Electronic structure, stability, and cooperativity of chalcogen bonding in sulfur dioxide and hydrated sulfur dioxide clusters: a DFT study and wave functional analysis

Density functional theory calculations and wave functional analysis are used to examine the (SO2)n and (SO2)n–H2O clusters with n = 1–7. The nature of interactions is explored by molecular electrostatic potentials, electron density distribution, atoms in molecules, noncovalent interaction, and energy decomposition analysis. The putative global minimum of SO2 molecules has a 3D growth pattern with tetrahedral. In the hydrated SO2 clusters, the pure hydrogen bond isomers are less stable than the O···S chalcogen bond isomers. The cluster absorption energy of SO2 on water increases with the size of sulfur dioxide, implying reactivity of sulfur dioxide with water increases with size. The presence of cooperativity was evident from the excellent linearity plot of binding energy/polarizability vs the number of SO2 molecules. Molecular electrostatic potential analysis elucidates the reason for the facile formation of S···O chalcogen than hydrogen bonding in hydrated sulfur dioxide. Atoms in molecule analysis characterize the bonds chalcogen and H bonds to be weak and electrostatic dominant. EDA analysis shows electrostatic interaction is dominated in complexes with more intermolecular chalcogen bonding and orbital interaction for systems with intermolecular H-bonding. The reduced density gradient (RDG) analysis of sulfur dioxide clusters has blue patches and green patches due to S···O chalcogen bonding O···O electrostatic interaction. The RDG analysis of hydrated sulfur dioxide clusters shows intensive blue patches and green patches for the existence of S···O chalcogen and hydrogen bonding respectively. Thus, the presence of strong electrostatic S···O chalcogen interaction and weak H bonds acts cooperatively and stabilize the hydrated sulfur dioxide clusters.

Synthesis of a tyrosinase inhibitor by consecutive ethenolysis and cross-metathesis of crude cashew nutshell liquid

A convenient and sustainable three-step synthesis of the tyrosinase inhibitor 2-hydroxy-6-tridecylbenzoic acid was developed that starts directly from the anacardic acid component of natural cashew nutshell liquid (CNSL). Natural CNSL contains 60–70% of anacardic acid as a mixture of several double bond isomers. The anacardic acid component was converted into a uniform starting material by ethenolysis of the entire mixture and subsequent selective precipitation of 6-(ω-nonenyl)salicylic acid from cold pentane. The olefinic side chain of this intermediate was elongated by its cross-metathesis with 1-hexene using a first generation Hoveyda–Grubbs catalyst, which was reused as precatalyst in a subsequent hydrogenation step. Overall, the target compound was obtained in an overall yield of 61% based on the unsaturated anacardic acid content and 34% based on the crude CNSL.

Coordination variability of Copper(I) in multidonor heterocyclic thioamides.

The chemistry of copper(I) with scarcely studied heterocyclic thioamides, namely, 2,4,6-trimercaptotriazine, purine-6-thione, 2,4-dithiouracil, 2-thiouracil and pyrimidine-2-thione is described. The interaction of 2,4,6-trimercaptotriazine (tmtH3) with [Cu(CH3COO)(PPh3)2] gave rise to a pair of bond isomers : [Cu(κ1N-tmtH2)(PPh3)2] (6a), [Cu(κ1N,κ1S-tmtH2)(PPh3)2] (6b) and with copper(I) bromide and PPh3, it has formed a trinuclear complex, [Cu3Br2(κ1N,κ1S,κ2S-tmtH2)(PPh3)6] (7) with anionic tmtH2- in these complexes. The 2,4-dithiouracil with copper halides (CuCl, CuBr) and PPh3 yielded dinuclear complexes: [Cu2(κ2Cl)(κ1S,κ1S-dtucH)(PPh3)4] (4) and [Cu2(κ2Br)(κ1S,κ1S-dtucH)(PPh3)4] (5) with unusual eight membered metallacyclic rings. Pyrimidine-2-thione (pymSH) coordinated to CuI as N,S-chelated anion yielding mononuclear complex, [Cu(κ1N,κ1S-pymS)(PPh3)2] (1), while 2-thiouracil (tucH2) with copper(I) chloride and PPh3 yielded a tetrahedral complex, [CuCl(κ1S-tucH2)(PPh3)2] (3). Purine-6-thione (purSH2) coordinated to CuI in two different modes yielding mono- and di-nuclear complexes, [Cu(κ1N,κ1S-purS)(PPh3)2].CH3OH (2a) and [Cu2(κ1N,κ2S-purS)2(PPh3)2] (2b). The existence of bond isomers (6a and 6b), synthesis of novel dinuclear ( 4 and 5) and rare trinuclear (7) complexes with unusual bonding patterns and uncommon chelation to CuI by pymS- in 1 are the novel features of the present study. Complexes have shown intense emission bands in the visible region, λmax 490 to 495 nm.

Synthesis of complex intermediates for the study of a dehydratase from borrelidin biosynthesis

Herein, we describe the syntheses of a complex biosynthesis-intermediate analogue of the potent antitumor polyketide borrelidin and of reference molecules to determine the stereoselectivity of the dehydratase of borrelidin polyketide synthase module 3. The target molecules were obtained from a common precursor aldehyde in the form of N-acetylcysteamine (SNAc) thioesters and methyl esters in 13 to 15 steps. Key steps for the assembly of the polyketide backbone of the dehydratase substrate analogue were a Yamamoto asymmetric carbocyclisation and a Sakurai allylation as well as an anti-selective aldol reaction. Reference compounds representing the E- and Z-configured double bond isomers as potential products of the dehydratase reaction were obtained from a common precursor aldehyde by Wittig olefination and Still–Gennari olefination. The final deprotection of TBS ethers and methyl esters was performed under mildly acidic conditions followed by pig liver esterase-mediated chemoselective hydrolysis. These conditions are compatible with the presence of a coenzyme A or a SNAc thioester, suggesting that they are generally applicable to the synthesis of complex polyketide-derived thioesters suited for biosynthesis studies.