Homoconjugated ketones with extended unsaturation: Wavelength-selective,regioselective, diastereoselective,and enantiospecific photochemical transformations of methyl 7-oxospiro[5.5]undeca-1,3-and -2,4,-diene-2-carboxylate

The title molecules were shown to photorearrange with remarkable selectivity. Wavelength variation steers the rearrangement modes, of which the [1,2]-acyl shift was found to be largely regioselective, diastereoselective, and enantiospecific. Chemical intercorrelation of products and mechanistic studies were carried out all along. The potential significance of these photochemical processes in selective synthetic schemes is discussed.     

Oligomerization of 1,2‐Ethanedithiol: An Expedient Approach to Oligothiaethylenethioglycols

Reactions of ethylenedithioglycol (ETG) with Na₂CO₃, K₂CO₃, and Cs₂CO₃ provided the oligothiaethylenethioglycols (nETG): di‐ (DETG), tri‐ (TrETG), tetra‐ (TETG), and pentathiaethylenethioglycol (PETG), along with higher polymers. The most efficient carbonate was K₂CO₃ and reactions using DETG and TrETG as starting materials—or their mixtures—were also found to afford similar species. This largely unknown oligomerization process was thoroughly explored and potential pathways were put forward. A convenient conversion of ETG to laboratory quantities of the otherwise scarce and/or expensive DETG, TrETG, TETG, and PETG oligomers, in organic or aqueous media was achieved. Notably, this straightforward reaction takes place without the addition of expensive or toxic metal catalysts and with pure water as the solvent, thereby highlighting its potential as a green chemical reaction. Moreover, the process opens up new approaches to dynamic combinatorial libraries (DCLs) of oligomers and macrocycles with manifolded nETG [(SCH₂CH₂)_nS] bridges.     

Complete enantiospecificity in the highly regioselective and stereoselective photo- and thermal rearrangements of some homoconjugated ketones

The oxa-di-π-methane (ODPM) photorearrangement of 2-(carbomethoxy)spiro [5.5]undeca-1,3-diene-7-one ($\text{1}$d) to $\text{2}$d and the latter's thermal rearrangement to $\text{3}$d were shown to be not only fully regio-and stereoselective but also to occur with complete enantiospecificity. An additional example, the 2-hydroxymethyl derivative ($\text{1}$f) was also studied and the two systems were chemically correlated.     

Novel bromination reagents · hexabromocyclopentadiene: bromination of activated saturated sites

Hexabromocyclopentadiene (HBC) readily brominates α-keto and benzylic sites, apparently by bromonium ion transfer.     

Steric effects in 5-membered rings—VIII : A conformational analytical study of cis- and trans-1,3-dichlorocyclopentane

A study of the title systems was performed using theoretical calculations, dipole moments (experimental and calculated) and NMR spectroscopy. The results were discussed in the light of literature data on related systems. It was concluded that a multitude of conformations occur in a potential well for the cis-isomer and that for the trans-one, an even flatter and more variegated distribution in a double minimum is indicated.     

Photochemical studies—XXI : Dimers of 3,4-(o,o'-biphenylene)cyclopentadienones: Thermal and photochemical behaviour

The 1,4-dimethyl-, 1,2,4-trimethyl and 1,2,4,7-tetramethyl substituted 3,4-(o,o'-biphenylene) cyclopentadienone-dimers (6a–6c) were prepared and found to undergo photochemically or thermally a 1,3-rearrangement to the centrosymmetrical diketones (11). Only the tetramethyl derivative (6a) undergoes at room temperature a fast, degenerate [3.3] Cope rearrangement with ΔG^≠₂₃₃ = 11.4 kcal mol.^?1 All these dimers and rearrangement products appear not to dissociate to their monomers, but react with dienophiles to give the adducts (12, 13). A stepwise mechanism involving diradical intermediates (19) is invoked. The “mixed dimer” (14) was also prepared and studied. The steric and electronic effects determining the behaviour of these compounds are discussed.     

Probing the anomeric effect. The diaminomethylene group: Calculations of N?C?N-containing molecular systems1

A study of systems containing the title moiety is described, with special reference to the anomeric effect. We have calculated ab initio, using Gaussian-80 with the 3-21G basis set, all basic conformations of methylene-diamine (H₂N? CH₂? NH₂) and its N-methyl derivative with full geometry optimization of energy minima and barriers. The structural data thus obtained, were then employed to parameterize Allinger's MM2-80 force field in a procedure similar to that described for oxygen derivatives, including hydrogen-bonding effects and C? N bond shortening in tertiary amines. This modification, termed MM2-AE was then used to calculate larger molecules, including N,N′-di- and N,N,N′,N′-tetramethyl-methylenediamine, various 1,3-diazane systems, and 1,4,5,8-tetraazadecalin derivatives of established (x-ray) structures. The results are discussed in light of their verificative and predictive power and appear to validate MM2-AE as a useful computational procedure.     

Modified MM2 scheme for computation of O?C?N-containing systems

A modification of Allinger's MM2 force field for the anomeric effect in O? C? N systems is presented. For optimal consistency, it was parameterized by alternate use of ab initio (3-21G level) and X-ray results to account for the energetic and structural manifestations of the effect in the gas or condensed phase, respectively. The results obtained with the modified force field are in good agreement with those from both theoretical and experimental methods. The parameterization scheme explicitly treats all structural parameters of the C? O? C? N? C moiety as well as C? N bond lengths in tertiary amines contained within an anomeric unit. In addition, it includes directional H-bond type interactions. A limited number of parameters is put forward, in accord with the general MM2 force-field approach. © 1993 John Wiley & Sons, Inc.     

Photochemical studies. Irradiation induced transformations of 1,2-dihydrophthalides

The photochemical behavior of $\text{cis}$-1,2-dihydrophthalide ($\text{1}$) and its 1,2-trimethylene derivative ($\text{4}$) was studied. Besides bicyclo[2.2.0]hexene formation in both cases, ($\text{1}$) transforms into its $\text{trans}$ isomer ($\text{3}$) whereas ($\text{4}$) undergoes a 1,2-shift to ($\text{7}$).