Proline-catalysed asymmetric ketol cyclizations: The template mechanism revisited

A modified template mechanism based on modelling studies of energy minimised complexes is presented for the asymmetric proline-catalysed cyclization of triketones1,2 and3 to the 2S,3S-ketols1a,2a and3a respectively. The template model involves a three-point contact as favoured in enzyme-substrate interactions. Our minimisation studies are in agreement with the divergent behaviour of the 6,5-, 6,6-and 6,7-bicyclic systems. They support the high 93.4%ee observed with the 6,5-bicyclic ketol and the lower 73%ee found with the 6,6-bicyclic ketol. The calculations also explain the lack of asymmetric induction with the 6,7-bicyclic system     

Nimonol and 6‐oxonimonol

The crystal structures of two intact limonoids, nimonol,7α‐­acetyl‐17α‐(3‐furyl)‐6α‐hydroxy‐4α,4β,8β‐trimethyl‐5α,18α‐androsta‐1,14‐dien‐3‐one (C₂₈H₃₆O₅), and 6‐oxonimonol,7α‐acetyl‐17α‐(3‐furyl)‐4α,4β,8β‐trimethyl‐5α,18α‐androsta‐1,14‐diene‐3,6‐dione (C₂₈H₃₄O₅), are reported. The molecular features are mostly the same in the two structures; however the orientations of the acetoxy group are different in the two structures. The packing in nimonol is due to O—H?O hydrogen bonds while in 6‐oxonimonol it is due to C—H?O hydrogen bonds.     

Samaderin B and C from Samadera indica

Samaderin B, or (1R,2S,5R,5aR,7aS,11S,11aS,11bR,14S)‐1,7,7a,11,11a,11b‐hexa­hydro‐1,11‐di­hydroxy‐8,11a,14‐tri­methyl‐2H‐5a,2,5‐(methan­oxy­metheno)­naphth­[1,2‐d]­oxepine‐4,6,10(5H)‐trione, C₁₉H₂₂O₇, and samaderin C, or (1R,2S,5R,5aR,7aS,10S,11S,11aS,11bR,14S)‐7,7a,10,11,11a,11b‐hexa­hydro‐1,10,11‐tri­hydroxy‐8,11a,14‐tri­methyl‐2H‐5a,2,5‐(methan­oxy­metheno)­naphth­[1,2‐d]­oxepine‐4,6(1H,5H)‐dione, C₁₉H₂₄O₇, were isolated from the seed kernels of Samadera indica and were shown to exhibit antifeedant activity against Spodoptera litura third‐instar larvae. The replacement of the carbonyl group in samaderin B by a hydroxy group in samaderin C causes conformational changes at the substitution site, but the overall conformation is not affected; however, the compounds pack differently in the crystal lattice.     

2′,3′‐Dehydrosalannol

The title compound, methyl (2aS,3R,5R,5aS,6S,6aS,8R,9aS,10aR,10bR,10cS)‐8‐(3‐furyl)‐2a,4,5,5a,6,6a,8,9,9a,10a,10b,10c‐dodeca­hydro‐3‐hydroxy‐2a,5a,6a,7‐tetra­methyl‐5‐(3‐methylbut‐2‐enoyl­oxy)‐2H,3H‐cyclo­penta­[4′,5′]­furo­[2′,3′:6,5]benzo[cd]­isobenzo­furan‐6‐acetate, C₃₂H₄₂O₈, was isolated from uncrushed green leaves of Azadirachta indica A. Juss (neem) and has been found to possess antifeedant activity against Spodptera litura. The conformations of the functional groups are similar to those of 3‐des­acetyl­salannin, which was isolated from neem kernels. The mol­ecules are linked into chains by intermolecular O—H?O hydrogen bonds.     

Simultaneous determination of Ru(III) and Rh(III) using octadecyldithiocarbamate by first-derivative spectrophotometry

A first-derivative spectrophotometric method is described for the simultaneous determination of Ru(III) and Rh(III) using octadecyl dithiocarbamate. The complexes are insoluble in water, but easily extractable into chloroform. Quantitative determination of Ru(III) and Rh(III) is possible in the ranges 0.5-6.0 mug ml(-1) and 1.0-10.0 mug ml(-1) respectively, with a standard deviation of +/-0.10. A statistical evaluation of the experimental results is reported.     

A new clerodane diterpene with antifeedant activity from Teucrium tomentosum

A new diterpene, viz. 3β‐acetoxy‐4α,18:15,16‐di­epoxy‐6β,12‐di­hydroxy­neocleroda‐13(16),14‐dien‐19,20‐olide, C₂₂H₂₈O₈, exhibiting antifeedant activity against Spodoptera litura was isolated from the aerial parts of Teucrium tomentosum and its structure is reported. One of the two fused rings has a distorted‐chair conformation and the other has a chair conformation. The mol­ecules in the crystal are stabilized via O—H...O and C—H...O hydrogen bonds.     

Synthesis of Novel Dispiro 1,4-Benzothiazine Hybrid Heterocycles Through 1,3-Dipolar Cycloaddition

The 1,3-dipolar cycloaddition of azomethine ylides generated in situ from the reaction of acenaphthylene-1,2-dione or isatins and α-amino acids to (E)-methyl/ethyl 2-(3-oxo-3,4-dihydro-2H-benzo[b][1,4]thiazin-2-ylidene)acetate led to the stereoselective formation of novel dispiro 1,4-benzothiazine hybrid heterocycles in good yields.     

Cyclic peptides from penicillium islandicum. A review and a reevaluation of the structure of islanditoxin

Dedicated to Professor John C. Sheehan on the occasion of his sixty-fifth birthday. The structures of cyclochlorotine and islanditoxin, cyclic peptides from Penicillium islandicum are reviewed. The peptide A isolated in these laboratories has been identified by gc/ms studies as cyclochlorotine. It is suggested that the structure of islanditoxin [cyclo-(L-dichloroprolyl-D-β-amino-β-phenylpropionyl-L-α-amino-n-butyryl-L-seryl-L-serine)] 1 should now be revised to that of cyclochlorotine, [cyclo-(L-dichloroprolyl-D-β-amino-n-butyryl-L-seryl-L-β-amino-β-phenyl-propionyl-L-serine)] 2 .