Assignment of absolute configuration of sulfinyl dilactones: Optical rotations and ¹H NMR experiment and DFT calculations

Abstract

In previous work, two sulfinyl dilactone compounds were synthesized as key intermediates in the development of new proteasome inhibitors. Two stereogenic centers were introduced during synthesis, including one chiral sulfoxide in a five-membered ring. The absolute configurations (AC) of these two compounds were hard to determine just based on experimental techniques. Therefore, theoretical calculations of chiro-optical and spectroscopic features were used to determine stereochemistry. Ab initio calculations at DFT/B3LYP/6-31G (d, p) level of theory obtained satisfactory agreement with the experimental results. Furthermore, there was good agreement between the calculations of optical rotations and hydrogen chemical shifts in ¹H NMR spectra for the final AC assignment as (1′S, 2R, 5R)- and (1′S, 2S, 5S)-5-[1′-(t-Boc-amino)-3′-methylbutyl]-1,3,2-dioxathiolane-2-oxide. This is a successful application of ab initio calculations for the AC assignment for small flexible molecules.

Introduction

The proteasome functionality is indispensable in cellular regulation and protein degradation of eukaryotic cells, and intensive investigations have been focused on the development of new compounds which could interact with the catalytic threonine to inhibit proteasomal proteolysis [1], [2]. Because of its sensitivity to nucleophilic attack, the sulfinyl dilactone functional group was rationally designed as the C-terminus of a potential proteasome inhibitor which could form an irreversible covalent adduct with the catalytic threonine to terminate proteasome function. To obtain a proteasome inhibitor containing a sulfinyl dilactone moiety, we first synthesized a protected aminoalkyl sulfinyl dilactone as the key intermediate [3]. Scheme 1 outlines the general synthetic route. During synthesis, two stereogenic centers were introduced in addition to the S carbon (C_α) already present in the starting material. A stereocarbon was introduced by dihydroxylation of olefin 5 to form diol 6, and a stereo sulfur was introduced by insertion of a sulfoxide moiety.

In a sulfoxide, the oxygen contributes unpaired p electrons to a d orbital of the sulfur, called d_π–p_π bonding (or d–π) bonding [4]. Two resonance structures can be used to describe the sulfoxide bond: R¹(R²)SO ↔ R¹(R²)S⁺O⁻. For sulfinyl dilactones with a sulfoxide in a five-membered ring, the pyramidal inversion barrier has been calculated to be >130 kcal mol⁻¹ [5], [6], [7], [8], and therefore stable, separable stereoisomers should be obtained under ambient conditions. N-protected aminoalkyl sulfinyl dilactones containing two undetermined asymmetric centers could form four possible diastereomers. To eliminate confusion, the S or R stereochemical descriptors can be labeled using subscript C for carbon or S for sulfur, so that the four diastereomers can be represented as S_CR_CR_S, S_CR_CS_S, S_CS_CR_S or S_CS_CS_S (see Fig. 1), with the middle descriptor referring to the ring carbon, and the initial descriptor referring to the acyclic carbon, which is always S based on the starting materials and synthetic route.

Experimentally, we obtained two stereometrically purified sulfinyl dilactones, compounds 1 and 2, from HPLC separation [3]. Our goal in this paper was to determine the absolute configuration (AC) of these two compounds. There were two difficulties: first, the needle-like crystals of the sulfinyl dilactones were too slim to be used for X-ray diffraction; second, there were no literature precedents to be used as reference for assigning the AC for a pyramidal sulfoxide in a five-membered ring. Therefore, ab initio calculations of chiro-optical and spectroscopic features were used to assist in the AC assignment. Density functional theory (DFT) has been extensively exploited to calculate chiro-optical properties such as optical rotation (OR) [9], [10], [11], [12], [13] and circular dichroism (CD) [14], and increasingly, consistency has been obtained between theoretical and experimental results [10]. So we used DFT to calculate OR at the sodium D line wavelength and then compared with experimental results. A medium-sized basis set was used to balance accuracy and computational cost. Before the ab initio calculations, the conformational space was investigated for these flexible diastereomers, in order to be able to include as many as possible of the stable conformations which contribute to the overall chiro-optical and spectroscopic features.

For spectroscopic features, the nuclear magnetic shielding constant at or close to the center of an anisotropic group can be employed to determine quantitatively the stereochemistry of potentially proximal nuclei [15]. A number of such applications have been published [16], [17], [18]. In the sulfinyl dilactone molecule, the introduction of the sulfoxide and amide groups into the molecular framework can cause anisotropy on nearby protons [19], and such anisotropic effects can be seen through comparison of the ¹H NMR spectra of compounds 1 and 2. Therefore, we further calculated the proton chemical shifts of the four diastereomers and compared them with the experimental ¹H NMR spectra to help specify the absolute configurations of 1 and 2.