Helvetica Chimica Acta

Crescent‐shaped polyamides composed of aromatic amino acids, i.e., 1‐methyl‐1H‐imidazole Im, 1‐methyl‐1H‐pyrrole Py, and 3‐hydroxy‐1H‐pyrrole Hp, bind in the minor groove of DNA as 2 : 1 and 1 : 1 ligand/DNA complexes. DNA‐Sequence specificity can be attributed to shape‐selective recognition and the unique corners or pairs of corners presented by each heterocycle(s) to the edges of the base pairs on the floor of the minor groove. Here we examine the relationship between heterocycle structure and DNA‐sequence specificity for a family of five‐membered aromatic amino acids. By means of quantitative DNase‐I footprinting, the recognition behavior of polyamides containing eight different aromatic amino acids, i.e., 1‐methyl‐1H‐pyrazole Pz, 1H‐pyrrole Nh, 5‐methylthiazole Nt, 4‐methylthiazole Th, 3‐methylthiophene Tn, thiophene Tp, 3‐hydroxythiophene Ht, and furan Fr, were compared with the polyamides containing the parent‐ring amino acids Py, Im, and Hp for their ability to discriminate between the four WatsonCrick base pairs in the DNA minor groove. Analysis of the data and molecular modeling showed that the geometry inherent to each heterocycle plays a significant role in the ability of polyamides to differentiate between DNA sequences. Binding appears sensitive to changes in curvature complementarity between the polyamide and DNA. The Tn/Py pair affords a modest 3‐fold discrimination of T⋅A vs. A⋅T and suggests that an S‐atom in the thiophene ring prefers to lie opposite T not A.

Antiparallel polyamides containing 1H‐pyrrole, 1H‐imidazole, and 3‐hydroxy‐1H‐pyrrole amino acids display a preference for minor‐groove binding oriented NC with respect to the 5′‐3′ direction of the DNA helix. We find that replacement of a central Py/Py pair with a β/β pair within a ten‐ring hairpin relaxes the orientation preference and, for some DNA sequences, causes the polyamide to prefer the opposite CN orientation. Substitution of the achiral γ‐aminobutanoic acid (γ) with either (R)(or S)‐2‐(acetylamino)‐4‐aminobutanoic acid moderates the orientation preference of the 2‐β‐2‐hairpin.

A novel class of odorants is described where the odor is associated with the interaction of two functional groups, one being an H‐donor (AH function), and the other an H‐acceptor (B function). Generally, odor occurs only if the distance between the two structural elements (AH/B system) is less than 3 Å. Bifunctional derivatives of the p‐menthane and iridane series served as models for deriving this rule. The stereospecificity of odor perception was an important prerequisite for its establishment.

By CNDO‐CI calculations we have found that dicarbonyl compounds exhibit only two n → π* transitions in the visible or near UV. region, instead of four as expected from simpler MO‐models. The dominant features of the long‐wavelength electronic spectra may be characterized by the relative energy of the two n and the two lowest π* orbitals. In general we distinguish between three cases:

Large splitting (about 2 eV) both between the n and the π* orbitals. The longest‐wavelength n → π* transition is shifted to the red. The higher transition remains roughly where it would appear in the monocarbonyl compound.

The splitting between the two n orbitals and the two π* orbitals depends not only on the distance between the two carbonyl groups, but also very strongly on their mutual position and on the nature of the molecular fragment connecting them. As one would expect, in α‐dicarbonyl compounds the splitting between the n orbitals is large, namely 1.8 eV. In some γ‐diketones such as P‐quinone, one still finds, however, a value of 0.7 eV. The splitting between the n orbitals gets significantly lowered if the two carbonyl groups are coaxial, but their 2_pπ nodal planes are perpendicular to each other. In planar β‐diketones the splitting between the π* orbitals is small, but may increase to an effective value of 0.4 eV when they are skewed.

Our calculations agree well with measured electronic spectra and CD. data. An electrochromism experiment on camphorquinone convincingly supports our conclusions on α‐dicarbonyl compounds.

