摘要:
The use of Breslow's remote functionalization paradigm for installation of an axial C-7 hydroxy group into a perhydrophenanthrene nucleus, with a view toward synthesis of bruceantin (1), has been investigated. The substrates that were evaluated were 9a-c, 11, 17, 18, and 20. Substrates 9a-c all undergo preferential functionalization at C-12. After oxidative cleavage of the initial photoproduct, ketones 18a-c were obtained in yields of 18-26% (36-41%, based on unrecovered starting material). Unsaturated substrate 11 undergoes remote functionalization exclusively at the secondary allylic position (C-12); enone 20 is obtained in 83% overall yield after oxidative cleavage of the initial photoadduct, 19a,b. Thus, in this system, C-12 appears to be the preferred site of intramolecular functionalization. Attempts to block reaction at this position by the use of saturated ketone 18, the derived ketal 17, or enone 20, were all unsuccessful. In the case of 18 the only photoproduct was the intramolecular pinacol. Enone 20 gave an exceedingly complex mixture, consisting of many products. Ketal 17 afforded the unusual macrocyclic lactone 21 in 33% yield. The main conclusion of this study is that it is difficult to extrapolate from the excellent regioselectivity observed by Breslow in the steroid system to the trans-anti-trans perhydrophenanthrene system, which is only slightly less rigid. A second factor which we believe is important in the system we have studied is the apparently minor perturbation of having an equatorial substituent at C-4. We postulate that this substituent, which was not present in the model steroidal systems investigated previously by Breslow, disfavors functionalization at C-7.