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.

The reductive cleavage of methyl vinyl-β-ionyl ether (1) or the deprotonation of 3,2Ј,6Ј,6Ј-tetramethyl-5-(1-cyclohexenyl)-1,3-pentadiene (2) gives rise to an organometallic C 15 species that combines selectively with a variety of electrophiles at the terminal chain position. Its reaction with aldehydes, however, is less clean. In particular, (E)-β-formyl-2-butenyl acetate gives the expected adduct 7a and, after dehydration, vitamin A acetate only in poor yield. The same is true for the analogous reaction with 2,7-dimethyl-2,4,6-octa-3-Methoxy-3-methyl-1-(2,6,6-trimethyl-1-cyclohexenyl)-1,4-pentadiene (1), quantitatively obtained by the treatment of vinyl-β-ionol with sodium hydride and methyl iodide in tetrahydrofuran or with dimethyl sulfate in the presence of sodium hydroxide under phase-transfer conditions, is readily cleaved by lithium metal or sodium/potassium alloy. The resulting heptatrienyl-type organometallic C 15 entity [1Ϫ3] can be trapped by standard electrophiles to afford, for example, the hydrocarbon 3, the silane 4 or the alcohol 5. As we have found more recently, the same C 15 species can be generated conveniently by the deprotonation of 3-methyl-5-(2,6,6-trimethyl-1-cyclohexenyl)-1,3-pentadiene (2) with sec-butyllithium or the superbasic mixture of butyllithium and potassium tert-butoxide. The required triene is easily prepared by a Wittig olefination of (E)-2-methyl-4-(2,6,6-trimethyl-1-cyclohexenyl)-2-butenal, [4Ϫ5] the key building block in the Isler/Hoffmann-LaRoche syntheses of vitamin A [5Ϫ7] and β-carotene. [7Ϫ8] (E)-3-Formyl-2-butenyl acetate is, along with vinyl-β-ionol, the major component of the Pommer/BASF route [9Ϫ10] to vitamin A. When this aldehyde was allowed to react with the organometallic C 15 species, regardless of [a] 3903 triendial, which ultimately affords β-carotene. Vitamin A acetate can also be prepared, this time in moderate yield, by functionalization through consecutive deprotonation, borylation, oxidation and acetylation of a C 20 pentaene hydrocarbon having the required skeleton. Both the C 15 and the C 20 organometallic key intermediates adopt spontaneously a zigzag-like outstretched conformation which, upon electrophilic trapping, directly and exclusively leads to the all-(E) configuration.

Abstract:The adduct obtained upon consecutive treatment of 1,4-dienes with butyllithium in the presence of potassium tert-butoxide, chlorotri(isopropyloxy)titanium and a carbonyl compound contains the a-hydroxyalkyl group invariably and exclusively linked to the 3-position of the former diene. When chlorotri(isopropyloxy)titanium is replaced by the Duthaler-Hafner reagent [(4R,5R)-chloro(cyclopentadienyl)(2,2-dimethyl-a,a,a',a'-tetraphenyl-1,3-dioxolane-4,5-dimethanolato-O,O α )titanium] the reaction does not only occur regioselectively but also with appreciably high enantioselectivity.

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