The photochemical goal in the production of vitamin Ds (Vit Ds) is to maximize the conversion of the provitamins (Pros) to the previtamins (Pres) while minimizing stoichiometric losses to undesirable over-irradiation products. The last step in the syntheses, the [1,7]-sigmatropic rearrangement of the Pres to the Vits, is thermally induced. The competition between cis-trans photoisomerization and photocyclization of the Pres is known to be highly excitation-wavelength specific. It inspired Havinga's nonequilibration of excited rotamers (NEER) principle and more recent alternative explanations. In contrast, the photochemistry of tachysterol has been reported to be relatively unexceptional with Tachy --> Pre quantum yields in the 0.10-0.13 range, independent of lambdaexc. Examination of the spectrum of the 25-hydroxy derivative of tachysterol (HOTachy) reveals vibronic structure between 295 and 260 nm and a broad structureless shoulder between 330 and 295 nm. These features are consistent with absorption by at least two Tachy conformers. We show that these conformers differ dramatically in their trans-to-cis photoisomerization efficiency. The Tachy --> Pre quantum yield at 313 nm (0.42) in degassed methanol solution is substantially higher than at 254 nm (0.12). On the basis of recent theoretical predictions, it is likely that 313 nm selectively excites the cEc HOTachy conformer which gives photoisomerization much more efficiently than do the other expected conformers (cEt, most abundant, and tEt). Efficient conversion of HOPro D3 to HOPre D3 is accomplished by a two-stage 254/313-nm irradiation sequence. Use of 313 nm in the second step is preferable to previously proposed much longer wavelengths that were hardly absorbed by HOTachy.

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