Pentadienyl Type Lithium and Potassium Species: The Regioselectivity of their Reactions with Electrophiles

Abstract

Seven structurally distinct pentadienyl type lithium and potassium compounds were screened against a variety of electrophiles in order to assess the regioselectivity of the trapping reactions. Organoborates and analogs thereof (fluorodimethoxyborane) proved to be perfectly regioreliable attacking only unsubstituted terminal positions and thus providing, after oxidation, exclusively primary allylic alcohols. 2,4-Pentadienyllithiums or -potassiums, that carry a methyl group at the 1-or 3-position, exhibit the same extreme regioselectivity towards halotrialkylsilanes or carbon dioxide. Although the unsubstituted parent compounds combine with such electrophiles still preferentially at the terminal position, considerable proportions of branched products are concomitantly formed as well (1/3-attack ratios ranging from 2:1 to > 20:1). Hydroxyalkylating and alkylating reagents such as formaldehyde, oxirane or butyl iodide invariably afford regioisomeric mixtures generally varying in composition between 3:1 and 1:3. The condensation reaction with halotrialkylsilanes appears to follow a concerted (S N 2-like) rather than an addition/elimination (ate complexmediated) mechanism.

Total Synthesis of (+)-Trienomycins A and F via C–C Bond-Forming Hydrogenation and Transfer Hydrogenation

“…Our retrosynthetic analysis of trienomycins A and F invokes a convergent assembly of Fragments A and B wherein ( E )-trimethyl-2,4-pentadienylsilane 15 serves as a linchpin through chelation controlled aldehyde addition 16 followed by diene-diene ring closing metathesis (RCM) (Scheme 1). 13b Fragment A is itself prepared using an enantio- and syn -diastereoselective ruthenium catalyzed crotylation developed in our laboratory, 17a which enables direct C-C coupling of a ( Z )-configured trisubstituted allylic alcohol obtained via Suzuki reaction of ( Z )-3-iodo-2-methyl-1-propenol and the indicated vinylarene 2 .…”

Evolution of multi-component anion relay chemistry (ARC): construction of architecturally complex natural and unnatural products

“…Metalation exploiting the Schlosser base,36 followed by iterative addition of linchpin 80 and prenyl bromide ( 108 ). In situ hydrolysis of the resultant TMS-silyl ether with 0.5 M sulfuric acid provided the desired tri-component ARC adduct 109 in 38% overall yield.…”

The first intramolecular silene Diels–Alder reactions

“…Consequently, we adopted the Ishikawa protocol for silene generation in which an acyl polysilane is treated with an alkyl lithium reagent to initiate the sila-Peterson olefination and afford the silene at much lower reaction temperatures (Scheme 3). 16 In earlier studies of the sila-Peterson reaction we demonstrated that the presence of LiBr promotes elimination of LiOSiMe and enhances the formation of silene. Consequently, initial attempts explored the use of MeLi•LiBr complex at -78 ˚C.…”

“…Intriguingly for the preparation of 19 deconjugation of methyl hepta-2,4-dienoate in an analogous fashion to that employed for ethyl sorbate followed by reduction afforded the desired hepta-3,5-dienol favouring the EZ isomer (EZ : EE 84 :16). Presumably, this reflects the kinetic protonation of an "endo W" 16 shaped tri-enolate anion as treatment of the EZ dienylester with a catalytic amount of iodine resulted in isomerisation to the corresponding E,E diene.…”