Pummerer rearrangements using chlorotrimethylsilane

Pummerer rearrangements of 3-carbomethoxythian-4-one $\text{S}$-oxide ($\text{1}$) have been investigated and chlorotrimethylsilane found to be the reagent of choice for preparing the corresponding α,β-unsaturated sulphides ($\text{2}$ and $\text{3}$). Related chlorotrimethylsilane-induced Pummerer rearrangements are also reported.     

Stable isobenzofuran endoperoxide and its use for oxidation of olefins ans aromatic compounds

1,3-Di-tert-butylisobenzofuran reacts with singlet oxygen with rate constant of 2.8 x 10⁹ M^?1 sec^?1 to give stable endoperoxide $\text{4}$. Thermolysis of the endopepoxide ($\text{4}$) in the ppesence of norbornene and naphthalene gave the corrpesponding epoxide and naphthols, respectively. Addition of Pd(OAc)₂ improved the yield of naphtols considepably.     

Comparison of reactivity of Ar(3Po) and Ar(3P2) metastable states. Products of quenching by some halogen-containing compounds

Vacuum UV emission from the products of quenching ofAr(³P_o) and Ar(³P₂) by several reagents has been compared. The large differences suggest that Ar(³P_o) and Ar(³P₂) preferentially yield respectively the D($\text{1}\text{2}$) and B($\text{1}\text{2}$) excited states of ArBr^★ or ArCl^★, which show specific, but very different, predissociation patterns.     

Stereochemical consequence in the elimination of β-hydroxyalkylsilanes: Stereoselective formation of (Z)- and (E)-alkylidene-γ-butyrolactones

Titanium(IV) chloride-mediated reaction of 4,5-dihydro-2-(trimethylsiloxy)-3-(trimethylsilyl)furan (1a) with acetaldehyde gave $\text{diastereomerically pure}$ (3S^*, 1′R^*)-4,5-dihydro-3-(1′hydroxyethyl)-3-(trimethylsilyl)-2(3$\text{H}$)furanone (2a), which afforded selectively either (Z)- or (E)-α-ethylidene-γ-butyrolactone (3a) under proper conditions. Facile isomerization of 2a into $\text{diastereomerically pure}$ (3S^*, 1′R^*)-4,5-dihydro-3-&{1′-(trimethylsiloxy)ethyl}-2(3$\text{H}$)furanone (4) suggests an intriguing stereochemical outcome from 2a to (E)-3a via an enolate of 4.     

The importance of spin relaxation in the delayed fluorescence of molecular crystals

Deactivation rate constants of spin-orbital excited Br atoms in the reactions Br(²P_{$\text{1}\text{2}$}) + O₂ → Br(²P_{$\text{3}\text{2}$}) + O₂ (k₁), and Br(²P_{$\text{1}\text{2}$}) + NO → Br(²P_{$\text{3}\text{2}$}) + NO (k₄) have been measured with a photodissociative IBr laser on the electronic transition ²P_{$\text{1}\text{2}$}?²P_{$\text{3}\text{2}$} in the Br atom (λ = 2.7 μm). The values obtained are (6.4 ± 1.8) × 10^?14 cm³ s^?1 and (1.9 ± 0.6) × 10^?12 cm³ s^?1, respectively. Comparison with published data leads to the conclusion that, contrary to a widely accepted point of view, the high rate constants for the quenching of excited halogen atoms are due to resonant energy transfer processes and not to the paramagnetic nature of the quencher.     

Yield of I(2Psol:12) in the photodissociation of CH2I2

The production of I(²P_{$\text{1}\text{2}$}) in the photolysis of CH₂I₂ has been studied optoacoustically at excitation wavelengths between 365.5 and 247.5 nm. Bands found at 32200 and 47000 cm^?1 correlate with I(²P_{$\text{3}\text{2}$}) whilst those at 34700 and 40100 cm^?1, which correlate with I(²P_{$\text{1}\text{2}$}), give final ²P_{$\text{3}\text{2}$}/²P_{$\text{1}\text{2}$} ratios of 1.75 and 1.1, respectively, after curve crossing.     

On the mode of incorporation of leucine samples labelled at positions 5 and 2 with 3H and 14C into the isoprenylated tryptophan derivative echinuline from aspergillusamstelodami

Feeding experiments with (4$\text{S}$) [5-³H;5-¹⁴C]$\text{L}$-leucine ($\text{1a}$) and with the (4$\text{S}$) [5-³H;2-¹⁴C]-isomer ($\text{1b}$) show that incorporation into the isoprenylated tryptophan derivative echinuline ($\text{3}$), produced by $\text{Aspergillus}$$\text{amstelodami}$, involves, to a large extent, loss of the fragment enbodying the carbon atom at position 2 of the fed amino acid.     

