Nazarov reagents for convergent and expedient synthesis of new steroids

Laboratoire de synthèse organique, Institut de pharmacologie de Sherbrooke, Université de Sherbrooke, Sherbrooke, Québec, Canada J1H 5N4.
Organic Letters (Impact Factor: 6.32). 05/2002; 4(7):1091-4. DOI: 10.1021/ol017274r
Source: PubMed

ABSTRACT [reaction: see text] A new methodology for the convergent synthesis of tetracyclic compounds was developed. Two new bicyclic Nazarov reagents of type II were synthesized, and their cycloaddition with 2-carbomethoxy-2-cyclohexenone I was studied. This cycloaddition afforded new interesting steroidal backbones.

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    ABSTRACT: Herein we report a convergent enantioselective synthesis of a polyfunctionalized ABCD tetracycle by using an anionic cycloaddition reaction between a chiral bicyclic CD Nazarov intermediate (see 6), derived from the (−)-Weiland–Mischer ketone, and an achiral cyclohexenone (see 5) adequately functionalized to furnish the ring A of pentacyclic triterpenes (Scheme 5). The chiral bicyclic CD Nazarov intermediate forms ring B upon cycloaddition with the achiral cyclohexenone to yield an ABCD tetracycle with a cis-anti-trans-anti-trans configuration (see 4). Further transformations on this adduct allowed reduction of the angular aldehyde function at C(10) to a Me group (→17) and introduction of an unsaturation at C(5)C(6) by using the ketone function at C(7) (→ 3; Scheme 6).
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