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