Reactivity differences between dimorphs in a crystalline phase norrish type II reaction

The dimorphs of α-adamantyl-p-chloroacetophenone (P2₁/n and C2/c) undergo solid state Norrish type II cyclobutanol formation with different stereoselectivity. Based on the X-ray crystal structure data, it is concluded that conformational factors rather than packing differences control the photoproduct stereochemistry.     

Analyse structurale en serile cyclobutanique : Partie 1. Dérivés monosubstitués et gem disubstitués du cyclobutane

Methylene bending mode analysis of some cyclobutane-d₂ molecules reveals that in the dissolved state (solvent CCl₄), bromocyclobutane occurs exclusively in a pseudo-equatorial form, whereas, under the same conditions, cyclobutanol and 1-bromocyclobutane carbonitrile exist both in pseudo-axial and pseudo-equatorial conformations. NMR spectroscopy confirms the results obtained for bromocyclobutane and leads to the conclusion that the pseudo-equatorial conformer is predominant in the case of cyclobutanol as well as in that of cyclobutane carbonitrile. A theoretical study of cyclobutanol in the gaseous state by the P.C.I.L.O. method gives results which are consistent with a pseudo-equatorial conformer.     

Preparation of 3-azetidinols with non-bulky 1-alkyl substituents

Cyclization of either the tetrahydropyranyl or trimethylsilyl ether of 1-(alkylamino)-3-chloro-2-propanols 1 followed by cleavage of the azetidinyl ether provides a general method for the preparation of 1-alkyl-3-azetidinols. Unhindered amines provide a more facile preparation of derivatives of 1 , or its ethers, than do hindered amines, while hindered derivatives of 1 undergo more facile ring closure.     

Photochemische Reaktionen 100. Mitteilung. Photochemistry of N-Acylimidazoles. V. Photolysis of the N-Acylimidazoles of Dehydroabietic Acid and of 13-Deisopropyl-10-epi-dehydroabietic Acid

The irradiation of 1-(dehydroabietoyl) imidazole (3) gave no Type II elimination product but yielded instead compounds 6 and 7 by migration of the imidazolylcarbonyl group from C(4′) to C(6′) of the abietan moiety, probably via a cyclobutanol intermediate. Similarly, irradiation of 1-(13′-deisopropyl-10′-epi-dehydro-abietoyl)imidazole (13) gave only a small amount of Type II fragmentation product 20 , the main products derived from γ-hydrogen abstraction being the cyclobutanol derivatives 16 and 17 .     

Structural parameter relaxation and pucker mode—internal mode coupling in the four-ring molecules cyclobutanol,cyclobutylfluoride and cyclobutylchloride

The present work is directed towards the investigation of the pucker—internal rotation modes of cyclobutanol and cyclobutanol-OD by the aid of a semirigid two-dimensional model. Based on spherical four-ring coordinates and general geometrical relations expressing wagging, twisting and rocking coordinates for CH₂ and CHX groups, experimental rotational constants of cyclobutanol, cyclobutylfluoride and cyclobutylchloride in the ground and excited pucker states are analyzed in terms of a two-dimensional four- ring pucker model with structural relaxation of CH₂ and CHX groups concerted with the ring puckering angles. The rotational data are found to be satisfactorily described by a one-dimensional ring pucker model including structural relaxation of the rocking angles of the CHX and the CH₂ groups. Then results of extended SCF computations (4-31G level) of the puckering—internal rotation potential of cyclobutanol are presented. These emphasize again the importance of structural relaxation of bond length and rocking type coordinates of the corner groups concerted with the puckering angle and serve as a basis for numerical solution of the two-dimensional pucker—internal rotation problem within the framework of a semirigid model. Predicted transitions and energy eigenstates will be correlated with far i.r. spectral data of cyclobutanol and cyclobutanol-OD observed in the 30–300 cm⁻¹ range and will be interpreted in terms of an alternative assignment, whose relation to an earlier analysis will finally be discussed.     

C−C Bond‐Forming Strategy by Manganese‐Catalyzed Oxidative Ring‐Opening Cyanation and Ethynylation of Cyclobutanol Derivatives

A novel C?C bond‐forming strategy employing manganese‐catalyzed ring‐opening of cyclobutanol substrates, followed by cyanation or ethynylation, is described. A cyano C1 unit and ethynyl C2 unit are regiospecifically introduced to the γ‐position of ketones at room temperature, providing a mild yet powerful method for production of elusive aliphatic nitriles and alkynes. All transformations described are based on a common sequence: 1) oxidative ring‐opening of cyclobutanol substrates by C?C bond cleavage; 2) radical addition to triple bonds bearing an arylsulfonyl group; and 3) radical‐mediated C?S bond cleavage.     

