Ergosterol diene protection

A similar strategy can also be used in a seven-step procedure of preparation of 3/3,25-dihydroxy-cholesta-5,7-diene from ergosterol giving a total yield of 30%. The 3-hydroxy function of ergosterol is protected as /-butyldimethylsilyl ether before the 5,7-diene system is treated with l,4-dihydrophthalazine-l,4-dione. In this case, the key step is a very mild method for the cleavage of the hetero Diels-Alder adduct using lithium naphthalenide <1999S1331>. 

Barton, D. H. R., A. Gunatilaka, T. Nakanishi, H. Patin, D. A. Widdowson, and B. R. Worth Synthetic Uses of Steroidal Ring B Diene Protection 22,23-Dihydro-ergosterol. J. Chem. Soc. Perkin 11976,821. 

Protection of steroidal 5,7-dienes (3, 223-224 6, 467 9, 372). The adducts of 1 with an ester of ergosterol can be obtained in >90% yield by reaction with 1 prepared in situ. The diene system can be regenerated by treatment with 2 N KOH in 95% ethanol. The derivative prepared from 2 is particularly readily hydrolyzed (98% yield), and consequently this reagent is generally preferable to 1. ... 

Ergosta-5,7,22,24(28)-tetraen-3jS-ol has been synthesised from ergosterol by ozonolysis to the 22-aldehyde and construction of the appropriate side-chain using a Wittig reaction. During these reactions, the 5,7-diene system was protected as the Diels-Alder adduct (546) with 4-phenyl-l,2,4-triazolin-3,5-dione, from which the homoannular diene could be regenerated in 99 % yield by reduction with lithium aluminium hydride. 

Protection of the ring-B diene system in ergosterol acetate (390) as its adduct (391), followed by bromination of the C-22 double bond to (392) and dehydro-bromination with l,5-diazabicyclo[4,3,0]non-5-ene (DBN) in toluene, gave ergosta-5,7,22,24(28)-tetraen-3)8-ol acetate (393). 

It would seem that ergosterol, provitamin D2 (la), would serve as an ideal precursor for side chain oxygenated vitamin D derivatives because of its ready availability from yeast and its preexisting 5,7-diene system. However, the reactivity of the diene moiety in its free or protected form (vide infra) has complicated the selective manipulation of the side chain. Eyley and Williams have studied the reactions of the aldehyde (43) obtained in modest yield by selective ozonization of... 

Iron carbonyl complexes continue to find use in organic synthesis. Protection of the ring b diene in ergosterol as the Fe(CO)3 complex allows selective reactions to be carried out on other parts of the molecule (Scheme 9). ° The 22,23-double bond in the complex (88 R = PhCO-) is readily hydrogenated over Adam s catalyst in the presence of a catalytic amount of benzyldimethylsilane in 94% yield. " The... 

The utility of tricarbonyliron fragments as protecting groups for 1,3-dienes has been demonstrated first by Barton et al. They showed that the diene system in the B ring of ergosterol can be protected by formation of the tricarbonyliron adduct to allow selective catalytic hydrogenation of the side chain A -double bond (Scheme 4-157). ... 

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