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Investigations relevant to semipinacol rearrangements for the formation of 1-azaspirocycles Easton, Leah Pearl

Abstract

Investigations related to semipinacol rearrangements for the formation of 1-azaspirocycles including substrate synthesis, reaction scope, and application to the Erythrina alkaloid skeleton are reported. Nickel(II) and chromium(II) salts are used to promote the addition of enol triflates derived from δ-valerolactam to aldehydes to produce substrates such as A. The use of N-Boc-protected triflate 1.21 results in higher yields than the corresponding N-Ts or N-Bz-protected analogues. Additions to aliphatic aldehydes are most efficient while electron-rich aldehydes prove less reactive. The optimized reaction conditions are also applicable to additions of 1.21 to cyclobutanone. The scope of the semipinacol rearrangement of siloxy-epoxides is extended to include substrates with larger rings. Using Lewis acid tin(IV) chloride the 6 to 7-membered and 7 to 8-membered ring expansions of the corresponding epoxides, 2.33 and 2.37, are possible. Two general strategies towards the Erythrina alkaloid skeleton which feature a sernipinacol rearrangement to form the spirocyclic ring junction are explored. However, both approaches are yet to provide a means to the total synthesis of the desired framework. During the course of investigations the semipinacol rearrangement of epoxide 3.59 involving a migration to a benzylic position is attained. [ Fomulas omitted ]

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