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

Abstract

Allene functional group in natural products isolated so far exists as a non-racemic form, but its axial chirality is difficult to elucidate. Allenes exhibit a characteristic antisymmetric C=C=C stretching mode...

Vibrational Circular Dichroism Studies on Axially Chiral Carbodiimides and Allenes

“…Although a detailed understanding of the VCD properties of allenes requires more studies on other types of allenes (e.g., tri-and tetrasubstituted allenes), our findings should facilitate studies on chiral allenes. 8…”

“…7 Our preliminary work on some chiral allenes found a characteristic VCD signal originating from an antisymmetric C=C=C stretching vibration at around 1950 cm –1 , as discussed in Section 3. 8 With these promising preliminary results on allenes in hand, we began to wonder whether the axial chirality of other X=Y=Z systems could be studied by VCD spectroscopy. In particular, we were interested in carbodiimides (N=C=N), as these are important in life science as coupling reagents for the formation of amide and ester bonds and in materials science as monomers for the synthesis of polycarbodiimides.…”

Vibrational circular dichroism spectroscopy in the C–D, XY, and XYZ stretching region

“…Consequently, VCD spectra in this region are not commonly reported even when measured molecules contain C–D, XY, or XYZ chromophores. For example, among 6 papers on VCD measurements on chiral allenes, 8 only 3 of them 8 a , e , f reported their CCC antisymmetric stretching VCD signal at around 1950 cm −1 . Note that low IR intensity does not necessarily lead to weak VCD signal.…”

Highlighting the Rich Chemistry of the Allenone Moiety

“…Among the family of electronically tuned allenes, allenones, constituted by an allene moiety and a ketone, have appeared as recurring motifs in organic synthesis, natural products and mechanistic investiga-tions, showing unique properties and synthetic applications. [10][11][12][13][14][15][16][17][18][19][20][21][22] During the last two decades, the chemistry of allenones has resulted in an increasing number of contributions describing both their synthesis and applications. Regarding reactivity, these systems can operate as Michael acceptors in nucleophilic additions, cyclization counterparts in cycloaddition reactions, or five-membered ring precursors through cycloisomerizations.…”