydroxide ion on the flavin ring had occurred preferentially at the 10a-position.

Treatment of 8, 8-dibromobicyclo [5.1.0] octanes (A) with a higher order organocuprate prepared from cuprous thiocyanate (method A) or cuprous cyanide (method B) and methyllithium, followed by the addition of methyl iodide in situ, readily afforded 8, 8-dimethylbicyclo [5.1.0] octanes (B) in good yields. The results are summarized in Table I. Interestingly, reaction of the cisdibromides (4 and 19) under the present conditions gave tricyclic compounds (15 and 16, and 20, respectively)