Regiospecific 6-Endo-Annulation of in Situ Generated 3,4-Dienamides/Acids: Synthesis of δ-Lactams and δ-Lactones

A novel and efficient method for the construction of alpha-(1,3-dithiolan-2-ylidene) delta-lactam and delta-lactone rings has been developed. It involves the reglospecific 6-endo-annulation of in situ generated 2-(1,3-dithiolan-2-ylidene)-3,4-dienamides/acids from the dehydrative coupling of alpha-oxo ketene dithioacetals with tertiary propargyl alcohols promoted by BF3 center dot Et2O. A range of alpha-(1,3-dithiolan-2-ylidene) delta-lactams and delta-lactones are obtained in good to high yields. In addition, indenes are prepared by using alpha-acetyl ketene dithioacetal as the precursor.