CAROTENOIDS WITH LESS THAN FORTY CARBON ATOMS 599 



One proof of the above structure is the observation that by Q!,a'-bromina- 

 tion of perhydrocrocetin followed by replacement of the halogen with 

 hydroxyl, a diglycol is formed which, on treatment with lead tetraacetate, 

 yields a diketone; the aldehyde group is completely absent. This proves 

 that the methyl groups are present in the a-position to the carboxyls.^' 



Better proof of the structure of crocetin has been presented by Karrer, 

 Benz, and Stoll,**^ who showed that synthetic perhydrocrocetin was identi- 

 cal with the perhydro compound prepared from the natural saffron dye- 

 stuff. 



b. Dimethylcrocetins. Kuhn and Winterstein^^* first demonstrated 

 that two types of dimethylcrocetin can be prepared by extraction and 

 exchange esterification of the natural crocetin gentiobioside (crocin) with 

 methyl alcohol. 



1. Stable dimethylcrocetin (7-crocetin, crocetin I). This is present in the larger 

 amount (17-60 g. per kilogram of drj' saffron). The dimethyl ester melts at 222°C. 1+ 

 appears to be the all-^rans isomer. ^^^ 



2. Labile dimethyl crocetin (crocetin II, pigment of Kuhn). Labile crocetin is known 

 only in the form of derivatives. The labile dimethyl ester can be prepared in compara- 

 tively small amounts from saffron (1 g. per kilogram of dry saffron); the dimethyl ester 

 melts at 141 °Cf, Zechmeister^^ believes that it contains one cis linkage. 



ds-Dimethylcrocetin can be readily changed to the trans or stable form 

 of the ester by heating the crystals of the cis form above the melting point 

 and cooling,^^^ by catalysis of petroleum ether solution with iodine,^*^ by 

 irradiation of petroleum ether solution,^^^ or by reduction of the labile 

 isomer followed by oxidation of the dihydromethylcrocetin formed as in- 

 dicated below: 



H20 . . 02 

 cis-Dimethylcrocetin > Dihydrodimethylcrocetin > irans-Dimethylcrocetin 



Pyridine 



c. Occurrence. Crocetin is present in the Crocus sativus (saffron), 

 largely in the form of a glucoside (crocin) along with considerable amounts 

 of carotene, lycopene, and zeaxanthin.^^-^^^-^^'^^^ Both cis and trans forms 

 are present. ^^^-^^ It has also been prepared from two other members of the 

 Crocus family : Crocus Candida var. luteus (yellow crocus) where it appears in 

 the petals, ^^ and Crocus neapolitanus (violet crocus) in which it is found in 

 the stigma.''^'^^ Flowers from other species such as mullein (flores ver- 

 hasci)*^^ (also referred to as Verhascum phlomoides, clasping mullein or 

 king's candle), the Nyctanthes arhor-tristis (night jasmine), ^^^''*" and the 

 Toona ciliata or Cedrela toona Roxb. (Burma toon or Indian mahogany 



«' R. Kuhn and A. Winterstein, Ber., 66, 209-214 (1933). 



*^ L. Schmid and E. Kotter, Monalsh., 59, 341-356 (1932). 



«« R. Kuhn and A. Winterstein, Helv. Chim. Ada, 12, 493-498 (1929). 



«' E. G. Hill and A. P. Sikkar, J. Chein. Soc, 91, 1501-1505 (1907). 



