Steroids, natural products synthesis, carbon

The androstane nucleus is sufficiently complex so as to effectively mle out total synthesis as a commercial source for starting materials. The great preponderance of steroid natural products includes carbon side chains at the 17 position. Preparation of androstane starting compounds thus relies on schemes for degrading those side... 

Consider, for example, the rapid and efficient construction of the perhydrophenanthrene skeleton 119 as exemplified by the conversion illustrated next [34]. The reaction is stereospecific, leading to the trans-anti-trans ring fused adduct in a 65% to 72% yield. Furthermore, C-C bond formation occurred despite the fact that in so doing, a very crowded environment consisting of vicinal quaternary carbon centers, is produced. There are very few methods that provide such scope. It takes little thought to imagine the application of this powerful transformation to the synthesis of steroids. Yet, the opportunity to apply the methodology to natural product synthesis has not been realized. 

Propiolaldehyde diethyl acetal has found numerous synthetic applications in the literature which may be briefly summarized. The compound has been utilized in the synthesis of unsaturated and polyunsaturated acetals and aldehydes by alkylation of metal-lated derivatives, " by Cadiot-Chodkiewicz coupling with halo acetylenes, " and by reaction with organocuprates. Syntheses of heterocyclic compounds including pyrazoles, isoxazoles, triazoles, and pyrimidines have employed this three-carbon building block. Propiolaldehyde diethyl acetal has also been put to use in the synthesis of such natural products as polyacetylenes " and steroids. ... 

Much like the work conducted by the Bogert and Cook groups, the Bardhan-Sengupta synthesis of phenanthrenes helped lead to the determination of the carbon framework for steroids and was used to prepare terpene-type natural products. 

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