Complete assignment of the ¹H and ¹³C NMR spectra and conformational analysis of bonellin dimethyl ester

Abstract

The use of a high-field NMR instrument (ν( 1 H) = 500 MHz) and 2-dimensional NMR techniques (HMQC, HMBC, ROESY) enabled us to fully assign the 1 H and 13 C chemical shifts of bonellin dimethyl ester. The β-pyrrolic proton of C-3 appeared as a broad singlet at δ = 8.93, whereas that of C-8 gave a quartet with δ = 8.69 and 4 J H-H = |1.28| Hz. The C-2 1 methyl protons appeared as a doublet with δ = 3.55 and 4 J H-H = |1.07| Hz, while the C-7 1 methyl protons afforded a doublet with δ = 3.51 and 4 J H-H = |1.28| Hz. These results suggest that the β-pyrrolic carbons of ring A belong to the aromatic 18 π-electron [18]diazaannulene delocalization pathway, whereas those of ring B remain outside the aromatic pathway. The broadening of the C-3 β-pyrrolic proton signal can be attributed to the allylic 3-CH -2 1 -CH 3 coupling and the 3-CH -21-NH coupling. At 330 K, the tautomeric exchange 21-NH a 23-NH b is fast and only one broad signal at δ = -2.49 is seen for these protons. The ROESY spectrum showed clear correlation signals between the 18 2 -CH 3 and 17 1 -CH 2 protons, the 18 2 -CH 3 and 17 4 -CH 3 protons, as well as between the 18 1 -CH 3 and 17-CH protons. These results are compatible with the previous assignment that the absolute configuration at C-17 is S. Application of spin simulation enabled us to determine the chemical shifts and the 3 J H-H coupling constants of the 17-propionate side-chain. The 3 J H-H -values were used to calculate the populations for the 17 1 -17 and 17 2 -17 1 rotamers. A relatively high population value of 0.41 was found for the 17 1 -17 g --rotamer, whose methoxycarbonylmethyl group points to the C-15 methine-bridge. This was interpreted as explaining the high tendency of bonellin to form anhydrobonellin. The rotational freedoms in the 13-propionate side-chain were studied by measuring the 1 H NMR spectra of the side-chain at temperatures between 300 and 195 K. At 300 K, the 13 1 -and 13 2 -CH 2 proton signals appeared as deceptively simple triplets, which at 195 K were split into complex multiplets. At 195 K, the signal arising from the 13 1 -CH 2 protons exhibited more splitting, which indicates that these protons have less rotational freedom than the 13 2 -CH 2 protons.

Amphiphilic chlorins and bacteriochlorins in micellar environments. Molecular design, de novo synthesis, and photophysical properties

“…73 A benzoic acid group at the 15-position was introduced by Suzuki coupling. Thus, treatment of C-T 5 Bacteriochlorins. To obtain tetrapyrroles with strong absorption in the 700-800 nm region, we chose one set of bacteriochlorins (Chart 4).…”

“…The S 1 excited-state decay characteristics of the amphiphilic chlorins and bacteriochlorins in DMF are generally similar to those of synthetic lipophilic chlorin [80][81][82] and bacteriochlorin 83 analogues studied previously in organic solvents (typically toluene). For example, the values of s S $ 4.6 ns, F f $ 0.16 and F isc $ 0.4 for bacteriochlorin BC-BA 5 are typical of synthetic free base bacteriochlorins that have the Q y absorption band in the 720-780 nm region, which denes the energy of the S 1 excited state. 83 Analogue BC-MePy 15 free base analogues can be traced largely to the heavy atom effect on intersystem crossing rate constant k isc .…”

“…The resulting suspension was centrifuged. The supernatant was removed leaving the title compound as a green solid (25 2,3,4,5-Tetrahydro-1,3,3-trimethyl-8-(phenylethynyl)dipyrrin (5). Following a procedure for detosylation of a Western half, 74 a sample of 4 (1.55 g, 3.48 mmol) was treated with TBAF (10.4 mL, 1.0 M in THF, 10.4 mmol, 3 equivalents) and the reaction mixture was stirred for 1 h at reux.…”

“…Bonellin, a naturally occurring chlorin found in a marine worm, is amphiphilic but also far less accessible than protoporphyrin IX. 4,5 Surfactant assemblies of micelles and bilayer membranes in aqueous solution are characterized by a profound change in polarity in moving from the aqueous medium to the core of the assembly; the static dielectric constant changes from $80 in bulk water to $2 to 4 deep in the hydrophobic milieu. 6 Accordingly, an amphiphilic molecule such as protoporphyrin IX is predisposed to organize with the polar, carboxylic acids (or carboxylates) at the aqueous-surfactant interface, and the remainder of the molecule projects into the core of the hydrophobic assembly.…”