The structures, relative stabilities, and vibrational wavenumbers of the two most stable conformers of serine, stabilized by the O−Hhydrogen bonds, have been evaluated by means of state-of-the-art composite schemes based on coupled-cluster (CC) theory. The socalled "cheap" composite approach (CCSD(T)/(CBS+CV) MP2 ) allowed determination of accurate equilibrium structures and harmonic vibrational wavenumbers, also pointing out significant corrections beyond the CCSD(T)/cc-pVTZ level. These accurate results stand as a reference for benchmarking selected hybrid and double-hybrid, dispersion-corrected DFT functionals. B2PLYP-D3 and DSDPBEP86 in conjunction with a triple-ζ basis set have been confirmed as effective methodologies for structural and spectroscopic studies of medium-sized flexible biomolecules, also showing intramolecular hydrogen bonding. These best performing double-hybrid functionals have been employed to simulate IR spectra by means of vibrational perturbation theory, also considering hybrid CC/DFT schemes. The best overall agreement with experiment, with mean absolute error of 8 cm −1 , has been obtained by combining CCSD(T)/(CBS +CV) MP2 harmonic wavenumbers with B2PLYP-D3/maug-cc-pVTZ anharmonic corrections. Finally, a composite scheme entirely based on CCSD(T) calculations (CCSD(T)/CBS+CV) has been employed for energetics, further confirming that serine II is the most stable conformer, also when zero-point vibrational energy corrections are included.
In this work, a new chemodivergent domino approach for the preparation of various saturated heterocycles, based on phase‐transfer catalysis (PTC), is presented. The versatile nature of doubly electrophilic substrates, showing both a Michael acceptor and a ketone, tethered by a heteroatom, enables three different domino reaction pathways. The nucleophile dictates the chemoselectivity of the reaction. Sulfa‐Michael/aldol, cyanide addition/oxa‐Michael and Michael/H‐shift/aldol processes, along with the variation of the tethering heteroatom, results in the formation of six different classes of saturated heterocycles. DFT calculations account for the observed chemo‐ and diastereoselectivity of the two most productive processes. Moreover, an extensive investigation on the sulfa‐Michael/aldol pathway was carried out, ultimately leading to the development of a new enantioselective domino approach to multi‐substituted piperidines based on PTC.
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