A comparative photoelectron spectroscopical investigation of the title compound (11) and its 3,4‐ and 7,8‐dihydro derivatives (9 and 10) indicates that a considerable ‘through bond’ interaction exists between the π‐orbitals in 11. The PE. spectra of the 3,4‐diaza‐analogue of 10 and 11, which contain a cis azo group in a four‐membered ring, yield a splitting Δ_n (4‐memb. ring) = 1.55–1.60 eV between the nitrogen lone‐pair orbital energies. This value contrasts with those obtained for a three‐membered ring analogue (3,3‐dimethyldiazirine (5), Δ_n (3‐memb. ring) = 3.55 eV) and for a five‐membered ring analogue (2,3‐diazanorbornene (7) Δ_n(5‐memb. ring) = 3.10 eV). The sequence Δ_n (3‐memb. ring) > Δ_n (4‐memb. ring) Δ_n (5‐memb. ring) is satisfyingly reproduced by MINDO/2‐ and EHT‐calculations for the model systems with n = 3,4,5. A similar trend can be deduced from MINDO/2‐calculations for cis‐diimid where Δ_n becomes minimal for a NNH angle φ ≈ 100°, whereas Δ_n for the corresponding trans‐structure goes through a maximum in this region. The experimental finding as well as the calculated results confirm the predictions made by Gimarc [15] who attributes the behaviour of Δ_n for cis‐azo groups to a ‘through‐bond’ interaction of the n₊‐orbital with a lowerlying NN σ‐orbital; this interaction becomes maximal for NNR angles of the size present in a fourmembered ring, e.g. in 12 or 13.

Small ω‐alkenyl radicals are known to cyclize exclusively via the unexpected Anti‐Markownikow pathway which leads to cycloalkyl‐methyl radicals as the observed reaction products. The strong kinetic preference diminishes, as the linking methylene chain becomes larger. Semiempirical calculations using MINDO/3‐UHF allow to classify the observed reaction pathways as individual probes between allowed (n = 1, route A) and electronically favourable (n = ∞, route M) routes of reactions.

The rearrangement of γ,γ‐dimethylallyl phenyl ether (1), contrary to previous reports, gives a mixture of 4‐(γ,γ‐dimethylallyl)‐phenol (2), 2‐(α,β‐dimethylallyl)‐phenol (3) and 2,2,3‐trimethylcoumaran (4). The ratio of the reaction products is dependent on the solvent and the reaction temperature. The mechanism for the reaction is discussed in terms of multiple sigmatropic rearrangements.

In position 3 (and 5) substituted phenyl‐γ‐methyl (and phenyl)‐allylethers when subjected to the thermal CLAISEN‐rearrangement not only yield the expected o‐allyl‐phenols but in addition considerable amounts of the P‐allylated phenols. Migration to the para‐position is favoured by non‐polar solvents. This para‐migration of the substituted allylgroup is attributed mainly to steric factors. With sterically hindered, in ortho‐ or para‐position allylated phenols there was furthermore found an allyl‐phenol‐rearrangement: on sufficient heating to 200° these substances are transformed to the thermodynamically more stable para‐ resp. ortho‐allylphenols with inversion of the allyl group.

The observed spin densities for the radical cation and anion of trans‐15,16‐di‐methyl‐dihydropyrene (I) are in good agreement with MO predictions based on unrestrained delocalisation in the peripheral 14‐membered π‐electron system. The symmetry of the MO's occupied by the unpaired electron in the two cases suggests that the central «butane» unit acts as an electron releasing group in the radical cation and as an electron accepting group in the radical anion.

Hiện nay có thể lựa chọn giữa hai cấu trúc I và III cho betanidin, nghiêng về cấu trúc I dựa trên việc hình thành một dẫn xuất di-O-acetyl được xác định rõ (II). Sự không ổn định của betanidin và khả năng biến đổi dễ dàng thành các sản phẩm khác ở độ pH cao được chỉ ra là do tính nhạy cảm của nó với oxy trong khí quyển. Betanidin và Isobetanidin có cùng cấu hình (cụ thể là S) tại C-2 và có thể chỉ khác nhau ở cấu hình tại C-15. Một con đường sinh tổng hợp khả dĩ được đề xuất.