On the first polyarsenic organic compound from nature: arsenicin A from the New Caledonian marine sponge Echinochalina bargibanti

Reported here is the first polyarsenic compound ever found in nature. Denominated arsenicin A, it was isolated along a bioassay-guided fractionation of the organic extract of the poecilosclerid sponge Echinochalina bargibanti collected from the north-eastern coast of New Caledonia. In defining an adamantine-type polyarsenic structure for this compound, deceptively simple NMR spectra were complemented by extensive mass spectral analysis. However, it was only the synthesis of a model compound that provided the basis to discriminate structure 4 from other spectrally compatible structures for arsenicin A; to this end, a comparative ab initio simulation of IR spectra for the natural and the synthetic compounds was decisive. Arsenicin A is endowed with potent bactericidal and fungicidal activities on human pathogenic strains. All this may revive pharmacological interest in arsenic compounds while prompting us to rethink the arsenic cycle in nature.     

One- and two-photon absorption and emission properties of a Zn(II) chemosensor

This paper presents the synthesis and two photon-induced absorption (TPA) properties of a functionalized distyrylbenzene (DSB) 1 containing a tetra-azacyclododecane (cyclen) receptor for Zn(II). The influence of Zn(II) on one- and two-photon absorption characteristics of 1 has been investigated in dimethyl sulfoxide. The experiments show that the TPA action spectrum of uncomplexed 1, at 750 nm employing nanosecond-long excitation pulses, is 5 times more intense than that of the complexed form. This moderate contrast between the bound and unbound species confirms, however, the potential of this design scheme for the development of molecular structures with enhanced sensitivity and contrast to be used as Zn(II) sensors through TPA-induced fluorescence microscopy.     

Green,lithium salt-free synthesis of 2-alkylated 1,4-benzenediols in hydroalcoholic media

Herein, we describe an efficient, green, and lithium salt-free synthesis of 2-substituted 1,4-benzenediol in water/ethanol as solvent and potassium carbonate as a base. Optimized reaction time, the required equivalent of base and the observed solvent effect are reported.     

On the Absolute Configuration of Penlanfuran and Related Sesquiterpenoids of the Sponge Dysidea fragilis from the North-Brittany Sea

The 1,4-addition of vinylmagnesium bromide/CuBr-SMe₂ to (+)-(4s)-cryptone ((+)- 6 ) in THF in the presence of Me₃SiCI gives the silylenol ether 7 which partly undergoes hydrolysis to ketone 8 on aqueous workup; residual 7 is hydrolyzed with pyridinium p-toluenesulfonate (PyT_sOH) to give 8 which is protected in situ with ethylene glycol and then ozonolized to give aldehyde (? )- 10. The latter, on addition of 3-lithiofuran followed by dehydration and deprotection with Py T_sOH, gives the unnatural (4S)-enantiomer (+)- 2 of noroxopenlanfuran. All processes, except 11 → (+)- 2 , are of good yield. On the basis of previous chemical transformations, this also establishes the (4R)-configuration for penlanfuran ((?)- 1 ) and other sesquiterpenoids of the sponge Dysidea fragilis from the North-Brittany sea. Non viable routes to (±)-noroxoperlanfuran ((±)-2) are also discussed.     

Entropy convergence in the hydration thermodynamics of n-alcohols

Using experimental data from the literature, entropy convergence in the hydration thermodynamics of n-alcohols is shown to occur at about 125 degrees C. The phenomenon is reproduced in a more-than-qualitative manner by means of a theoretical approach that accounts for the entropy contributions associated with (a) creation of a cavity in water, (b) turning on solute-water van der Waals interactions, and (c) turning on the solute-water H-bonding potential. The density of water and the effective size of water molecules with their temperature dependence play the pivotal role for the occurrence of entropy convergence, together with the property of the alcohol hydroxyl group to form the same number of H-bonds with water molecules regardless of the length of the alkyl chain.     

Rogiolenyne A,B, and C: The First Branched Marine C15 Acetogenins. Isolation from the Red Seaweed Laurencia microcladia or the Sponge Spongia zimocca of II Rogiolo

It is shown here that the red seaweed Laurencia microcladia, collected off the torrent II Rogiolo, south of Livorno, contains rogiolenyne A (= (?)-(1R*,2S*,3R*,5S*,7S*)-2-(bromomethyl)-5-[(Z)-1-chlorohex-3-en-5-ynyl]-3-ethyl-4,8-dioxabicyclo[5.1.0]octane; (?)- 1 ) while the sponge Spongia zimocca, which grows in the same small area, contains rogiolenyne B (= (?)-(2R*3R*,4R*,5R*,7S*)-3-(bromomethyl)-5-chloro-7-[(Z)-1-chlorohex-3-en-5-ynyl]-2-ethyloxepan-4-ol; (?)- 4a ) and its acetate, rogiolenyne C((?)- 4b ). These structures, which are based on extensive NMR and MS data and on chemical transformation, are the first examples of branched marine C₁₅ acetogenins. Biogenesis of (?)- 1 in L. microcladia is thought to involve C(12) extrusion form a C₁₅ linear tetraen-1-yne precursor via H⁺-induced cyclopropane ring closure, followed by Br⁺-induced cyclopropane ring opening, aided by C–O^? attack (Scheme 2). It is also proposed that transfer of (?)- 1 to S. zimocca is followed by epoxide ring opening by Cl^? to give (?)- 4a and acetylation to give (?)- 4b .     

