Bicyclic rings, synthesis from enamines

A key step in the synthesis in Scheme 13.11 was a cycloaddition between an electron-rich ynamine and the electron-poor enone. The cyclobutane ring was then opened in a process that corresponds to retrosynthetic step 10-IIa 10-IIIa in Scheme 13.10. The crucial step for stereochemical control occurs in Step B. The stereoselectivity of this step results from preferential protonation of the enamine from the less hindered side of the bicyclic intermediate. 

Bicyclic dienic products recently synthesized by this method (Scheme 36) are lactams 144a and b (07T8267) and enamine ester 145 (09CEJ4565). The precursor of the latter substance originates from a four-component synthesis followed by y-allylation. The alternative y-propargylation and subsequent ring-closing enyne metathesis performed under an ethene atmosphere lead to the 8-vinyl derivative of product 145. 

B. Heterocyclic - A number of ring-aza steroids are conveniently prepared by total synthesis. The 6,7-diaza steroid was prepared from a bicyclic enamine intermediate with m-methoxydiazonium fluoroborate which was used to introduc the adjacent nitrogens prior to completion of the rest of the steroidal skeleton. The enamines of g-tetrolone and 6-methoxy -tetralone were treated with a-bromoacetate and then treated with hydrazine hydrate followed by either malonyl dichloride or propiolactone to give 13,14-diozo steroids. By this sequence the D-homo-diaza steroidal system could also be prepared.The 8,13-diazaestranes were prepared by... 

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