The structures of the two isomers of monodehydro[14]annulene

Variable temperature ¹H nmr spectrometry has shown that the “unstable” isomer of monodehydro[14]annulene possesses the symmetrical di-$\text{trans}$ configuration $\text{1}$ or $\text{2}$, and the “stable” isomer (the precursor of [14]annulene) possesses the tri-$\text{trans}$ configuration $\text{4}$.     

Interactions of singlet oxygen with 2,5-dimethyl-2,4-hexadiene in polar ano non-polar solvents evidence for a vinylog ene-reaction

2,5-Dimethyl-2,4-hexadiene ($\text{1}$)was studied as a singlet oxygen acceptor in various solvents. $\text{1}$undergoes concomitantly the three well-known modes of singlet oxygen reactions: (1) the ene-reaction to give the allylic hydroperoxide $\text{3}$, (2) the (4+2)-cycloaddition to give the endoperoxide $\text{4}$, and (3) the (2+2)-cycloaddition to give the dioxetane $\text{2}$. Beyond that (and in contrast to simple olefins), there are (4) “physical” quenching and (5) a “vinylog ene-reaction” to give the twofold-unsaturated hydroperoxide $\text{5}$. The latter reaction represents a novel mode of singlet oxygen interaction with a substituted 1,3-diene. - Kinetic analysis shows that “physical” quenching, endoperoxide and vinylog ene-product formations proceed with solvent-inde pendent rates; the rates of dioxetane and ene-product formations, however, are solvent-dependent. - A mechanism (Scheme 3) is proposed, according to which endoperoxide formation is due to a concerted singlet oxygen reaction with the s-cis-conformational isomer $\text{1b}$; with the s-trans-isomer $\text{1a}$, “physical” quenching and the vinylog ene-reaction proceed via a non-polar singlet diradical intermediate, whereas the ene-product formation occurs via a per epoxide-like transition state. In aprotic solvents, the dioxetane is mainly formed via a “tight-geometry intermediate”, in methanolic solution via a solvent-stabilized zwitterion; the latter is also responsible for the formation of the methanol-addition product $\text{6}$.     

Flavonoids, 39. The reaction of 3-mesyloxyflavanones with O- and S-nucleophiles,synthesis of 3-thiocyanato- and 3-acetylthio-flavanones

The action of either cyanate or acetate ions on $\text{trans}$-3-mesyloxyflavanones $\text{1}$ led to the formation of flavones $\text{2}$ and aurones while the reaction with the respective S-nucleophiles (thiocyanate and thioacetate ions) resulted in $\text{cis}$-and $\text{trans}$-3-thiocyanato- and 3-acetylthioflavanones ($\text{6}$ and $\text{11}$). The participation of the concurrent nucleophilic substitution, β-elimination and elimination with ring-contraction is dependent on the “hard-soft” character of the nucleophile and the increase of the “soft” character favours the displacement.     

Synthesis and revised structure of the o-succinylbenzoic acid coenzyme a ester,an intermediate in menaquinone biosynthesis

Synthesis of the two isomers $\text{2}$, $\text{3}$ of the mono coenzyme A ester of o-succinylbenzoic acid ($\text{1}$, OSB, i.e. 4-(2-carboxyphenyl)-4-oxobutanoic acid) and enzymic conversion of $\text{3}$, to 1,4-dihydroxy-2-naphthoic acid $\text{7}$ shows that as opposed to previous assumptions the “aliphatic” rather than the “aromatic” carboxyl group in o-succinylbenzoic acid $\text{1}$ is activated during vitamin K₂ biosynthesis in Escherichia coli and Mycobacterium phlei.     

Thermal rearrangements of some indole alkaloid derivatives

Under both static and flow thermolysis conditions, several compounds with an “aspidosperma” framework rearranged to “vinca” derivatives. Thus (-)1,2-dehydroaspidospermidine ($\text{4}$) rearranged to (-) gave vincane $\text{14}$. Compound $\text{6}$ rearranged to vincamine ($\text{13a}$) and 16-epi vincamine ($\text{13b}$) under either condition ; increasing the temperature resulted in formation of apovincamine ($\text{19}$) (pyrolysis) or vincamone thermolysis).     

Synthesis of “capped porphyrins”

The synthesis of “capped porphyrins” ($\text{10}$), ($\text{18}$), and ($\text{28}$), and their (chloro)iron(III), iron(II), cobalt (II), and zinc(II) complexes is reported. These complexes serve as models for the active site of the oxygen binding haemoproteins. In addition to reversible binding of dioxygen by each of the iron (II) porphyrin complexes, the 1-methyl-imidazole-(“C₃-capped porphyrin”) iron (II) complex ($\text{23}$) reacts reversibly with carbon monoxide, in solution at 25°C.     

Reaction of aldehydes and ketones with t-butyl bromide-dimethyl sulphoxide

Reacting aldehydes and ketones with the “Bu^tBr-Me₂SO” system produces the corresponding α-bromoderivatives $\text{2}$. In the case of ketones, is possible, bromination is obtained exclusively at the more highly substituted α-position. With slight modifications of the reaction conditions (add to obtain “in situ” formation of either dimethyl(2-oxo-2-phenylalkyl)sulphonium salts $\text{3}$ or of α-methylthioderivatives $\text{4}$. Dimethyl(l-m bromide ($\text{3h}$) during crystallization undergoes spontaneous resolution of the two enantiomers, as demonstrated by single crystal X-ray analysis a     

An adamantane rearrangement. A new pathway

Tricyclo(4.2.2.o^1,5)decane ($\text{7}$) in the presence of AlBr₃ rearranges partly “forwards” to adamantane ($\text{1}$) and partly “backwards” to tetrahydrodicyclopentadiene ($\text{2}$, largely the oxo isomer). Intermediate $\text{14}$, characterizing the $\text{7}$→exo-$\text{8}$→$\text{14}$→$\text{3}$→$\text{1}$ forward pathway, is found only in small amounts. The detection of a new intermediate, $\text{12}$, also shows that a second major rearrangement route from $\text{7}$ to $\text{1}$ is being utilized (see dashed lines in Figure 1).     

