Asymmetric synthesis of densely functionalized medium-ring carbocycles and lactones through modular assembly and ring-closing metathesis of sulfoximine-substituted trienes and dienynes

Abstract

An asymmetric synthesis of densely functionalized 7-11-membered carbocycles and 9-11-membered lactones has been developed. Its key steps are a modular assembly of sulfoximine-substituted C- and O-tethered trienes and C-tethered dienynes and their Ru-catalyzed ring-closing diene and enyne metathesis (RCDEM and RCEYM). The synthesis of the C-tethered trienes and dienynes includes the following steps: 1) hydroxyalkylation of enantiomerically pure titanated allylic sulfoximines with unsaturated aldehydes, 2) α-lithiation of alkenylsulfoximines, 3) alkylation, hydroxy-alkylation, formylation, and acylation of α-lithioalkenylsulfoximines, and 4) addition of Grignard reagents to α-formyl(acyl)alkenylsulfoximines. The sulfoximine group provided for high asymmetric induction in steps 1) and 4). RCDEM of the sulfoximine-substituted trienes with the second-generation Ru catalyst stereoselectively afforded the corresponding functionalized 7-11-membered carbocyles. RCDEM of diastereomeric silyloxy-substituted 1,6,12-trienes revealed an interesting difference in reactivity. While the (R)-diastereomer gave the 11-membered carbocyle, the (S)-diastereomer delivered in a cascade of cross metathesis and RCDEM 22-membered macrocycles. RCDEM of cyclic trienes furnished bicyclic carbocycles with a bicyclo[7.4.0]tridecane and bicyclo[9.4.0] pentadecane skeleton. Selective transformations of the sulfoximine- and bissilyloxy-substituted carbocycles were performed including deprotection, cross-coupling reaction and reduction of the sulfoximine moiety. Esterification of a sulfoximine-substituted homoallylic alcohol with unsaturated carboxylic acids gave the O-tethered trienes, RCDEM of which yielded the sulfoximine-substituted 9-11-membered lactones. RCEYM of a sulfoximine- substituted 1,7-dien-10-yne showed an unprecedented dichotomy in ring formation depending on the Ru catalyst. While the second-generation Ru catalyst gave the 9-membered exo 1,3-dienyl carbocycle, the first-generation Ru catalyst furnished a truncated 9-membered 1,3-dieny carbocycle having one CH 2 unit less than the dienyne.