Preparation of an analogue of orbicuside A,an unusual cardiac glycoside

A steroid containing a multi-linked glycoside, analogous to the bufadienolide orbicuside A, has been prepared. The key steps were (i) the preparation of a 2α-allyloxycarbonyloxy-3β-hydroxy steroid, (ii) a Ferrier reaction between the steroid and a rhamnal derivative, (iii) removal of protecting group and oxidation of the 2-hydroxy group, (iv) dihydroxylation of the pseudoglycal from the sterically more hindered side and finally (v) ring closure by acetal formation under acidic conditions.     

Synthesis and X-ray crystal structures of metal complexes of three isomeric bibenzodiazines: Discrete and polymeric assemblies

Thirteen complexes of three isomeric bibenzodiazines were prepared and X-ray crystal structures were determined for seven of these. It is shown that 2,2′-biquinazoline prefers to form mononuclear complexes with chelation involving the less hindered nitrogen donors. Complexes of 4,4′-biquinazoline failed to provide suitable crystals for X-ray structure determination. 2,2′-Biquinoxaline is shown to offer two modes of coordination; in the first it acts as a “stepped-linear” bridging ligand using the two less hindered nitrogens, while in the second mode it chelates to copper(I) centres using its more hindered nitrogens. The presence of the fused benzo rings leads to a higher propensity for π–π stacking of the heterocyclic ligands within such complexes.     

Electrostatics and aggregation: how charge can turn a crystal into a gel

The crystallization of proteins or colloids is often hindered by the appearance of aggregates of low fractal dimension called gels. Here we study the effect of electrostatics upon crystal and gel formation using an analytic model of hard spheres bearing point charges and short range attractive interactions. We find that the chief electrostatic free energy cost of forming assemblies comes from the entropic loss of counterions that render assemblies charge-neutral. Because there exists more accessible volume for these counterions around an open gel than a dense crystal, there exists an electrostatic entropic driving force favoring the gel over the crystal. This driving force increases with increasing sphere charge, but can be counteracted by increasing counterion concentration. We show that these effects cannot be fully captured by pairwise-additive macroion interactions of the kind often used in simulations, and we show where on the phase diagram to go in order to suppress gel formation.     

SmI2-mediated radical cyclizations directed by a C-Si bond

The use of a silicon stereocontrol element in cyclobutanol and cyclopentanol-forming cyclizations mediated by SmI(2) results in excellent diastereocontrol. The C-Si bond in the products of cyclization provides a versatile handle for further manipulation. An asymmetric route to cyclization substrates involving copper-catalyzed silyl transfer has also been developed.     

Stereoselective control in the Staudinger reactions involving monosubstituted ketenes with electron acceptor substituents: experimental investigation and theoretical rationalization

The stereoselectivity of the Staudinger reactions involving monosubstituted ketenes with electron acceptor substituents was investigated experimentally by determination of the product stereochemistry and theoretically via DFT calculations. The results indicate that imines preferentially attack the less sterically hindered exo-side of the ketenes to generate zwitterionic intermediates. Subsequently, for cyclic imines, the intermediates undergo a conrotatory ring closure directly to produce β-lactams, while for linear imines, the imine moiety of the intermediates isomerizes to more stable intermediates, which further undergo a conrotatory ring closure to afford trans-β-lactams. The steric hindrance and the isomerization, rather than the torquoelectronic effect, play crucial roles in controlling the stereoselectivity in the practical Staudinger reactions involving monosubstituted ketenes with electron acceptor substituents, although the unaccessible borylketene with a powerful electron acceptor group controls the stereoselectivity torquoelectronically, in theory.     

In pursuit of pestalotiopsin a via zirconocene-mediated ring contraction

[reaction: see text] An asymmetric route from the epimeric beta-hydroxy esters 4 and 5 to the densely functionalized (+)-10 and (-)-10, respectively, is described. Either cyclobutanol can be made available as the predominant product. The levorotatory antipode has been transformed into the advanced intermediate 21 bearing side chains destined to become incorporated into the cyclononene ring of the title compound (1).     

Photochemische Reaktionen. 99. Mitteilung. Photochemistry of N-Acylimidazoles. IV. Structural Factors Leading to Norrish Type II Elimination and to Cyclobutanol Formation in the Photolysis of Acylimidazoles

Conformational factors leading to Type II elimination and to cyclobutanol formation were studied in the irradiation of N-acylimidazoles with simple acyl groups as well as of their photochemical acyl migration products.     

Synergism of hindered amine light stabilizers and UV-absorbers

The synergism of hindered amine light stabilizers (HALS) and UV-absorbers is very important in the practical photo-stabilization of polymers. As very little is known, however, the relationship between the composition of the mixture of the two kinds of light stabilizer and its photo-stabilizing efficiency was investigated in four common polymers. A high level of synergism was observed in polypropylene, high density polyethylene and ABS resin. A moderate synergistic effect was obtained in polystyrene. The maximum efficiency was observed at mixing ratios of hindered amine light stabilizer to UV-absorber of about 75:25 in polypropylene and high density polyethylene, 90:10 in ABS resin and 80:20 in crystal polystyrene.The synergism is due to different stabilizing mechanisms of hindered amine light stabilizer and UV-absorber, and is explained by the diffusion of very effective hindered amine light stabilizer from the polymer bulk, which is protected by the UV-absorber, towards the surface layer where photo-oxidation proceeds.     

