Stewart McLean

Total assignment of 13C and 1H spectra of three isomeric triterpenol derivatives by 2D NMR: an investigation of the potential utility of 1H chemical shifts in structural investigations of complex natural products

It is shown that a recently proposed indirect heteronuclear shift-correlated pulse sequence, XCORFE, can be used to unambiguously assign the 13C spectra of three isomeric (C30H50O) triterpenols: taraxasterol (1a), pseudo-taraxasterol (2) and lupeol (3). This sequence gives excellent resolution combined with sensitivity far in excess of that given by 13C-13C connectivity experiments. Direct heteronuclear shift-correlated spectra are used to totally assign 1H spectra for 1a, 2, 3 and taraxasteryl acetate (1b). 1H chemical shifts are mainly sensitive to local environment and often show values which are characteristic of a particular environment. Knowledge of 1H chemical shifts and splitting patterns also places useful constraints on assignment of 13C chemical shifts for 13C1Hn units. It is strongly recommended that natural products chemists routinely use 2D NMR to assign 1H chemical shifts of complex organic derivatives in order to build up a data bank of 1H spectral data.

Hydrogen migration in cyclopentadienes

Abstract A kinetic study of the rearrangement of 5-methylcyclopentadiene to 1-methylcyclopentadiene has been made, using NMR to monitor the reaction. In the absence of a strong base, the rearrangement took place through an intramolecular 1,2 hydrogen shift and showed the characteristics of a homogeneous reaction following the first order rate law. The activation energy and entropy were determined from the temperature dependence of the rate. Base did not catalyze this process, but, in the presence of a sufficiently strong base, an alternative process involving the intermediacy of the methylcyclopentadienide anion became an important route for isomerization. A study of the rearrangement of 1,5-dimethylcyclopentadiene was also made. Results from these experiments and other more approximate data serve as a basis for a discussion of the mechanism of the rearrangement.

Substitution in the cyclopentadienide anion series: Methylation of the cyclopentadienide and methylcyclopentadienide anions

Abstract It is shown that the cyclopentadienide anion can undergo an electrophilic substitution. In methylation reactions it was possible to stop the reaction at the intermediate 5-substituted cyclopentadiene stage. In the methylcyclopentadienide anion, the methyl group directs an electrophile to the adjacent carbon atom.

PRENYLATED BENZOPHENONE DERIVATIVES FROM CARIBBEAN CLUSIA SPECIES (GUTTIFERAE). PLUKENETIONES B-G AND XEROPHENONE A

Abstract Six new prenylated benzophenone derivatives plukenetiones B-G (2–7) have been isolated from the fruits of the Barbadian plant Clusia plukenetii. These structures were elucidated by the use of 2D NMR spectroscopic methods. The regiochemistry of xerophenone A from Clusia portlandiana has been revised.

Comparison of13C Resolution and Sensitivity of HSQC and HMQC Sequences and Application of HSQC-Based Sequences to the Total1H and13C Spectral Assignment of Clionasterol

It is demonstrated that HSQC gives considerably better 13C resolution and sensitivity than HMQC for CH2 groups of natural products, particularly when combined with linear prediction. Similarly, coupled HSQC spectra provide a useful method for the determination of 1H multiplet structure and consequent assignment of individual CH2 protons as axial or equatorial in fused cyclohexane rings. These and related techniques are used to assign 1H and 13C spectra of the marine sterol clionasterol.

Plukenetione A. An unusual adamantyl ketone from Clusia plukenetii (guttiferae)

Abstract The structure of plukenetione A from Clusia plukenetii Urban has been shown by spectroscopic methods to be 1-benzoyl-8,8-dimethyl-3,5-bis(3-methyl-2-butenyl)-6-(2-methylpropenyl)tricyclo[3.3.1.1 3,7 ]decane-2,4,9-trione.

GLABRESCOL. A UNIQUE SQUALENE-DERIVED PENTA-THF DIOL FROM SPATHELIA GLABRESCENS (RUTACEAE)

Abstract The structure of glabrcscol from Spathelia glabrescens Planch has been shown by spcctroscopic methods to be a meso form of 2,23-dihydroxy-2,6,10,15, 19,23-hexamethyl-3,7,11,15,19-penta(oxacyclopentyl)tetracosane.

A prenylated benzopyran derivative from peperomia clusiifolia

The structure of a new compound, clusifoliol, isolated from whole plants of Peperomia clusiifolia has been established as 3,4-dihydro-2,7-dimethyl-6-(3-methyl-2-butenyl)-2-(4-methyl-1,3-pentadienyl)-2H-1-benzopyran-5-ol by spectroscopic methods.

GEODIAMOLIDES H AND I, FURTHER CYCLODEPSIPEPTIDES FROM THE MARINE SPONGE GEODIA SP.

Abstract Two new cyclodepsipeptides, geodiamolide H (3) and I (4), were isolated from the marine sponge Geodia sp. The structures of 3 and 4 were determined by 2D NMR spectroscopy while the absolute stereochemistry of 4 was established by X-ray crystallography. Geodiamolide H 3 was cytotoxic to some human cancer cell lines.

Cassane diterpenes from Caesalpinia bonducella

Abstract Two new cassane diterpenes, named caesaldekarins F and G, were isolated and identified from the roots of Caesalpina boducella . The recently reported caesaldekarin C was also isolated from the roots of this plant. The structures of caesaldekarins F and G were established by the use of 2D NMR spectroscopy.