Configuration interaction calculations for the N2 molecule and its three lowest dissociation limits

A series of multi-reference double-excitation CI calculations is reported for the N₂ molecule in its equilibrium conformation as well as for its three lowest dissociation limits ⁴S + ⁴S, ⁴S + ²D and ⁴S + ²P respectively. Special emphasis is placed upon satisfying the requirement that the molecular CI energies at large internuclear separations must converge to the same values as are obtained in an analogous treatment for the corresponding atomic limits. In the most extensive CI undertaken the lowest three dissociation limits are calculated to occur at 9.33, 12.08 and 13.39 eV respectively compared to the experimental values of 9.90, 12.29 and 13.48 eV. The advantages of employing bond-centered functions in obtaining the proper balance between the bound molecule and its dissociated atoms are underscored in the calculations.     

Synthesis of gibberellins A8 and A56 from gibberellin A3

A mixture of gibberellin A₃ derivatives with 1(10)-ene-2,3-diol and 1(10)-ene-2,3-diol (2:5) groups, readily obtained from gibberellin A₃, has been used for a new and simple synthesis of gibberellin A₈ and its esters. The hydrolysis of GA₃ and the iodolactonization of a mixture of the 2-epimers was carried out in aqueous solution in a single flask, as also was a synthesis of GA₅₆ from GA₃ by a method that we have modified. The mixture of 1-iodides of GA₈ and GA₅₆ was separated by chromatography on SiO₂ in the form of methyl or p-bromophenacyl esters which were then deiodinated and the methyl or p-bromphenacyl ester of GA₈ was isolated. Free GA₈ was obtained by the dephenylation of the latter ester. By two-dimensional NMR spectroscopy we succeeded in assigning all the signals in the¹³C and¹H NMR spectra of the methyl esters of GA₈ and GA₅₆. In an attempt to obtain GA₅ methyl ester by the action of trimethylchlorosilane/sodium iodide on the 2,3-diol system in GA₅₆ methyl ester, the 8,13-epimer of the latter was formed, the structure of its molecule being established from the results of X-ray structural analysis.Novosibirsk Institute of Organic Chemistry, Siberian Division of the Russian Academy of Sciences. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 663–669, September–October, 1994.     

The reaction of gibberellins A1 and A7 with neutral manganese dioxide

Neutral manganese dioxide brings about oxidative lactonization and oxidative decarboxylation of gibberellins A₁ and A₇. The presence of a δ¹⁶-double bond is necessary for the occurrence of oxidative lactonization at C-15.     

Synthesis of coumermycin A1

A concise synthesis of the antibiotic coumermycin A₁, a natural product isolated from streptomyces, was achieved. In a key step, a selectively protected noviose sugar was prepared from novobiocin through a transglycosylation reaction with acetone.     

NQR and phase transitions in mixed A2BX6 compounds

³⁵Cl NQR frequency and phase transition temperature were investigated in the mixed crystals (A_xK_1?x) SnCl₆ (A = Rb, NH₄) and K₂ (Re_x Sn_1?x) Cl₆. The frequency and the intensity of the NQR signal were measured in the temperature range 100 K to 300 K for different chemical compositions. Phase transition temperatures were determined from the modifications in the line pattern. The observed change of the transition temperature as a function of concentration x can be fitted by the relation T_c(x) = T_c(O) (1 + a x)/(1 + b x). This formula is explained in the context of the virtual crystal approximation and the Landau theory.     

Configuration interaction calculation of the potential curves for the C3 molecule in its ground and lowest-lying Πu states

Ab initio SCF and Cl calculations are reported for the C₃ molecule using a basis set of double-zeta plus polarization quality. Potential curves are obtained for the symmetric stretch and bending and antisymmetric stretch vibrational coordinates for the ground and 3σ_u → l π_g^3,1Π_u excited states of this system in order to calculate the intensity distributions for the associated electronic transitions. The calculated T₀ value for the ¹Π_u ← X?¹ ∑⁺_g transition of 3.03 eV is in quite good agreement with the location of the origin of the 4050 Å (3.06 eV) band system in C₃, confirming its previous assignment to this electronic transition; the lifetime of the ¹H_u upper state is also obtained in the CI treatment. A value of 2.04 eV is calculated for the corresponding ³Π_u ← X?¹ ∑⁺_g origin, which result in turn suggests that the weak feature starting at 2.10 eV (5900 Å) should be assigned thereto.     

The 1B1, 1B2, 1A1, 1A2, 3A′(2B2), and 2A1 states of the 1,1-difluoromethane ion studied using multiconfiguration second-order perturbation theory

The 1²B₁(X²B₁), 1²B₂, 1²A₁, 1²A₂, 2²B₂, and 2²A₁ states of the ion were studied using CASPT2 and CASSCF methods. Calculations suggest that one should consider the 3²A′ state instead of 2²B₂. The CASPT2 T₀ calculations predict the energy ordering of 1²B₁(X²B₁), 1²B₂, 1²A₁, 1²A₂, 3²A′, and 2²A₁, which is in line with the experimental results by Pradeep and Shirley. The CASPT2 T₀ values for the 1²B₂, 1²A₂, 3²A′, and 2²A₁ states are close to the experimental values. The F-loss and H-loss dissociation processes were studied at the CASPT2//CASSCF level. The energy levels of low-lying states of are compared.     

