Reactivity of N‐Boc‐Protected Amino‐Ynones in the Presence of Zinc Chloride: Formation of Acetylenic Cyclic Imines and Their Palladium Complexes

Abstract

Amino‐ynones can be seen as precursors of various heterocyclic rings. Although γ‐amino‐ynones have been used to prepare exocyclic vinylogous amides in the presence of Brønsted acids, we describe here the use of zinc chloride as a weak Lewis acid for their conversion into acetylenic cyclic imines. This reaction, which was attempted for a diverse range of γ‐amino‐ynones, proved to be robust and efficient in most cases and was easily extended to δ‐amino‐ynones to yield various acetylenic cyclic imines. Depending on the workup procedure, the latter compounds could be isolated either as free organic compounds or as zinc complexes. The ability of these cyclic imines to form complexes with palladium is also reported.

Chirality in gold(III) homodimeric complexes

“…In the presence of 1 equivalent of AuCl3 the vinylgold complex 4 was obtained in 30% yield (estimated purity = 90%) whereas the use of NaAuCl4 led to 4'. As previously reported 9 HCl was generated in situ. If 4 was treated with 1 equivalent of AuCl3 in CDCl3 the formation of aurocycle 5 was observed.…”

“…According to the literature procedure, 9 di-protected (S)-4hydroxymethylpyrrolidin-2-one 1 was reacted with lithium phenyl acetylide to give N-Boc amino-ynone 2, which was cyclized into ligand 3 in the presence of 5 equivalents of a 1M ZnCl2 solution in ether (Scheme 1).…”

A Zinc-Mediated Deprotective Annulation Approach to New Polycyclic Heterocycles

“…It is well known that zinc (II) compounds are able to promote Boc deprotection [ 49 , 50 , 51 , 52 , 53 , 54 ]. In particular, an excess of ZnBr 2 has been successfully employed for the deprotection of N -Boc secondary amines [ 52 ] as well as of tert -butyl esters [ 53 , 54 ].…”

Synthesis of 3-substituted indolizidines from amino-ynones derivatives

“…In previous publications, 1,2 the reactivity of ynones bearing N-Boc protected primary amines I in the presence of electrophilic reagents was reported. Treatment with methanesulfonic acid (CH 3 SO 3 H), gave pyrrolidine exocyclic vinylogous amides via Meyer-Schuster rearrangement, 1 whereas zinc chloride (ZnCl 2 ) furnished acetylenic cyclic imines.…”

“…In both cases a cyclic carbinolamine intermediate was formed, which underwent either a Meyer-Schuster rearrangement with protic acid 1 or dehydration with Lewis acid. 2 In this paper, we present an extension of this study to amino-ynones having a secondary Boc-protected amine. Thus, when applied to the piperidine derivatives II, reaction with CH 3 SO 3 H or ZnCl 2 resulted in formation of vinylogues of indolizidin-3-ones III or very stable cyclic enamine-zinc complexes.…”