148 VITAMIN D GROUP 



and Migliardi^^ dehydrogenated cholesteryl acetate with quinone in boiling 

 acetic acid during exposure to filtered light (to activate the quinone) and 

 obtained a yield of 7-dehydrocholesterol amounting to 30 %. Sah*^ made the 

 provitamin by heating cholesteryl acetate with ordinary quinone and sev- 

 eral homologs and analogs thereof. 



A new semisynthesis of provitamin D, based on the selective bromination 

 of sterols at position 7 by means of N-bromosuccinimide, has been described 

 by Henbest et al.,^^ Bide et al.,^^ andBuismanei al.^^ Cholesteryl acetate gives 

 7-bromocholesteryl acetate, and this, when heated in diethylaniline, splits 

 off hydrogen bromide, leaving 7-dehydrocholesteryl acetate, from which 

 the free provitamin is obtained by saponification. Yields up to 30 % have 

 been claimed. Schaaf" has patented some technical details of the reaction, 

 claiming yields up to 40 %. Redel and Gauthier^* bring about the dehydro- 

 bromination by heating with collidine, and Ruigh and Gould^^ use quinal- 

 dine, which they claim gives 50 % better yields than diethylaniline. Schaaf ^^ 

 effects a further improvement in yield by employing quinaldine diluted 

 with aromatic hydrocarbons. 



C. THE PROVITAMINS D IN NATURE 



1. Isolation 



Only rarely does a provitamin D occur in such abundance, or so free 

 from other sterols, that it can be obtained in fair purity simply by the 

 recrystallization of unsaponifiable extracts. The exceptions are ergosterol 

 in certain fungi, and the new provitamin D^ in the ribbed mussel. 

 Modiolus. ^^ Usually it is necessary to employ special techniques, such as 

 digitonin precipitation to concentrate the sterol fraction, esterification with 

 suitable agents for the double purpose of protecting the provitamin from 

 destruction and enhancing fractionation, and above all, chromatographic 

 separation of the provitamin or its esters. Esterification with 3,5-dim- 

 trobenzoyl chloride in pyridine is a characterization procedure borrowed 



82 F. P. Mazza and C. Migliardi, Quaderni nidriz. 8, 86 (1941). 



S3 P. P. T. Sah, Rec. trav. chim. 59, 454 (1940). 



8* H. B. Henbest, E. R. H. Jones, A. E. Bide, R. W. Peevers, and P. A. Wilkinson, 



Nature 158, 169 (1946); E. R. H. Jones and R. W. Peevers, British Pat. 574,432 



(1946). 



85 A. E. Bide, H. B. Henbest, E. R. H. Jones, R. W. Peevers, and P. A. Wilkinson, 

 /. Chem. Soc. 1948, 1783. 



86 J. A. K. Buisman, W. Stevens, and J. van der Vliet, i?ec. trav. chim. 66, 83 (1947). 



87 K. H. Schaaf, U. S. Pat. 2,542,291 (1951). 



88 J. Redel and B. Gauthier, Bull. soc. chim. France 15, 607 (1948). 



89 W. L. Ruigh and D. H. Gould, U. S. Pat. 2,546,787 (1951). 

 '"K. H. Schaaf, U. S. Pat. 2,546,788 (1951^. 



" H. G. Petering and J. Waddell, /. Biol. Chem. 191, 765 (1951). 



