Diels’ hydrocarbon

Although many sterols and bile acids were isolated in the nineteenth century, it was not until the twentieth century that the stmcture of the steroid nucleus was first elucidated (5). X-ray crystallographic data first suggested that the steroid nucleus was a thin, lath-shaped stmcture (6). This perhydro-l,2-cyclopentenophenanthrene ring system was eventually confirmed by the identification of the Diels hydrocarbon [549-88-2] (4) and by the total synthesis of equilenin [517-09-9] (5) (7). 

Diels hydrocarbon 123, indicating the steroidal nature of the alkaloid. 

Dihydro-17-methyl-l5/f-cyclopenta[a]phenanthrene Diels hydrocarbon. C Hlt mol wt 232.33. C 93.06%, H 6.94%. Prepd from cholesterol, cholic acid, or suitable sapogenins Diels, Rickert, Ber, 60, 267 (]935) starting with 2 -acetyl -phenanthrene Gamble ef al. J. Chem, Soc. 1935, 443, 644. 

As shown in Fignre II.B-2, pyrolysis of cholesterol la yields chrysene 111, Diels hydrocarbon IV —a methyl-cyclopentaphenanthrene—and nnmerous other PAHs. Both PAHs noted have also been identihed in pyrolysates of the major tobacco phytosterols [ Wynder et al. (4356), Van Dnnren (4022)]. More recently in the early 1970s, Hecht et al. (1560) discnssed the generation of chrysene and alkylchrysenes by pyrolysis of phytosterols. 

Several PAHs other than chrysene and Diels hydrocarbon (Fignre XXV-1) were subsequently identihed in sterol pyrolysates. In 1959, Wynder et al. (4355,4356) reported that PAHs were generated at both temperatures when tobacco sterols were pyrolyzed in air at 720°C and 850°C. At these temperatures, the pyrolysates constituted 28% and 22%, respectively, of the phytosterols pyrolyzed B[a]P constitnted... 

In the same year that he identified carbon suboxide, Diels began to investigate cholesterol with E. Abderhalden. The structure of this Hpid had not yet been determined. He was the first to study the products of the selenium dehydrogenation of cholesterol and isolated a hydrocarbon (CigHig), which became known as "Diels hydrocarbon." This substance proved to possess the basic steroidal ring structure its subsequent synthesis by Diels in 1935 (almost 30 years after he started this work) led to the rapid elucidation of the structures of a vast array of steroidal sex hormones, saponins, cardiac glycosides, bile pigments, and adrenal cortical hormones, such as cortisone. 

The exceptional difficulties that had to be overcome compelled attention to be devoted first to the solution of simpler problems, in the first place, to the synthesis of dehydro derivatives of the steroids. As early as 1932-1933 (Scheme 32), the total synthesis had been achieved of 1,2-cyclopentanophenanthrene and its 3 -methyl derivative, the "Diels hydrocarbon, which is frequently formed in the dehydrogenation of natural steroids. During the next few years, methods were developed for obtaining tricyclic intermediates and this enabled an approach to be made to the synthesis of the simplest representative of the natural steroids, equilenin (2). Equilenin contains only two centers of asymmetry (C13 and C14), which simplified the stereochemical problems and made it possible to use classical methods for the construction of rings. This synthesis was first effected by Bachmann et al. (Scheme 2) in 1939, all four theoretically possible enantiomers and two racemates of equilenin being obtained. 

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