Douglas J. Brecknell

New Sesquiterpene and Brominated Metabolites from the Tropical Marine Sponge Dysidea sp.

2D NMR spectroscopic data are reported for 6-hydroxyfurodysinin-O-methyl lactone (3), 2-(2′,4′-dibromophenoxy)-4,6-dibromoanisole (4), and dehydroherbadysidolide (8), all isolated for the first time from Dysidea sp. Revised NMR assignments are presented for the compounds dysetherin (1) and furodysinin-O-methyl lactone (2), previously reported from Dysidea herbacea. The full relative stereochemistry of spirodysin (12) is defined for the first time.

Synthesis and absolute stereochemistry of hagen's-gland lactones in some parasitic wasps (Hymenoptera:Braconidae)

Abstract Efficient syntheses and enantioselective gas chromatography have confirmed the structures and established the absolute stereochemistry of some novel bicyclic lactones (tetrahydrofurofuranones) in species of parasitic wasps (Hymenoptera:Braconidae). The co-occurring γ-lactones, octan-4-olide and dodecan-4-olide, have the ( R )-configuration.

New Non-Contiguous Polypropionates from Marine Molluscs: A Comment on their Natural Product Status

The structure of the rearranged polypropionate ester siserrone A (8), isolated from Siphonaria serrata, was investigated by standard spectroscopic methods and the relative stereochemistry determined by ROESY spectroscopy and chemical degradation studies. Base-catalysed rearrangement of denticulatin A (13) yields the polypropionate ester (16) while careful work-up of S. baconi yields the siphonarins, but not the baconipyrones, as earlier reported. The natural product status of polypropionate ester metabolites is discussed.

Structures of lanthanide shift reagent complexes by molecular mechanics computations

Abstract Molecular mechanics calculations have been applied to the structure determination of 7-coordinate lanthanide complexes. To circumvent problems in defining oxygen—lanthanide—oxygen bond angles, the energy of angle deformations at the metal center are not evaluated explicitly. Instead the standard approach to molecular mechanics calculations is modified by including 1,3-nonbonded interactions between atoms that are both bonded to the metal center. Geometry optimization for two known lanthanide complexes afforded structures that are in reasonable agreement with X-ray crystal structures, and small discrepancies are attributed to cyrstal packing forces.

Some simple cyanoformaldehyde phenylhydrazones

The triazole ring of 2-phenyl-1,2,3-triazole-4-carboxylic acid is cleaved by alkaline treatment. The synthesis and stereochemistry of some simple cyano-formaldehyde phenylhydrazones is discussed.

Sylvestrene nitrosochloride and derived amino oximes

Configurational and conformational effects in amino oximes derived from sylvestrene (m-mentha-6, 8-diene) nitrosochloride are discussed in terms of o.r.d. and H n.m.r. observations.

Tri-O-benzylidene-D-glucitol

The known tri-O-benzylidene derivative of D-glucitol has structure (la) while a new diastereomer has structure (lb).

Benzylidene derivatives of mannitol

Further work is reported on the known 1, 3(R):4, 6(R)-di-O- and 1, 3(R):2, 5:4, 6(R)-tri-O-benzylidene derivatives of D-mannitol. New 1, 3(R)-mono-O-, 1, 2(R):4, 6(R)-di-O-, 1, 2(S):4, 6(R)-di-O-, 1, 2(R):5, 6(R)-di-O-, 1, 2(S):5, 6(S)-di-O-, 1, 2(S):5, 6(R)-di-O-and 1, 3(R):2, 5(S)-di-O-benzylidene derivatives are reported.

HALOGENATED TERPENOIDS. XXIX. THE 1-BROMO 1-BROMOMETHYL CYCLOHEXYL SYSTEM

Bromination of methylene groups exocyclic to cyclohexyl systems normally affords two isomeric products; the axial 1-bromo equatorial 1-bromomethyl compound and the axial 1-bromomethyl equatorial 1-bromo derivative. Free energy differences between these two isomers, and the conformations adopted by the axial 1-bromomethyl group, have been explored by n.m.r. spectroscopy, by X-ray crystallography and by MM3 calculations. Evidence is presented to show that the ax-bromomethyl group exists primarily as those rotamers which site the bromine atom synclinal to the vicinal bromine. The A value for a bromomethyl group in this system is then similar to that of an unsubstituted methyl group.

1,2-Distannylation and -disilylation of cyclic allenes

Cyclo-1,2-dienes(C9–C13) are converted into the synthetically promissing 2,3-bis(trimethylsilyl)- and 2,3-bis(trimethylstannyl)cycloalk-1-enes by disilylation with lithium chlorotrimethylsilane in tetrahydrofuran, and Pd0-catalysed addition of hexaalkyldistannes, respectively.