High level ab initio and density functional theory study of bond selective dissociation of CH3SH and CH3CH2SH radical cations

Selective bond dissociation energies for CH₃SH and CH₃CH₂SH radical cations were evaluated with G1, G2, G2MP2, B3LYP, BLYP, and SVWN computational methods. It was determined that both G2 and CBSQ evaluate very accurate bond dissociation energies for thiol radical cations, while gradient-corrected BLYP computes the best energies of three employed DFT methods. For the CH₃CH₂SH radical cation, new, higher than previously estimated selective bond dissociation energies were suggested. Received: 10 September 1997 / Accepted: 9 September 1998 / Published online: 11 November 1998     

The Dynamics of Laser-Induced Changes in Human Skin Autofluorescence—Experimental Measurements and Theoretical Modeling

To study the temporal dynamics of human skin autofluorescence photobleaching, we measured the autofluorescence spectral changes of skin in vivo during continuous exposure to 442 nm (He-Cd) laser light. Integral intensities were calculated for various spectral wavelength bands and plotted as a function of time. Mathematical analysis of the time function revealed a double-exponential photobleaching process: I(t) = a exp(-t/τ¹,) + b exp(-t/τ²) + c, in which t¹, and t² differed by an order of magnitude. A hypothesis for the mechanism of the double-exponential photobleaching dynamics was proposed and evaluated using Monte Carlo modeling of light propagation in the skin and autofluorescence escape from skin. By combining the fluorophore microdistributions, Monte Carlo simulation results and the variation in fluorescence decrease parameters (a, b, c, τ¹τ²) with increasing exposure intensities a biophysical explanation for the double-exponential photobleaching function was elucidated. The fast decrease term corresponds to laser-induced photobleaching in the stratum corneum, while the slow decrease term represents fluorophore changes in the dermis. The measured autofluorescence photobleaching dynamics can be used to determine the fractional contributions of different skin layers to the total autofluorescence signal measured in vivo.     

1H nmr study of hydrogen bonding of fluorinated alcohols with ethers

The hydrogen bonding of TFE and HFIP with t-BuOCH₃ (1) and PhOCH₃ (2) was studied using ¹H nmr. The Δ δ values of carbon-bound hydrogens in 1 and 2 show good linear correlation with the alcohol acidity.     

The transformations of 4-heteroarylaminomethylene-5(4H)-oxazolones into dehydropeptide derivatives

2-Phenyl-4-heteroarylaminomethylene-5(4H)-oxazolones 3 , which were prepared from the corresponding N,N-dimethyl-N'-heteroarylformamidines 1 and hippuric acid 2 in acetic anhydride, react with amino acids giving dehydropeptide derivatives 4, 5 , and 6 as products. Dehydration of N-protected peptides 7–10 , containing glycine at the C-terminal, followed by the reaction with formamidines 1 gave 2-substituted-4-heteroarylaminomethylene-5(4H)-oxazolones 11–14 .     

Alkyl (E,Z)-2-(2-benzoyl-2-ethoxycarbonyl-1-ethenyl)amino-3-dimethylaminopropenoates in the synthesis of fused pyrimidinones. A facile route to 3-aminoazino-4H-pyrimidin-4-ones

Alkyl (E,Z)-2-(2-benzoyl-2-ethoxycarbonyl-1-ethenyl)amino-3-dimethylaminopropenoates 1a,b react with het-eroarylamines 2 to give alkyl 2-(2-benzoyl-2-ethoxycarbonyl-1-ethenyl)amino-3-heteroarylaminopropenoates 3-13 . These were cyclized into fused 3-(2-benzoyl-2-ethoxycarbonyl-1-ethenyl)amino-4H-azolo- (or azino)-pyrim-idin-4-ones 14-18 . 2-Benzoyl-2-ethoxycarbonyl-1-ethenyl group can be easily removed from 3-(2-benzoyl-2-ethoxycarbonyl-1-emenyl)amino-8-memyl-4H-pyrido[1,2-a]pyrimidin-4-one ( 14 ) to give 3-amino-8-methyl-4H-pyrido[1,2-a]pyrimidin-4-one ( 19 ). The structure of 1a was confirmed by X-ray analysis.     

