5 - Ring Syntheses which Involve Formation of Three or Four Bonds

Abstract

This chapter discusses the ring syntheses which involve formation of three or four bonds. Formation of 1,2, 3,4, and 1,5 bonds or 1,2, 2,3, 3,4, and 1,5 bonds are also discussed. These two synthetic approaches are formally similar and thus it is convenient to combine them. Both condense an α-functionalized ketone or aldehyde with an amine or ammonia and an aldehyde. Although 1,2-dicarbonyl substrates (especially unsymmetrical benzils) are often difficult to make, there are a number of approaches which may be appropriate. Propane-1,3-dithiol reacts with aldehydes to give cyclic thioacetals (in 52-91% yields) which form stable dithiane anions when treated with butyllithium. Subsequent quenching with an acid chloride followed by mercury(II) chloride treatment gives a 1,2-dicarbonyl species. Alternatively, substitution of an aldehyde for the acid chloride gives rise eventually to an α-hydroxycarbonyl derivative.