Conformational Analysis of Marine Polyhalogenated β-Chamigrenes Through Temperature-Dependent NMR Spectra

Temperature-dependent ¹H- and ¹³C-NMR spectra reveal that polyhalogenated marine β-chamigrenes or synthetic derivatives thereof which are trans-diequatorially, substituted at C(8) and C(9), such as rogiolol ((?)- 2 ), obtusol ((+)- 3 ), and their acetates (+)- 1 and (?)- 4 , undergo slow ring- A chair-chair inversion. Conformational equilibria and kinetics are investigated with the aid of synthetic model compounds and molecular-mechanics calculations. Thus, steric repulsions between Br_ax–C(2) and H_eq–C(7) are seen to disfavour thermodynamically conformers 1b , 2b , 3b , and 4b , which can only be detected through cross-saturation transfer, while additional steric repulsions between Me_ax–C(1) and OH_ax–C(3) make conformer 8b of obtusol epimer so scarcely populated that it can not be detected. In agreement, with (+)- 9 and (+)- 10 , which have a trigonal C(2), two conformers can be directly observed by NMR. The kinetic barriers, which are seen to arise mainly from steric repulsions between H_ax–C(14) and the axial H or halogen atoms at C(8) and C(10), are calculated and discussed with respect to well documented exocycliemethylidene-substituted cyclohexane(ene) systems. This helps to rationalize why in rogiolol acetate ((+)- 1 ) ring B is unusually inert towards Zn/Et₂O/AcOH which causes bromohydrine-group elimination from ring A.     

PCl3 mediated cyclization: Synthesis,at room temperature,of N‐alkenyl derivatives of 1,4‐phthalazinedione

The use of a PCl₃/hydrazone/dicarboxylic acid combination can be applied in an efficient one‐pot procedure for the synthesis at room temperature of the title ring compounds that are also a new family of stable enamines. The phthalazinediones 7 were obtained in good yields by reaction of PCl₃ with phthalic acid and hydrazones 1a,b,c containing hydrogen atoms only in one α position. Two structurally isomeric phthalazinediones (7 and 8) were obtained with hydrazones 1d,e,f,g containing hydrogen atoms in the two different α positions. When we used a methyl ketone hydrazone 1c,d,e,f,i, it was possible also to isolate the isomer containing an N‐alkenyl group with two terminal hydrogen atoms. Where E,Z isomers are possible, the exclusive formation of the E isomer was always observed. As a rule, changing the order of addition of reagents gave almost identical results; however, in the case of phthalic acid, it is better to use the procedure in which PCl₃ is added last to prevent the formation of the corresponding anhydride. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 291–296, 1999     

Chlorodiazaphospholines as intermediates in the synthesis of 1,2-dihydro-2-alkenyl-3H-pyrazol-3-ones and 2-pyrrolylacetates

New derivatives of 1,2-dihydro-2-alkenyl-3H-pyrazol-3-ones 6 and 7 have been synthesized at room temperature by a one-pot two-step reaction of PCl₃, a ketone methylhydrazone, and a β -keto ester. With ketone methylhydrazones bearing at least a phenyl group in the α -positions to the CN bond and in the second step β -keto esters, such as propionyl- or butyrylacetates, we obtained 2-pyrrolylacetates 8 as unexpected products together with pyrazolones 7 . The ratio of the two products depends on the nature of the groups in the α -positon to the CN bond. A chlorodiazaphospholine 1 is the key intermediate of this new reaction, and a plausible mechanism of formation of the azaheterocycles is reported.     

The hydroxyl radical reaction rate constant and products of cyclohexanol

The gas phase reaction of the hydroxyl radical (OH) with cyclohexanol (COL) has been studied. The rate coefficient was determined to be (19.0 ± 4.8) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ (at 297 ± 3 K and 1 atmosphere total pressure) using the relative rate technique with pentanal, decane, and tridecane as the reference compounds. Assuming an average OH concentration of 1 × 10⁶ molecules cm⁻³, an atmospheric lifetime of 15 h is calculated for cyclohexanol. Products of the OH + COL reaction were determined to more clearly define COL's atmospheric degradation mechanism. The observed products and their formation yields were: cyclohexanone (0.55 ± 0.06), hexanedial (0.32 ± 0.15), 3‐hydroxycyclohexanone (0.31 ± 0.14), and 4‐hydroxycyclohexanone (0.08 ± 0.04). Consideration of the potential reaction pathways suggests that each of these products is formed via hydrogen abstraction at a different site on the COL ring. The products and their relative amounts are discussed in light of the predicted yields for each reaction channel. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 108–117, 2001