Unexpected behavior of the reaction between 1,2-diaza-1,3-butadienes and 3-dimethylaminopropenoates: a useful entry to new pyrrolines, pyrroles, and oxazolines

We observed a nucleophilic attack by the ene-amino carbon of 3-dimethylaminopropenoates at the terminal carbon of the azo-ene system of 1,2-diaza-1,3-butadienes. In tetrahydrofuran at 65 degrees C, this attack produced 1-aminopyrrolines with a high degree of cis-stereoselectivity by means of an unusual zwitterionic adduct intermediate followed by intramolecular ring closure. In toluene under reflux, 1-aminopyrrolines produced oxazoline-fused 1-aminopyrrolines. Oxazoline-fused 1-aminopyrrolines were directly obtained by reaction of 1,2-diaza-1,3-butadienes with 3-dimethylaminopropenoates in toluene under reflux. The ring opening of oxazoline-fused 1-aminopyrrolines in acidic or basic media provides highly substituted 1-aminopyrroles. 5-Unsubstituted 1-aminopyrrole derivatives were obtained from 1-aminopyrrolines under basic conditions by loss of dimethylamino and ester groups. We discuss the plausible mechanisms of the ring closure and opening.     

1,3-Dipolar cycle-addition of 2-diazoalkanes to pyridazin-3(2H)-one derivatives. The formation of 3H-pyrazolo[3,4-d]pyridazin-4(5H)-one and 3H-pyrazolo[3,4-d]pyridazin-7(6H)-one derivatives

1,3-Dipolar cycloadditions of diazoalkanes to pyridazin-3(2H)-ones 1–7 and pyridazin-3(2H)-thiones 8 and 9 are regioselective producing 3H -pyrazolo[3,4-d]pyridazin-4(5H)-ones 15–19, 27–29 and 34–38 as the major products. In some instances, the isomeric 3H-pyrazolo[3,4-d]pyridazin-7(6H)-ones, such as 20 and 23 were isolated as the minor products. From 3 and 6 the primary 3a,7a-dihydro cycloadducts 25 and 26 , and rearranged 1,2-dihydro intermediate 31 were isolated. From 10 and 1-diazoindane the isomeric exo- and endospiro products 39 and 40 were formed.     

AM1 computational study of cycloaddition reaction of vinyl sulfene generated from thiete 1,1-dioxide

Theoretical studies of the cycloaddition reaction between vinyl sulfene, norbornene and related systems are presented. Since the parameterized AMI semiempirical method was used, the generated data are only of qualitative value. The vinyl sulfene reactivity was assessed by using frontier molecular orbital (FMO) energy gaps, bond orders, and the charge distribution of the reactants. To obtain more information, as far as the reaction outcome and its feasibility, the transition state structures and activation barriers for competitive pathways are computed.     

Theoretical study of azide anion addition to nonpolar and polar double and triple bonds

Transition structures for the 1,3-polar addition of azide anion to hydrogen cyanide, formaldimine, nitrogen, cis- andtrans -diazene, ethylene and acetylene were obtained at the MP2/6-31+G^* theoretical model. The additions can be divided into two groups: addition to a triple bond, giving rise to an aromatic heterocyclic product, and addition to a double bond, forming a non-aromatic product. All transition structures correspond to a concerted mechanism for the polar cycloaddition. Symmetrical dienophiles, apart from cis-diazene, give rise to synchronous transition structures. The anomaly is explained in terms of strong n-n repulsion of the reactants in the transition structure. The reactivity of the compounds can be rationalized in terms of the bond orders of the newly forming bonds, from the frontier orbital energy differences and from the charge transfer from the azide anion to the dienophile. The quantitative correlation of the reactivity has been judged on the basis of the activation energies of the reactions calculated at MP2/6-31+G^* and MP3/6-31+G^*. It is predicted that the addition of azide to nitrogen is the slowest and that the additions to hydrogen cyanide and acetylene have the lowest barriers, in agreement with literature data.     

Amino acids in the synthesis of heterocyclic systems. Synthesis of γ-(5-(1,2,4-Triazinylidene))-α,β-dehydro-α-amino acid derivatives and 6H-pyrido[1,2-d][1,2,4]triazin-6-ones

5-(1,2,4-Triazinyl) substituted enamines 3 react with 5(4H)-oxazolones 4 in acetic anhydride to give acetylated products 5 , while in toluene-acetic acid mixture nonacetylated products 9 are formed. Both types of products were isolated as (E,Z) mixtures. Compounds 5 and 9 rearrange into 6H-pyrido[1,2-d]-[1,2,4]triazin-6-ones 12 by heating in formic acid or in xylene, respectively. Compounds 5 are transformed in the presence of nucleophiles, such as sodium alkoxides or sodium amides via anionic form 10 into corresponding esters 13 and amides 14 of γ-(5-(1,2,4-triazinylidene)) substituted derivatives of α-amino-2-butenoic acid, which exist in 2-(Z),4-(Z) form.