On the origin of quasi-racemic aplysinopsin cycloadducts, (bis)indole alkaloids isolated from scleractinian corals of the family Dendrophylliidae. Involvement of enantiodefective Diels-Alderases or asymmetric induction in artifact processes involving adventitious catalysts?

Reported here are two novel quasi-racemic (bis)indole alkaloids, cycloaplysinopsin A (5) and cycloaplysinopsin B (6), isolated from tropical Indo-Pacific (Comoros, Philippines) scleractinian corals of the family Dendrophylliidae. Although their structures suggest a Diels-Alder cycloaddition origin from aplysinopsin-type precursors, neither experiments, nor theory allowed us to clearly distinguish an enzymatic process with scarce enantioselection from the intrusion of an adventitious catalyst in the coral extracts, where the chiral environment caused a slight asymmetric induction.     

Activity coefficients of electrolytes from the E.M.F. of liquid membrane cells. II - multicharged electrolyte solutions

The activity coefficients of Co(en)₃Cl₃ and K₂SO₄ were measured by means of a cell with ion-exchange liquid membranes following the method described in paper I. The results prove that this method is even more valuable with multicharged salts than with 1-1 electrolytes. The values obtained are precise and reliable down to dilution limits never before accessible, e.g., 4×10^–5 mol-kg^–1 in Co(en)₃Cl₃. High dilution levels are of particular importance when dealing with highly charged electrolytes since the trend at higher concentrations often leads to errors both in extrapolation to infinite dilution and in the absolute activity coefficients. As an application, the activity coefficients of [Co(en)₃]₂(SO₄)₃-suspected to be wrongly evaluated in past literature-were measured, and their values at low concentrations were actually lower than those quoted before.     

From Epiraikovenal,an Instrumental Niche-Exploitation Sesquiterpenoid of Some Strains of the Marine Ciliated Protist euplotes raikovi,to an unusual intramolecular tele-dienone-olefin [2+2] photocycloaddition

It is shown that strains of the marine ciliate Euplotes raikovi are subtly variable in their production of secondary metabolites. Strains GA8 and 39W from Mediterranean and SB8 from Californian coasts produce the sesquiterpenoid epiraikovenal ( 3 ), while strains GA8 and SB8 also produce secoepiraikovenal ( 4 ), which play an instrumental niche-exploitation role and have also taxonomic significance. Comparison of 3 and 4 with raikovenal ( 2 ) and its putative biogenetic precursor 1 , which have similar roles in the conspecific strain Morl from Casablanca coast in the Atlantic Ocean, inspired us the first case of intramolecular tele-dienone-olefin [2+2] photocycloaddition, exemplified here by the transformation of 1 into ent- 3 . This served also to unequivocally clarify the stereochemical relationship between 3 and 2 .     

Xenicane Diterpenes Revisited: Thermal (E)→(Z) isomerization and conformational motions. A unifying picture

Kinetic and equilibrium studies show that typical xenicanes such as dictyolactone ( 1 ) and 4-hydroxydictyolactone ( 3 ) undergo slow conformation medium-ring flipping between the predominant trans-( 1a or 3a ; Me(20) trans to H–C(3)) and the minor cis-conformers ( 1b or 3b ; Me(20) cis to H–C(3); see Scheme 1). The formation of the latter is inhibited in heterocyclic-ring-opened congeners such as 18-acetoxy-4-hydroxydictyo-19-al ( 7 ). Molecular-mechanics calculations suggest that typical-xenicane cis-conformers are disfavoured by mainly C(4)–C(5) torsional strain. This is confirmed by the observation of two sizably populated cis-and trans-conformers for the unnatural 4-oxoxenicanes 10–12 . Unusually facile thermal (E)→(Z) isomerization of xenicanes 1,3,10–12 , and 7 is also observed (→ 13–17 and 9 , resp.; Scheme 3), reflecting great strain relief in the transition state. Conflicting results in the literature now fit into this scheme which provides a basis for unravelling recognition phenomena with these biologically active systems.     

Bretonin A and Isobretonin A,Unique Glycerol Derivatives Isolated from a Demosponge of Brittany Waters

It is shown that an unidentified marine demosponge of Brittany contains two unique lipids consisting of glycerol etherified by a C₁₂ trienic linear alcohol and esterified by 4-hydroxybenzoic acid. The latter is attached to the secondary position of glycerol in bretonin A (= 3-[((4E, 6E, 8E)-dodeca-4,6,8-trienyl)oxy]-2-(4-hydroxy-benzoyl)propan-1-ol; 1a ), and to the other primary position of glycerol in isobretonin A ( = (+)-3-[((4E, 6E, 8E)-dodeca-4,6,8-trienyl)oxy]-1-(4-hydroxybenzoyl)propan-2-ol; (+) -2 ). The structures are based on NMR and MS data, including the ones of the acetylation product (?)- 1b of 1a .     

(η3‐Allyl‐2κ3C)(chloro‐1κCl)(μ‐N,N′‐diethyldithioxamidato‐1:2κ4S,S′:N,N′)[diphenyl(2‐pyridyl)phosphine‐1κP]palladium(II)platinum(II) chloroform solvate

The title compound, [PdPtCl(C₃H₅)(C₆H₁₀N₂S₂)(C₁₇H₁₄NP)]·CHCl₃, was obtained by deprotonation of the initial platinum(II) complex of the di­thio­xamide and subsequent reaction with [Pd(η³‐C₃H₅)(μ‐Cl)]₂. Both metal atoms exhibit a square‐planar coordination geometry, with the two planes forming a dihedral angle of 21.7 (2)°. The di­thio­xamide bis‐chelating bridge is flat.     

Surface modification of proteins by covalent binding of acrylic polymers

Surface modification of enzymes for a potential use in therapy was obtained with a new type of tailor-made copolymers ofNacryloylmorpholine andN-acryloxysuccinimide. The first monomer was designed to confer solubility on the polymer, whereas the second was used to give it reactivity toward protein amino groups. The reactivity of polymers of different composition towards amino acid derivatives and model proteins, such as catalase and ribonuclease-A, is described. Water soluble and catalytically active enzyme derivatives were obained using copolymers prepared with a mixture of N-acryloxysuccinimide andn-acryloylmorpholine in a 1:99 molar ratio. At increasing molar ratio (3:97, 10:90) extensive crosslinking between polymer and enzymes takes place, yielding insoluble adducts.     

Quantum mechanical calculations of conformationally relevant 1H and 13C NMR chemical shifts of calixarene systems

[graphs: see text] QM GIAO calculations of 13C and 1H chemical shift values of the ArCH2Ar group have been performed, using the hybrid DFT functional MPW1PW91 and the 6-31G(d,p) basis set, on some representative calixarenes and on a series of simplified calixarene models allowing derivation of chemical shift surfaces versus phi and chi dihedral angles. A good reproduction of experimental data was obtained. The applicability of chemical shift surfaces in the study of calixarene conformational features is illustrated.