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The reactions of (trans-2-phenylcyclopropyl) ethyne, (trans,trans-2-methoxy-3-phenylcyclopropyl) ethyne, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl) ethyne, with either aqueous sulfuric acid or tris (trimethylsilyl) silane or AIBN are investigated. Protonation and addition of the silyl radical occurred at the terminal position of the alkyne giving an alpha-cyclopropyl-substituted vinyl cation or radical, respectively.

A novel Nazarov reaction is described in which 2-chloro-3-silyloxypentadienyl cations capable of electrocyclization are generated from easily available dichlorocyclopropane precursors, as an alternative method for generation of Nazarov-type pentadienyl systems which may permit novel trapping pathways or greater functional group compatibility. Examples are presented of an apparent sequential 2pi electrocyclic opening/4pi electrocyclic closure using alkenyldichlorocyclopropanes as pentadienyl cation precursors.

Dichlorocyclopropanation of 2-amino-1,3-dienes affords 1-alkenyl-1-amino-2,2-dichlorocyclopropanes which undergo silver-assisted 2-π electrocyclic opening to furnish 3-aminopentadienyl cations. Nazarov-type cyclization of these intermediates leads to cyclopentenone iminium salts, which provide allylic amines upon reduction. This process, the imino version of the traditional Nazarov reaction, can also be combined with an interrupted Nazarov domino process to give polycyclic amines. [ABSTRACT FROM AUTHOR]

Readily available five- and six-membered lactones and N-sulfonyllactams undergo efficient addition of t-butyl propiolate, and the resulting adducts undergo cycloisomerization to six- and seven-membered cyclic ethers or amines in the presence of pyridinium acetate. The ring expansion process occurs in generally good yields and is proposed to involve a nucleophilic catalysis mechanism.

2-Siloxy-2-alkenyl-1,1-dichlorocyclopropanes with aryl-terminated side chains undergo silver-assisted electrocyclic opening/Nazarov cyclization. The resulting 2-siloxycyclopentenyl cations are intercepted by the pendant arenes to furnish tricyclic adducts in moderate to good yields. In cases where the arene trap was tethered through the cyclopropane unit, a new mode of trapping occurred to generate unique bridged carbon frameworks.