Cleavage of activated cyclic amines: Unprecedented approach toward 2-substituted cyclobutanones

We report, for the first time, ring opening of activated four- to six-membered cyclic amines followed by an intramolecular expansion of cyclopropanol to cyclobutanone via carbocation intermediate. In the case of a N-tosylaziridine ester, a cyclobutanol was formed in a stereospecific manner during the Kulinkovich reaction step.     

Oxatriquinane and oxatriquinacene: extraordinary oxonium ions

Oxatriquinane, a fused, tricyclic alkyl oxonium ion of unprecented stability, was synthesized in five steps from 1,4,7-cyclononatriene. It survives reflux in H2O, chromatography, and attack by alcohols, alkyl thiols, halide ions, and hindered amine bases. The X-ray crystal structure shows longer C-O bond distances and more acute C-O-C bond angles than any reported alkyloxonium salt. The corresponding oxatriquinene and oxatriquinacene were also synthesized and represent the first examples of stable allyl oxonium species.     

A revised mechanism for the structure-specific gas phase enol cation-cyclobutanol reaction

Ion cyclotron resonance studies using deuterium labelled substrates have revealed a more complex mechanism than previously proposed for the structure-specific ion/molecule reaction of the [C₂H₄O]^+˙ and [C₃H₆O]^+˙ enol ions with neutral cyclobutanol. This requires the intermediacy of a species in which all of the methylene groups present in both the reactant ion and neutral can become equivalent.     

Fragmentation des δ-Dicetones,Analogies Avec le Comportement Photochimique

The mass spectra of thirty δ-diketones are reported. Their main pathways have been rationalized with the aid of deuterium labelling, metastable ions and high resolution mass measurements. Special attention has been given to ions resulting from the expulsion of neutral molecules such as water preceded, or followed by the loss of ethylene or alkenes. The expulsion may involve an intermediate cyclobutanol ion which would be analogous to the photochemical reaction observed by Yang.     

Photostabilizing activity of tertiary hindered amines

The influence of the N-alkyl group of tertiary hindered amines on the photostabilization of polymers was studied. The photostabilizing effects of the tertiary amine derivatives of 4-benzoyloxy-2,2,6,6-tetramethylpiperidine ( 1a ) in polypropylene were compared. All tertiary amine derivatives having α-H to hindered N showed higher effectiveness than 1a . Model liquid phase photoxidations were carried out by irradiating (UV-lamp) the solutions of tertiary hindered amines containing tert-butyl hydroperoxide as a photoinitiator. The tertiary hindered amines were oxidized more easily than corresponding parent hindered amine and converted to the parent amine, which was identified as its salt, resulting from the carboxylic acid produced from the N-alkyl group by oxidation. The thermal reaction of the tertiary hindered amines with tert-butyl hydroperoxide was also studied in the liquid phase. The tertiary hindered amines decomposed tert-butyl hydroperoxide more rapidly than the parent secondary hindered amine, and generated the parent amine. It was also found that the photostabilizing effects of tertiary hindered amines for polyolefins were higher than that of the parent secondary hindered amine.     

Stereoselective synthesis of 2-aminocyclobutanols via photocyclization of alpha-amido alkylaryl ketones: mechanistic implications for the Norrish/Yang reaction.

A series of chiral N-acylated alpha-amino p-methylbutyrophenone derivatives 1a-1h was synthesized from alpha-amino acids via a three-step procedure. These substrates were photolyzed in benzene and gave Norrish II and Norrish I cleavage products as well as the N-acylated 2-aminocyclobutanols that derive from gamma-hydrogen abstraction and 1,4-triplet biradical combination (Yang cyclization). The products were formed with characteristic Yang/cleavage ratios. The quantum yields for the photodecomposition of the N-acetyl-protected substrates 1b,e,f were moderate (12-26%); the diastereoselectivities of the cyclobutanol formation were remarkably high for all substrates. High diastereospecificity was observed for the isoleucine derivatives (2S,3S)-1g and allo-(2S,3R)-1g; the Yang reaction dominated the photochemistry of allo-1g, whereas 1ggave preferentially Norrish II cleavage. The role of hydrogen bonding as one of the stereodirecting effects was demonstrated by comparison of the cyclization efficiency of the valine derivative 1e with 1h,i,j. Also, aromatic beta-keto esters gave the Yang cyclization products in low yields. The diastereoselectivity of the cyclobutanol formation was rationalized by a three-step mechanism where every step is connected with one distinct stereochemical induction mechanism: (a) diastereoselective hydrogen abstraction, (b) conformational equilibration of the 1,4-tetramethylene biradicals (as calculated by semiempiric methods) controlled by hydrogen bonding, and (c) diastereoselective biradical combination (versus cleavage) influenced by spin-orbit coupling controlled intersystem crossing geometries.