Temperature dependence of collisional deactivation of Tl(62P32)

The rate coefficients for the collisional deactivation of Tl(6²P_{$\text{3}\text{2}$}) by several gases has been determined at 300°K. This data is compared with that previously obtained at higher temperatures and the Arrhenius parameters calculated. Both the overall rate coefficients and temperature effects display trends similar to those observed for 1(5²P_{$\text{1}\text{2}$}) relaxation. The deactivation of Tl(6²P_{$\text{3}\text{2}$}) by O₂ is shown to proceed by a process involving an equilibrium with Tl(6²P_{$\text{1}\text{2}$}) and electronically excited oxygen, probably O₂(¹Δ_g).     

Isolation ofΔ9(11)-sterols from the sea cucumber psolusfabricii. Implications for holothurin biosynthesis

- 14α-Methyl-5α-cholest-9(11)-en-3β-ol ($\text{2}$) and 4α,14α-dimethyl-5α-cholest-9(11)-en-3β-ol ($\text{3}$) have been isolated from the sea cucumber $\text{Psolus}$$\text{fabricii}$ and characterised by ¹H NMR, ¹³C NMR and mass spectrometry. Lanost-9(11)-en-3β-ol ($\text{4}$) has also been tentatively identified. The relevance of this series of Δ⁹⁽¹¹⁾-sterols to holothurin biosynthesis is briefly discussed.     

A synthesis of the aromatic segment of rifamycin s

The aromatic segment $\text{15}$ of rifamycin S ($\text{1}$) was synthesized from the cyclohexadienone $\text{2}$ or $\text{ii}$ in high overall yield.     

Inept-29Si NMR study of a TiCl4-mediated reaction of an enol silyl ether

Condensation of 1,3-bis(trimethylsiloxy)-1-methoxybutadiene ($\text{1}$) and 2-phenyl-2,2-dimethoxyethanal ($\text{2}$) under TiCl₄ condition gave the γ-hydroxycyclopentenone product $\text{3}$. The reaction was followed by INEPT²⁹Si NMR. Implication to the mechanism of the reaction was discussed.     

The aromatic cope rearrangement: Activation parameters

The activation parameters (ΔH^≠ = 26.7 ± 4.0 kcal/mol and ΔS^≠ = 17.5 ± 9.8 eu) have been measured for the Cope rearrangement of $\text{cis}$-1-($\text{m}$-hydroxyphenyl)-2-vinylcyclopropane.     

Perhalogenmethylmercapto-heterocyclen,IX (perchlormethylmercapto)- und (perfluormethylmercapto)pyridine

Methods of substituting pyridine by perhalogenomethylmercapto groups are discussed. The side chain chlorination of 2-(methylmercapto)pyridine leads gradually to 2-(trichloromethylmercapto)pyridine hydrochloride ($\text{1a}$) and 2-(trichloromethylmercapto)pyridine ($\text{1b}$). Neither a direct reaction of pyridine with CF₃SCl nor the way over a Grignard reaction or a sulfenylcarboxylate lead to CF₃S-substituted products. Reactions of pyridine and bromopyridines with Hg(SCF₃)₂ yield 1:1-adducts ($\text{2a-d}$) only. Lithium tetrakis(1,2-dihydro-1-pyridyl)aluminate (LDPA) reacts with CF₃SCl to give 3-(trifluoromethylmercapto)pyridine ($\text{3}$); in addition a disubstituted product can be identified massspectroscopically. ¹H- and ¹⁹F-NMR-spectra are reported.     

Studies on amino acids and peptides VIII : Synthesis and crystal structure of two monothiated analogues of Boc-Gly-S-Ala-Aib-OMe

The model tripeptide Boc-Gly-S-Ala-Aib-OMe ($\text{2b}$) and the two monothiated analogues Boc-Gly(¹ψ²CSNH)-S-Ala-Aib-OMe ($\text{2c}$) and Boc-Gly-S-Ala(²ψ³ CSNH)-Aib-OMe ($\text{2a}$) were synthesized. Peptide $\text{2a}$ was obtained by thiation of $\text{1a}$ using 2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane 2,4-disulfide, Lawesson's Reagent (LR), followed by deprotection of the Boc group and coupling with Boc-Gly-OH. Thiation of $\text{2b}$ with LR regiospecifically transformed the protected tripeptide to the monothiated analogue $\text{2c}$. X-Ray diffraction analysis showed that the type-III β-turn formed by the reference peptide $\text{2b}$ is preserved in the monothiated analogue $\text{2a}$; conversely,the structure of the isomeric tripeptide $\text{2c}$ is partially extended.     

