Formation of Acetylenic Acetals by Ring Opening of 1,1,2-Trihalocyclopropanes under Phase-Transfer Conditions.
摘要:
A number of substituted 1,1-dibromo- and 1,1-dichlorocyclopropanes with an additional chlorine or bromine atom attached to C-2 were synthesized in reasonable to good yields by dihalocarbene addition to the corresponding alkenes under phase-transfer conditions. When the trihalides were treated with 50% aqueous sodium hydroxide in the presence of ethanol, triethylbenzylammonium chloride and dichloromethane, most of the compounds underwent ring opening and afforded mixtures of acetylenic acetals, usually in good yields. The reaction most likely involves cyclopropene intermediates, which in some cases also rearrange to a minor extent to the corresponding vinylcarbenes and afford alpha,beta-unsaturated aldehydes.
Cyclopent-2-en-1-one is a versatile electrophilic cyclopentadiene equivalent in reactions with 1-bromo-1-lithiocyclopropanes. The synthetic sequence (outlined in Scheme 1) has been applied to the synthesis of functionalized 7-X-7,8-dihydrocalicenes 13c (Scheme 3) and 13d (Scheme 4). 7-Bromo-7,8-dihydrocalicene (13d) is considered to be a promising precursor of the so far unknown parent calicene (2). A similar sequence has been realized for 7-(chloromethyl)-7,8-dihydrocalicene (21a, Scheme 5) which, under appropriate conditions, could give 7-methylcalicene (16).