Tachysterol, photoisomerization

An X-ray crystal structure determination of calciferol (vitamin D-2,71) showed that steric crowding in the s-cis diene system resulted in a twisted conformation with a dihedral angle of 53° [59], On irradiation with a mercury lamp, it was partially converted into ergosterol (72) and tachysterol (73) [60, 61]. When a solution of calciferol in light petroleum containing a trace of iodine was exposed to diffuse daylight, the vitamin was photoisomerized to (74) [62],... 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and coworkers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally (Ea ca 6.5 kcal mol 1) to the more stable tEc rotamer of tachysterol (tEc-T equation 58)230. The rate of this conversion is viscosity dependent. They identified this intermediate as the cEc rotamer, produced by selective excitation of the cZc rotamer of previtamin D. In a re-examination of the low temperature ,Z-photoisomerization of previtamin D as a function of excitation wavelength, Fuss and coworkers have suggested an alternative mechanism, in which tEc-1 is produced directly from cZc-P and cEc-T directly from tZc-P (equation 59)103. This mechanism involves isomerization about both the central double bond and one of its associated single bonds—the hula-twist mechanism of Liu and Browne101 — and involves a smaller volume change than the conventional mechanism for ,Z-isomerization. The vitamin D system has also been the subject of recent theoretical study by Bemardi, Robb and Olivucci and their co workers232. 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and co workers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally ( a ca 6.5 kcal rnol ) to the more stable tEc rotamer of tachysterol (lEc-T equation 58)230 jjjg this conversion is viscosity dependent. They identified this intennedi-... 

The idea that the various more or less planar conformers of a conjugated trlene—or of a polyene in general—upon n - n excitation do not interconvert during their short (singlet) excited-state lifetime arose in the early days of the study of the photoisomerization reactions in the vitamin D field (14, 63). Its introduction served the purpose of rationalizing the values of the quantum yields of isomerization of prevltamln D and tachysterol into their primary photoproducts. 

To enable photosensitized isomerization of tachysterol (6) into previtamin D (5) for the industrial preparation of vitamin D (7), use has been made of H20-soluble photosensitizers [36], easier to remove in the workup of the crude irradiation product. Noteworthy is the efficient photoisomerization of 1 into 5 in the hydrophobic microdomains of aqueous poly(sodiumsty-rene sulfonate-co-2-vinylnaphthalene) [37]. The reaction is initiated by the singlet-singlet energy transfer [Naphthalene] -r o[l] [Naphthalene] + J[l]. That 6 is practically absent is... 

The experimental data that have been presumed to be incompatible with the NEER principle derive from studies by Dauben and co-workers of the dependencies of cis-trans-photoisomerization (to tachysterol,... 

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