Cycloaddition of single oxygen and triazolinedione (TAD) with spirocyclopentadienyl-1,3,5-cycloheptatrienes1: tropilidene versus norcaradiene reactivity

4-Methyl-1,2,4-triazoline-3,5-dione (MTAD) gives with 7-spiro-fluorenyl-1,3,5-cycloheptatriene ($\text{1d}$) initially he norcaradine-type urazole ($\text{N3d}$) which at ambient temperatures rearranges into ($\text{6d}$), while ¹0₂ gives the tropilidene-type endoperoxide ($\text{T4d}$).     

The octant rule. XVI1. COnformation and circular dichroism of syn and anti 2-methylbicyclo[3.1.0]hexan-3-ones,thujone and isothujone

(lS,5S)-exo-2(R)-Methylbicyclo[3.1.0]hexan-3-one ($\text{1}$) and (lS,5S)-endo-2(S)-methylbicyclo[3.1.0]hexan-3-one (2) were synthesized and their circular dichroism (CD) spectra run. Conformational analysis based on molecular mechanics calculations and Karplus equation analyses of vicinal H¦H NMR coupling constants indicate boat-like sofa conformations for both $\text{1}$ and $\text{2}$, with very little ring distortion from the symmetry of the parent bicyclo[3.1.0]hexan-3-one. The lone dissymmetric ψ-axial and ψ-equatorial methyl groups of $\text{1}$ and $\text{2}$, respectively, are both octant consignate. The natural product analogs of $\text{1}$ and $\text{2}$, (?)-3-isothujone ($\text{3}$) and (+)-3-thujone ($\text{4}$) were prepared and examined similarly. Their α-methyl perturbers dominate the CD n-π^★ Cotton effects.     

(4+2)-cycloaddition of nitroalkenes with ynamines; formation of a 4H-1,2-oxazine 2-oxide derivative

The formation of a thermally unstable (4+2)-cycloadduct, a 4H-1,2-oxazine 2-oxide derivative ($\text{1}$), from the reaction of 1-nitrocyclopentene with 1-phenyl-2-(1-pyrrolidinyl)acetylene has been proven by the structure elucidation of isoxazole derivative $\text{3}$ which results from thermal rearrangement and by the structure determination of the 1,3-dipolar adducts $\text{5}$ of $\text{1}$ with electron-deficient acetylenes.     

Assymetric reduction of ketones using LiAlH4 modified with chiral 1,2-aminodiols.txt

Four chiral aminodiols from ring opening of ($\text{S}$)-propylene oxide and ethylene oxide wiht n-butylamine and ($\text{R}$) or ($\text{S}$)-α-methylbenzylamine were used to modify LiAlH₄. Assymetric reduction of acetophenone and propiophenone gave the highest percent enantiomeric excess when the modifier was made from ($\text{S}$)-oxide and ($\text{S}$)-amine.     

Protonation of acenaphthoquinone. A novel peripheral 10 π-electron aromatic system

Acenaphthoquinone ($\text{1}$) undergoes ppotonation to afford the respective double protonation product $\text{2}$. The proton and carbon NMR parameters of $\text{2}$ are discussed.     

A 29Si, 13C and 1H NMR study of the interaction of various halotrimethylsilanes and trimethylsilyl triflate with dimethyl formamide and acetonitrile,a comment on the nucleophile induced racemisation of halosilanes

The ²⁹Si, ¹³C and ¹H NMR spectra of $\text{1}\text{1}$ mixtures of Me₃SiI and Me₃SiOSO₂CF₃ with DMF in CD₂Cl₂ show signals that are consistent with the formation of Me₃SiO$\text{C}\text{+}$H(NMe₂)X^? but not with penta- or hexa-coordinate silicon species. The spectra of a $\text{1}\text{1}$ mixture of Me₃SiBr and DMF show a rapidly exchanging, equilibrium mixture of Me₃SiO$\text{C}\text{+}$H(NMe₂)Br^? and starting materials. No strong evidence for salt formation between DMF and Me₃SiCl was obtained. The spectra of Me₃SiX (X = I, Br, Cl, OSO₂CF₃) in CD₃CN indicate that neither adduct formation nor extra coordination at silicon is significant.