Overirradiation products

III. "OVERIRRADIATION PRODUCTS" FROM 7-DEHYDRO-CHOLESTEROL BISCHOLESTADIENOLS... 

A further example of vinylcyclobutene formation from an E-trlene is found among the overirradiation products of the E/Z-isomerlc pair of compounds, previtamin D and tachysterol, to be discussed in Section VII. 

Comparing the recently obtained results with the situation In the field as reviewed 10 years ago (1), It Is seen that the last major white area In vitamin D photochemistry, that of the overIrradiation products, has been disclosed. A whole series of compounds, partly formed via predictable pathways and partly originating from reactions novel In trlene photochemistry, now complete the stoichiometric picture. 

Havinga and Bots (1954) were the first to describe a method for the preparation of C -labeled cholecalciferol. Cholecalciferol-3-C was obtained by irradiation of 7-dehydrocholesterol synthesized from labeled cholesteryl acetate. The photochemical conversion from 7-dehydrocholesterol to cholecalciferol is accompanied by the formation of several other products such as lumisterols, tachysterols, and prechole-calciferol. A solution of NaNOs in water (0.4 ) effectively absorbs ultraviolet rays of wavelengths shorter than 250 mji which are supposed to promote die formation of side products. In order to minimize the formation of overirradiation products the ethereal solution of 7-dehy-drocholesterol-3-C is irradiated till 44.5% has been converted. The Havinga-Bots irradiation vessel, unique in itself, consists of three annular chambers made of quartz enclosing the irradiating lamp. 7-Dehydrocholesterol-3-C has been prepared recently from diolesterol-3-C (Kulkami et al., 1963) by the appKcation of an isocaproate adaptation (Nes et al., 1956) of the allylic bromination of cholesterol with 2V-bromosuccinimide followed by dehydrobromination. 

In recent years, it has been suggested that not all the excitation wavelength dependence of quantum yields in the vitamin D field can be accounted for by the NEER principle. We consider below the competing mechanisms that have been proposed to account for photochemical observations and describe some of the strategies that have been employed to improve the photochemical production of the previtamins from the provitamins. Optimization of the previtamin yields improves vitamin yields as the latter are formed thermally from the previtamins via 1,7-suprafacial hydrogen shifts. Readers interested in the rich photochemistry leading to overirradiation products should consult previous reviews. ... 

How long should the irradiation be allowed to proceed Overirradiation may produce secondary photolysis products as a result of primary product absorption. It is best in cases where the products are likely to absorb to monitor the reaction as a function of time. 

A number of examples are shown in Scheme 5. Where more than one mode of cyclization is possible, mixtures of products may result, and overirradiation may result in dechlorination. This method has also been used for the synthesis of chloromethoxydibenzofurans. ... 

The submitters emphasize the importance of terminating the irradiation before all of the /V-oxide is consumed. Overirradiation gives rise to a more complicated mixture of products from which the product can no longer be isolated by the simple extraction procedure described. 

The submitters followed the course of the irradiation by TLC analysis on silica gel with 5% (v/v) methanol in chloroform as developing solvent. Since some by-products have Rf values coincident with the /V-oxide, this spot will not completely disappear and caution must be exercised to avoid overirradiation. 

Since diones have appreciable absorption at longer wavelengths than most organic compounds, including their products of reaction, it is usually feasible to perform photoreactions using simple filters so that light is absorbed only by the dione. The initial excited species, at least, can then be clearly defined. Proper choice of wavelength can also ensure irreversibility of reactions and eliminate problems of overirradiation. 

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