Assignment of Absolute Configuration of Bromoallenes by Vacuum-Ultraviolet Circular Dichroism (VUVCD)

Abstract

A new application of vacuum-ultraviolet circular dichroism (VUVCD), which enables the measurement of CD spectra in the vacuum-ultraviolet region (140–200 nm), for the assignment of the absolute configurations of bromoallenes is described. Bromoallene moieties are found in natural products obtained from many marine organisms. To date, the absolute configuration of bromoallenes has been assigned almost exclusively with Lowe’s rule, which is based on specific rotation. However, exceptions to Lowe’s rule have been reported arising from the presence of other substituents with large specific rotations. For the unambiguous assignment of the absolute configuration of the bromoallene moiety with its characteristic absorption wavelength at 180–190 nm due to the π–π* transition, VUVCD was applied to four pairs of bromoallene diastereomers prepared by modifying the synthetic scheme of omaezallene. The VUVCD spectra clearly showed positive or negative Cotton effects around 180–190 nm according to the configuration of the bromoallene employed, revealing the potential of VUVCD for determining absolute stereochemistry.

Enantioselective Total Synthesis of Multifidene, a Sex Pheromone of Brown Algae

“…The resultant labile aldehyde 14 was transformed into triolefin 6 in 48% yield over two steps from 13. The ring-closing metathesis reaction with the Grubbs 1st generation catalyst [35] proceeded regioselectively to produce the desired cyclopentene 15 after the removal of the TES group under mild acidic conditions [36]. We rationalized this regioselectivity by kinetic control.…”

Scope and limitations of absolute configuration determination of allenic natural products using the CCC stretching VCD signal

“…The sign of the VCD signal may be changed in the presence of other asymmetric centers. Thus, care must be taken in the interpretation of the VCD spectrum of allenic molecules with multiple asymmetric centers, and for such molecules, the use of multiple analytical methods such as ECD spectroscopy 4 is recommended. The studied set of allenes is not comprehensive and mostly focused on 1,3-disubstituted allenes partially because of difficulties in the preparation of allenic molecules.…”

“…In fact, exceptions to this rule are reported by some of the authors. 3 Electronic circular dichroism (ECD) signals originating from allene π–π* transition (usually observed in a region shorter than 250 nm) have been shown to be indicative of its AC, 4 but the ECD spectrum of allenes with UV-active chiral substituents has to be interpreted with great care. Although chemical correlation 5 and total synthesis 6 have elucidated the AC of chiral allenes, other methods have to be explored.…”