Stereoselective syntheses of the glycosidase inhibitors hyacinthacine A2, hyacinthacine A3 and 5-epi-hyacinthacine A3

Stereoselective syntheses of the naturally occurring glycosidase inhibitors hyacinthacines A₂ and A₃ are reported. In the case of hyacinthacine A₂, the pyrrolizidine system was created from an acyclic precursor via a double cyclization procedure with a one-pot formation of two C-N bonds. In the case of hyacinthacine A₃, the two C-N bonds were created in separate steps. In addition, the non-natural epimer at C-5 of hyacinthacine A₃ was obtained.     

Total synthesis of fluvirucinine A1

An efficient and highly stereocontrolled convergent synthesis of fluvirucinine A₁ is reported herein. In fluvirucinine A₁ both C₅-C₁₃ and C₁-C₄ fragments were accessed from a common intermediate 6 derived from (S)-Roche ester in 15 and 7 steps, respectively. The key steps involve Evans asymmetric alkylation, Sharpless asymmetric epoxidation, amidation and a ring-closing metathesis reaction (RCM) for macrocyclization.     

Total synthesis of natural (+)-hyacinthacine A6 and non-natural (+)-7a-epi-hyacinthacine A1 and (+)-5,7a-diepi-hyacinthacine A6

Naturally occurring (1S,2R,3R,5R,7aR)-1,2-dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine [(+)-hyacinthacine A₆, 2] together with unnatural (1S,2R,3R,7aS)-1,2-dihydroxy-3-hydroxymethylpyrrolizidine [(+)-7a-epi-hyacinthacine A₁, 3] and (1S,2R,3R,5S,7aS)-1,2-dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine [(+)-5,7a-diepi-hyacinthacine A₆, 4] have been synthesized from a DALDP derivative [5, (2R,3S,4R,5R)-3,4-dibenzyloxy-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine], as the homochiral starting material. The synthetic process employed took advantages of Wittig methodology followed by internal lactamization, in the case of (+)-7a-epi-hyacinthacine A₁ (3), and reductive amination for (+)-hyacinthacine A₆ (2) and (+)-5,7a-diepi-hyacinthacine A₆ (4).     

Synthesis of thromboxane A2 analogs—1 : (±)-Dimethanothromboxane A2

The stable thromboxane A₂ analog (±)-dimethanothromboxane A₂ 1 was synthesized from bicyclo[3.1.1]heptane 2 via the tricyclic compound 4.     

Configuration interaction study of H2 using a modified slater-type orbital

A non-standard 1s basis function, χ = (1 + ar²) exp(?ζr), is used to approximate the ground state of the hydrogen molecule in a configuration interaction framework and using numerical integration over MO's. Results are compared to the traditional 1s STO.     

Stereoconvergent synthesis of C1-C17 and C18-C25 fragments of bafilomycin A1

The effective syntheses of the enantiomerically pure C1-C17 2 and C18-C25 3 fragments as promising synthetic intermediates of bafilomycin A₁, 1 have been achieved.     

Toward carboxylate group functionalized A4, A2B2, A3B oxaporphyrins and zinc complex of oxaporphyrins

Series of new oxaporphyrins were isolated from the reaction of furan-1,4-diol, pyrrole, and an aldehyde under Lindsey’s conditions, which gives easy access to ester group functionalized oxaporphyrins. The ester substituents can be readily hydrolyzed to terminal carboxylic acid in the presence of KOH. The Zn(II) oxaporphyrins have been synthesized from the reaction of free base with ZnCl₂ and fully characterized by variable temperature NMR, 2D NMR, and single crystal X-ray diffraction studies.     

Synthesis of 7-thiologibberellin A3 and A1 and their photochemical ring closure to 7→15α-thiololactones

The thiogibberellin analogs $\text{3}$ and $\text{4}$ were synthesized via sulfhysdrolysis of the carboxylic acid anhydrides $\text{1}$ and $\text{2}$ and transformed photochemically to the corresponding 7→15α-thiololactones $\text{5}$ and $\text{6}$.     

Theoretical study of the N20 molecule

In the study of the N₂₀ molecule with an I_h symmetry group, the following methods were applied: 6-31G, 3-21G and STO-3G ab initio and PM3 semiempirical MO methods. Both geometrical optimization and frequency calculations are reported. Results of optimized bond distances (d_N---N), first ionization potential, ΔH_a, ΔG_a and bond energy, for the cases of 6-31G, 3-21G, and PM3 showed that the N₂₀ molecule is a highly stable compound with a delocalized N---N single bonded cluster structure.     

Synthetic studies on bafilomycin A1: stereoselective synthesis of the enantiopure C1-C11 fragment

The synthesis of the enantiopure C₁-C₁₁ fragment of bafilomycin A₁ has been achieved with a 4% overall yield over 18 steps from (R)-(+)-citronellol. Key steps involve Sharpless asymmetric epoxidation, Miyashita reaction of a γ,δ-epoxymethacrylate with trimethylaluminum in the presence of water, bis-OTMS selective Swern oxidations, Corey-Fuchs alkyne formation, Negishi's carbometalation, and stereoselective formation of the C₂-C₃ trisubstituted bond of the conjugated diene by a Wittig-type olefination of the α,β-unsaturated C₃-C₁₁ aldehyde with the ylide derived from the readily available phosphonium salt [Cl⁻, Ph₃P⁺CH(OMe)COOMe].     

Synthesis of thromboxane A2 analogs—2 : (±)-Thiathromboxane A2

The total synthesis of (±)-11a-methano- 9,11-thiathromboxane A₂(1), the sulfur analog of thromboxane A₂ is described