Large lanthanide-induced chemical shifts in cobalt complexes of enethials (α,β-unsaturated thioaldehydes)

Eu(fod)₃-, Yb(fod)₃- and Pr(fod)₃-induced chemical shifts of the ‘thioaldehydic’ protons in enethial ligands complexed to a cobalt cyclopentadienyl group are unusually large and in the same direction (10–30 ppm downfield per mole of shift reagent per mole of substrate). The shifts of the protons induced by Eu(fod)₃ and Pr(fod)₃ in the enethial ligands show an alternation in sign on proceeding away from the sulfur atom. In contrast to the results with the fod reagents, the ytterbium and lanthanum shift reagents Yb(thd)₃ and La(thd)₃ caused only small shifts of protons in the 2-phenylpropenethial ligand. No induced shifts with the Eu or Pr reagents were observed for a cyclopentadienyl cobalt complex of dithioglyoxal. The induced shifts in these enethial complexes may be caused by varying blends of complex formation, contact and pseudocontact shifts. Caution is advised in assigning origins to lanthanide induced shifts in such organometallic systems.     

Reinforced self-assembly of hexa-peri-hexabenzocoronenes by hydrogen bonds: from microscopic aggregates to macroscopic fluorescent organogels

Hexa-peri-hexabenzocoronene derivatives (HBCs) that have hydrogen-bonding functionalities (either amido or ureido groups) adjacent to the aromatic cores have been synthesized to study the effects of intracolumnar hydrogen bonds on the self-assembly behavior of HBCs. The hydrogen bonds effectively increased the aggregation tendency of these compounds in solution. In the bulk state, the typical columnar supramolecular arrangement of HBCs was either stabilized substantially (1 a, 1 b, 2 a, and 2 b), or suppressed by dominant hydrogen-bonding interactions (3). For some of the compounds (1 a, 2 a, and 2 b), the supramolecular arrangement adopted in the liquid-crystalline state was even retained after annealing, presumably owing to the reinforcement of the pi-stacking interactions by the hydrogen bonds. Additionally, the combined effect of the hydrogen bonds and pi-stacking of the aromatic moieties led to the formation of fluorescent organogels, whereby some derivatives were further investigated as novel low molecular-mass organic gelators (LMOGs).     

Multicomponent synthesis of 6H-dibenzo[b,d]pyran-6-ones and a total synthesis of cannabinol

A multicomponent domino reaction that affords 6H-dibenzo[b,d]pyran-6-ones is reported. The overall transformation consists of six reactions: Knoevenagel condensation, transesterification, enamine formation, an inverse electron demand Diels-Alder (IEDDA) reaction, 1,2-elimination, and transfer hydrogenation. Both the diene and dienophile for the key inverse electron demand Diels-Alder (IEDDA) step are generated in situ by secondary amine-mediated processes. In most cases, the yields (10-79%) are considerably better than those obtained using a stepwise process. This methodology is employed in a concise total synthesis of cannabinol.     

A study of the aldol reaction between enolates derived from the iron acetyl complex [(η-C5H5)Fe(CO)(PPh3)COCh3] and 2,3-O-lsopropylidene-D-glyceraldehyde

Diethylaluminium enolates derived from the iron acetyl complex [(η⁵-C₅H₅)Fe(CO)(PPh₃)COCH₃] undergo highly diastereoselective aldol reactions with the homochiral aldehyde, 2,3-O-isopropylidene-D-glyceraldehyde with the matched and mismatched pair reactions being readily identified. In both these reactions the observed diastereoselectivities may be rationalised in terms of the Masamune model for double asymmetric induction. Similarly the tin (II) enolates react in a predictable way, showing complementary diastereoselectivity, although effects attributed to enolate aggregation may suppress the mismatched pair reaction. However, the Masamune model cannot predict the results obtained with lithium enolates, where addition to the electrophile may occur under either chelation or non-chelation control. In the former case, both reagents reverse their selectivities as the initial two control elements are not mutually accommodating.