Theoretical conformational analysis of cyclic hexadepsipeptides. Enniatins

Chemistry of Natural Compounds - The spatial conformation of the “polar” form of enniatin B has been established. A conformation of the N1 type has been proposed for the...     

Theoretical conformational analysis of cyclic hexadepsipeptides. Enniatins

Conclusions The spatial conformation of the polar form of enniatin B has been established. A conformation of the N₁ type has been proposed for the nonpolar form.Institute of the Chemistry of Natural Compounds, Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 616–622, September–October, 1970.     

Conformational analysis of six-membered cyclic carbonates

The conformational isomerization of 2-oxo-1,3-dioxane and its methyl analogs was investigated by means of ab initio RHF//6-31G(d,p) and MP2//6-31G(d,p) quantum-chemical methods. It is shown that in comparison with 1,3-dioxanes the potential energy surface of the mentioned compounds has a fewer number of stationary points and includes two minima corresponding to conformers of sofa or distorted sofa configuration and one maximum corresponding to 2,5-twist form. The value of potential barrier of interconversion of cyclic carbonates is significantly lower than that of the analogously substituted 1,3-dioxanes.     

Mass spectra of some six-membered cyclic phosphites

A comparative analysis of the mass spectra of a number of 1,3,2-dioxaphospholanes and phosphorinanes was made in order to ascertain the dependence of several pathways of dissociative ionization on the size and degree of alkylation of a phosphorus-containing ring. The principal differences in the mass spectra of five- and six-membered cyclic phosphites are due to the high (as a consequence of the presence of strain) probability of cleavage of the P-O bond in the dioxaphospholane ring. The possibility of the use of mass spectrometry for the establishment of the composition of the hydrocarbon fragment of the ring is demonstrated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1620–1623, December, 1974.     

Asymmetric Kinugasa reaction involving six-membered cyclic nitrones

Kinugasa reactions between terminal acetylenes and six-membered ring nitrones when one or both components are chiral, proceed in a low to moderate yield and with a high diastereoselectivity affording mostly one, dominant β-lactam product. The first step of the reaction is controlled by the configuration of the nitrone, whereas the protonation of the C-7 center of the carbacepham skeleton in the second step depends on: a) the configuration of the bridgehead carbon atom formed in the first step, b) epimerization process in the presence of a base, and c) on the configuration of the stereogenic center in the acetylenic partner. In the case of the nitrone derived from dihydroisoquinoline, the reaction proceeds in a more complex way affording not only β-lactams, but also products derived from the alternative regio-1,3-cycloaddition, or nucleophilic addition of the acetylene to the double bond of the nitrone.     

Theoretical study of the conformational features of triosmium clusters

The article summarizes the results of the study of conformational states of triosmium clusters with organic ligands. Special attention is paid to the structural features of molecular carbonyl cluster complexes with M _n (CO) _m L organic ligands. The effect of the structure surface charge, solvent, cluster crystalline state, and intermolecular and intramolecular interactions on the cluster conformational state is also determined.     

Theoretical study of the conformational energy hypersurface of cyclotrisarcosyl

The multidimensional Conformational Potential Energy Hypersurface (PEHS) of cyclotrisarcosyl was comprehensively investigated at the DFT (B3LYP/6-31G(d), B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p)), levels of theory. The equilibrium structures, their relative stability, and the Transition State (TS) structures involved in the conformational interconversion pathways were analyzed. Aug-cc-pVTZ//B3LYP/6-311++G(d,p) and MP2/6-31G(d)//B3LYP/6-311++G(d,p) single point calculations predict a symmetric cis-cis-cis crown conformation as the energetically preferred form for this compound, which is in agreement with the experimental data. The conformational interconversion between the global minimum and the twist form requires 20.88 kcal mol-1 at the MP2/6-31G(d)//B3LYP/6-311++G(d,p) level of theory. Our results allow us to form a concise idea about the internal intricacies of the PEHSs of this cyclic tripeptide, describing the conformations as well as the conformational interconversion processes in this hypersurface. In addition, a comparative analysis between the conformational behaviors of cyclotrisarcosyl with that previously reported for cyclotriglycine was carried out     

Theoretical study of the ozonolysis of allyl acetate: mechanism and kinetics

The potential energy surface (PES) and mechanism of the reaction of allyl acetate (AAC) with O₃ are investigated by using density functional theory (DFT) and ab initio (MP2 and CCSD(T)) methods. The kinetics and main product branching ratios over the temperature range of (200–2,000 K) and at various pressures are obtained by employing multichannel Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The results show that the main products are 2-oxoethyl acetate and formaldehyde. Two channels are found for the decomposition of primary ozonides: one path is corresponding to the 2-oxoethyl acetate + CH₂OO formation (R1); the other channel products formaldehyde + CH₃C(O)OCH₂CHOO (R2). In the whole temperature range, R1 is calculated to be preferable and its product yield accounts for 60–77% of the total while R2 is found to contribute 20–40% to the total product yield. The overall rate constants show pressure independence and strong positive temperature dependence.     

Stereoselective synthesis of 2,3-unsaturated-aza-O-glycosides via new diastereoisomeric N-Cbz-imino glycal-derived allyl epoxides

Diastereoisomeric D,L-N-Cbz-imino glycal-derived allyl epoxides 5 and 6 have been synthesized, and their addition reactions with alcohols examined. The reactions lead to the corresponding 2,3-unsaturated-aza-O-glycosides through a new, completely regioselective 1,4-addition process which proceeds with complete substrate-dependent stereoselectivity.     

Selective formation of six-membered cyclic sulfones and sulfonates by C-H insertion

[reaction: see text] Carbethoxy diazosulfones and sulfonates, easily available from corresponding sulfones and sulfonates, undergo C-H insertion with preferential formation of six membered cyclic sulfones and sulfonates.     

Theoretical conformational study of the molecular structures of some bipyridine cardiotonics

Conformational features of amrinone and milrinone have been examined viaab initio (at STO-3G and 3–21G levels) MO theory. The calculated results suggest twisted structures for both the molecules. The higher barrier to rotation of the pyridine ring and restricted conformational flexibility of milrinone are observed to be two significant factors responsible for its greater cardiotonic activity as compared to that of amrinone.     

Theoretical conformational analysis of rutin

Abstract  

The structural properties of rutin were determined by using a computational multistep progression. In the first step a stochastic strategy based on a molecular mechanics calculation was adopted to obtain a preliminary screening of the low-energy rutin conformations. The most stable structures obtained by the random conformational search were used as a starting point for an Austin Model 1 (AM1) semiempirical optimization. After this treatment, 16 structures characterized by a more stable frontal in respect to back disposition of the glycosidic chain of rutin were identified. To validate the results found from the stochastic search a molecular dynamics simulation was carried out. The results evidenced the presence of a global minimum highly stabilized by a hydrogen bond between the hydroxyl in the 3′ position of the B ring and the endocyclic oxygen of the rhamnose unit followed by approximately 8 kJ mol⁻¹ less stable local minima with similar energy values. Finally, the reliability of the molecular model was confirmed by comparing the calculated electronic absorption spectrum with that measured on a methanolic rutin solution.     

Selective five- and six-membered cyclic amine syntheses via capture of episulfonium ions

Amide nitrogens open episulfonium ions to form pyrrolidines or piperidines selectively, depending on the nitrogen substituent, in either reversible or irreversible reactions.