Stereoselective Cyclization of Functionalized 1,n-Diynes Mediated by [X-Y] Reagents [X-Y = R3Si-SnR′3 or (R2N)2B-SnR′3]: Synthesis and Properties of Atropisomeric 1,3-Dienes

The borylstannane [-N(Me)CH2CH2(Me)N-]B-SnMe3 is a superior reagent capable of effecting bisfunctionalization-cyclization in several highly functionalized 1,n-diynes, 1,n-enynes, and 1,n-allenynes (including 1,2-dipropargylbenzenes, 2,2'-dipropargylbiphenyls, 4,5-dipropargyldioxolanes, and 1,4-dipropargyl-beta -lactams) where the more well-known silylstannanes fail. Variable-temperature NMR studies showed that conformational restraints imposed by selected backbones increase the activation barrier for the helical isomerization in (Z,2)-dienes that are generated in the cyclization of the diynes. In the biphenyl and dioxolane systems, the reactions proceed with surprisingly good regio- and stereoselectivity. The resulting diazaborolidine derivatives are hydrolytically unstable but can be isolated by recrystallization or precipitation. For further synthetic applications, it is advantageous to convert these compounds in situ into the corresponding dioxaborolidines with either retention of the Me3Sn group or replacement of this group via halodestannylation. The configurations of the vinyl moieties are preserved in these reactions. Highly functionalized dibenzocyclooctadienes, which adorn the carbon frames of several important cytotoxic natural products, can be synthesized using this chemistry.