Lumisterol to Tachysterol Photoisomerization in EPA Glass at 77 K. A Comparative Study

Abstract

We present a comparative study of the photoisomerizations of lumisterol (Lumi), previtamin (Pre), and provitamin D (Pro) to tachysterol (Tachy) at 77 K in EPA (5:5:2 ether, isopentane, and ethanol by volume) glass. Fluorescence, fluorescence excitation, and UV spectra, measured in the course of these reactions, were analyzed using singular value decomposition with self-modeling (SVD-SM). This represents an extension of previous work that led to the conclusion that in the EPA glass Pre exists as an s-cis,s-cis-conformer (cZc-Pre) which gives, exclusively, an unstable s-cis,s-cis-conformer of Tachy (cEc-Tachy) and Pro gives mainly the tEc-Tachy, that corresponds to a stable s-trans,s-cis-conformer. ( Redwood , C. ; et al. J. Phys. Chem. Lett. 2013 , 4 , 716 - 721 . ) The surprising result was that the major Pre photoproduct from Pro also has a tZc-Pre conformation instead of the expected cZc-Pre conformation. Accordingly, the Pre to Tachy cis-trans photoisomerization proceeds via a conformer specific one-bond-twist (OBT) process as proposed by Havinga ( Maessen , P. A. ; et al. Angew. Chem. Int. Ed. Engl. 1983 , 22 , 718 - 719 . Maessen , P. A. ; et al. Angew. Chem. Int. Ed. Engl. 1983 , 22 , 994 - 1004 . Maessen , P. A. Ph.D. Thesis, State University at Leiden, Leiden, The Netherlands, 1983. ). The role of the EPA glass in controlling conformer distributions and reaction outcomes is further explored by the extension of the studies to Lumi, whose structure differs substantially from that of its stereoisomer, Pro. Initially, the light-induced conrotatory ring openings of Pro and Lumi are expected to give cZc-Pre conformers that differ in the relative orientation of the double bond dihedral angles that define the chiral axis of the triene moiety: (-)cZ(-)c-Pre and (+)cZ(+)c-Pre, respectively. In the case of Pro, much of the cZc-Pre proceeds to tZc-Pre, the precursor of tEc-Tachy. In contrast, we show that under the same conditions most cZc-Pre formed from Lumi retains the cZc-conformation and isomerizes to cEc-Tachy. cZc-Pre from Lumi was not detected by fluorescence, but UV absorption measurements establish its formation as an essential intermediate to Tachy. Aided by theoretical calculations of conformer UV and CD spectra, we conclude that fluorescent thermodynamic Pre and nonfluorescent Pre from Lumi are both (+)cZ(+)c-Pre conformers. They differ in the orientation of the OH in the A ring, pseudoequatorial in the former and pseudoaxial in the latter. The most likely major photochemical sequences starting from Pre and Lumi are (+)cZ(+)c-Pre-eq-OH → (+)cE(+)c-Tachy-eq-OH and Lumi → (+)cZ(+)c-Pre-ax-OH → (+)cE(+)c-Tachy-eq-OH.

Previtamin D:Z–Ephotoisomerizationviaa Hula-twist conical intersection

“…8 are more consistent with the CI mechanism and also those in ref. 9 seem compatible with it. Also further related questions will be considered such as the NEER principle and conformer control, volume constraints and reactions in the hot ground state.…”

“…[13][14][15] After the in situ preparation of cZc-Pre (according to the assumption) that photoisomerized to tEc-Tachy (via HT), the Garching group 1 reassigned thermal Pre to tZc-Pre (because a different product requires a different reactant), consistent with the sign of the circular dichroism [13][14][15] and the assignment of Dauben,16 so that the reaction was tZc-Pre -cEc-Tachy (hence also HT) in the Leiden experiment. This assignment will be maintained here, although the Florida and Leiden groups 8,9,[13][14][15] argued in favor of ''thermal Pre = cZc-Pre''.…”

“…Furthermore, the OH group of the cyclohexene A ring can be equatorial or axial, which also has some influence on the torsions in the chromophores. By various quantum chemical methods Dmitrenko and coworkers [9][10][11][12] calculated for these conformers of Pre and Tachy the relative energies and conformer equilibria at room temperature, structures (in particular twist angles), UV and CD spectra, mainly in the gas phase. The energy differences are small, so that solvent interactions may be more important to control which conformer dominates.…”

“…In the matrix experiments in Garching and Florida 1,8,9 the reactant (Pre) was produced in situ by photochemical opening of the cyclohexadiene B ring of Dehy 1,8 or Lumi. 9 In ref. 1 it was assumed that the resulting conformer has the cZc structure, which has still a ring-like form, and Z-E isomerization was found to produce the tEc conformer of Tachy (see the structures in Scheme 1); from cZc to tEc means rotation of two bonds, hence HT.…”

“…8 In 2017 they investigated in the same way the photoisomerization of lumisterol (Lumi) to Pre and Tachy. 9 (In Lumi and Dehy the diene chromophores form a right-handed and left-handed screw, respectively. The same difference must be assumed for the -nonplanar -triene chromophores of the two resulting cZc-Pre-s.) They found from Lumi only one Pre conformer (cZc) but two (or three) Tachy conformers, whose identification seems to be less clear-cut than on starting from Dehy.…”