Cyclotrimerization of cyclic diynes via metal atom cocondensation; revised structure for the reported “percyclophane-4”

A compound previously assigned the “percyclophane-4” structure $\text{3}$ has been shown to be the triyne $\text{2}$.     

The selenide systems of niobium and tantalum

The selenide systems of niobium and tantalum were investigated under high pressures in the composition limits of MX₂ to MX₃. Two distinct phases, NbSe₂ and NbSe₃, were obtained in the niobium system. In the tantalum system, there were three phases, TaSe₂, “TaSe₃,” which has been thought of as a triselenide, and stoichiometric TaSe₃. The ratio of $\text{Se}\text{Ta}$ in “TaSe₃” was 2.8. Conversions of the triselenides under high pressure to NbSe₂ or TaSe₂ were observed at the starting composition of $\text{Se}\text{M}\text{= 2.0}$ and to “TaSe₃” at $\text{Se}\text{Ta}\text{= 2.8}$.     

Two new methods for the synthesis of C-glycosides

Thiophenyl glycosides are converted to C-glycosides by reaction with allyl or methallyltri-$\text{n}$-butylstannane using both “one-electron” and “two-electron” procedures, which give different stereoselectivities in some cases.     

The diastereospecific aprotic conjugate addition reactions of allylic anions-mechanistic aspects.

A ten-membered cyclic “chair-chair” - or “$\text{trans}$-decalyl”-like TS is proposed to account for the diastereospecific aprotic conjugate addition reactions of allylic carbanions bearing polar, charge-stabilizing groups.     

On the biogenesis of gliotoxin. Synthesis of 3-(β-aminoethyl)benzene oxide.

The synthesis of 3-(β-aminoethyl)benzene oxide ($\text{7}$ is described. The failure of $\text{7}$ to ring close ($\text{7}$→$\text{8}$) is attributed to the low “nucleophilic susceptibility” of the arene oxide.     

Novel synthesis of n,n'-bis (α-isocyanato alkyl) diazenes

The conversion of aliphatic ketazines $\text{1}$ with cyanic acid into the “criss cross” cycloadducts, the triazolo-triazolediones $\text{2}$, followed by oxidative ring opening provides an efficient synthetic route to the title compounds $\text{4}$.     

Synthesis of (1,5-cis-5,6-trans-6,7-trans)-2-oxa-3-methoxy-6-ethoxycarbonyl-7-benzoyl oxy bicyclo /3.3.0/ octane : An useful synthon in the prostaglandins field

The target molecule ($\text{11}$), a precursor of PG, has been prepared by a novel fully stereoselective methodology “via” cyclopentandione ($\text{2}$), starting from ethyl acetoacetate and ethyl-2-bromo-pentenoate.     

Minor and trace sterols in marine invertebrates 30. Isolation,structure elucidation and partial synthesis of 26-methylstrongylosterol and 28-methylxestosterol - two marine sterols arising by a novel quadruple biomethylation sequence

Two new sterols, 26-methylstrongylosterol ($\text{4}$) and 28-methylxestosterol ($\text{5}$), arising from a hitherto unknown quadruple biomethylation sequence, together with the biogenetically important “missing link”, durissimasterol ($\text{6}$), were isolated from the Indo-Pacific sponge $\text{Strongylophora}$$\text{durissima}$.     

Novel (4 + 1) fragment combination approach to chiral cyclopentanoids from tartaric acid

A $\text{chiral}$ cyclopentanoid building block $\text{29}$ has been synthesized in “one pot” cyclization process from epoxides $\text{22}$ - $\text{25}$ (which are readily accessible from (R,R)-(+) tartaric acid) with the carbanion derived from phenylthioacetonitrile (PhS-CH₂-CN).     

Two new inert carbon biradicals of the triphenylmethyl series. Synthesis,conformation and spin exchange

The synthesis and some properties of two new “chemically inert” free biradicals, the $\text{perchloroethynylene}$- (III) and $\text{perchlorovinylenebis-4-triphenyl}$-$\text{methyl}$ (IV), are described. They correspond to hydrocarbon analogues known to exist, at least predominantly, in a singlet, quinonoid form.     

α-Phenylselenenylation d'aldehydes - synthese simple d'α-cetoaldehydes a fonction cetone protegee

Morpholinophenylselenenamide $\text{2}$ add to β-phenylselenoenamines $\text{1a}$ and yield β,β-bis(phenylseleno) enamines $\text{4}$. These compound undergo hydrolysis to form bis(phenylseleno) ethanal $\text{5a}$. Also, $\text{2}$ react with enolic aldehydes to give α-phenylselenoaldehydes $\text{3}$. This reaction allow a “one step” synthesis of cetoselenoacetals $\text{5}$ of α-cetoaldehydes with good yields.     

Stereoselective total syntheses of (±)-sinularene and of (±)-5-epi-sinularene via intramolecular type-I-“magnesium-ene” reaction

(±)-Sinularene ($\text{1}$) as well as (±)-5-epi-sinularene ($\text{9}$) were synthesised in a stereocontrolled manner from the norbornene $\text{2}$ in overall yields of 4% and 8%, respectively. The key step $\text{5}$ → $\text{6}$ involves a regio- and stereoselective intramolecular “magnesium-ene” reaction.