A mild route to α‐alkoxyacetylenes mediated by lewis acids and synthetic routes to 10‐, 11‐, and 12‐membered ring enediyne carbocycles

Abstract

Abstract The condensation of silyl substituted acetylenes with dimethoxy acetals via a modified “Mukaiyama‐type reaction” to afford α‐alkoxyacetylenes is described. These propargylic ethers 17–21 were synthesized from the reaction of aluminum or zinc acetylides with acetals mediated by Lewis acids. In appropriate cases, when a second acetal was present in the initial acyclic product ( 24 ), a subsequent intramolecular acetal‐ene (Prins) cyclization ensued to afford the silylalkylidenealkoxycyclopentane ( 25 ). Enediyne species were unreactive under these conditions, even in an intramolecular case. Routes to 11‐ and 12‐membered carbocyclic enediyne compounds ( 43, 44 ) were developed via an intramolecular pinacol coupling of the dialdehydes 39 with sammarium diodide/HMPA. The requisite double bond was introduced using the thiocarbonate expulsion method with trimethyl phosphite. A route to the highly substituted 10‐membered ring enediyne 50 is also described based on the use of an isopropylidene acetal tether control group to facilitate an intramolecular chromium (II)/nickel (II)‐mediated coupling of the iodoacetylene aldehyde 49 .