C. 011 Natural Product Degradation During Extraction and/or Chromatography The shrub Baeckea frutescens (Myrtaceae) has long been used in traditional medicine for the treatment of rheumatism and snake bites, and a recent examination of a dichloromethane extract of the aerial parts revealed the presence of a number of structurally unique compounds. The major component of the extract was shown to have structure 1.
Give a mechanistic explanation for this transformation. O O ~ Me O R R R R 1 R = H, alkyl, vinyl 2 83. m-Terphenyls from Pyrones Alkali-catalysed reaction of the pyrones 1 with acetophenones ArlCOMe in DMF at room temperature results in smooth transformation into the m-terphenyls 2 (50-66%) and the pyranylideneacetates 3 (0-28%). Give mechanisms to account for the formation of 2 and 3 from 1. SMe SMe O2Me Ar Ar Ar~ ]~ CO -12Me Al Ar1 1 2 84. CO2Me 3 A Cascade Reaction Treatment of the readily accessible ester 1 with either Ac20/TsOH/refluxing xylene or TFAA/Et3N/CH2C12/RT or TMSOTf/Et3N/CH2C12/RT results in smooth conversion into the indoline 2 in 50-75% yield depending on the conditions employed.
Me ~N.. CH2CO2Et Me EtO2CCH 2~ N O )=( CHN2 CO 2Me CO 2Me O O O Mex 9 OH MeO 2 71. H 2Me 9 EtO 2CCH 2N Me02c / NCO 2Me Pyrrolo[ 1,2-a]benzimidazole Synthesis by Ortho Nitro Interaction Pyrrolo[1,2-a]benzimidazoles are of interest as potential antitumor agents, and one convenient and short route to such compounds involves treatment of the readily available 1-(onitroaryl)pyrrolidines 1 with ZnC12/Ac20 at 110~ for several hours. This gives the 3-acetoxy derivatives 2, usually in modest yield, and the initial cyclisation reaction is known to involve a ZnC12-catalysed internal redox reaction leading to an o-nitroso iminium ion.
Advanced Problems in Organic Reaction Mechanisms by McKillop