Reagent-controlled enantioselective homoaldol reaction with chiral 1-oxyallyllithium derivatives. Enantio-divergent tuning by achiral titanium reagents

Generation, structure and reactivity of tertiary organolithium reagents

Tertiary organolithium reagents have great potential in natural product synthesis. Recent progress in organolithium generation has made them accessible. Their utility and stereoselectivity in synthesis is discussed.

Asymmetric synthesis of cis-1,2-dialkenyl-substituted cyclopentanes via (-)-sparteine-mediated lithiation and cycloalkylation of a 9-chloro-2,7-nonadienyl carbamate

Herein we report our comprehensive results in enantioselective cyclopentane synthesis via stereogenic allyllithium compounds. The described cycloalkylation reaction starts with a (-)-sparteine-mediated asymmetric deprotonation of the 2,7-alkadienyl carbamate 7e and leads to the enantioenriched (80% ee) and diastereomerically pure (dr = 99:1) cis-1,2-divinyl-cyclopentane 8, by a subsequent cyclization and elimination of lithium chloride. The reaction mechanism has been investigated by silylation and lithiodestannylation experiments and was found to represent a completely regioselective anti-S(N)'S(E)'-reaction. Trapping of the vinyllithium intermediate 12 with various electrophiles under retention of the configuration at the double bond extends the field of application for this cyclization. We also applied this reaction as the key step in the enantioselective synthesis of (+)-dihydromultifidene (17).

Highly enantioselective syntheses of anti homoaldol products by (-)-sparteine-mediated lithiation/transmetalation/substitution of N-Boc allylic amines

[reaction: see text](-)-Sparteine-mediated lithiation/transmetalation/substitution of N-Boc allylic amines provides anti-configured homoaldol precursors in yields of 38-85% and enantiomeric ratios of 83:17-99:1. Subsequent O-protection and hydrolysis allows access to O-protected homoaldol adducts in good yields. The absolute configurations of the homoaldol products have been assigned by calculation of optical rotations and by X-ray crystallography of derivatives. A stereochemical course of reaction for the lithiation/transmetalation/substitution sequence is proposed.