Eine vielseitige und effiziente Synthese anellierter Cyclopentane durch stereoselektive Cycloaddition von Allylsilanen und Cycloalkenylmethylketonen

Abstract

Nützliche Vorstufen für cyclopentanoide Naturstoffe und mittlere Ringe lassen sich durch die Titelreaktion (a) gewinnen. Es wird postuliert, daß an dieser neuen Anellierungsreaktion eine kationische 1 2‐Silyl‐Verschiebung über ein Kation mit pentavalentem Silicium beteiligt ist.

Cycloadditions of allylsilances. Part 10. Stereoselective Construction of Ring Systems by Cycloaddition Reactions of allyltriisopropylsilance

“…Only the cycloadditions with the 1acetylcyclopentenes 4a and 4d afforded the syn diastereoisomer as a minor by-product. This loss of diastereoselectivity is explained by a steric effect: in the products anti-5a and anti-5d the bulky triisopropylsilyl group is in the endo position of the bicyclo[3.3.0]octane framework, representing the sterically more hindered diastereoisomers [I, 9,131. Further applications of the [3+2] cycloaddition of all yltriisopropylsilane and a,p-unsaturated ketones to the synthesis of the bicyclo[3.3.0]octane ring system are described below.…”

“…With bulky alkyl groups R at the silicon atom the cycloadduct 2 was obtained in better yields [ 101. Using allyltriisopropylsilane l b as the reagent, the cycloadduct 2b was obtained in 86% yield as a single diastereoisomer with anti stereochemistry provided that optimized reaction conditions (addition of 1.5 equivalents of l b at -78 "C and subsequent reaction for 19 h at -20 "C) are applied [ 1,9]. The prefixes anti and syn denote the position of the triisopropylsilyl group relative to the carbonyl group.…”

“…The titanium tetrachloride promoted nucleophilic attack of the allylsilane 1 at the P-position of the enone generates a p-silyl cation (Scheme 2). This intermediate is stabilized by forming a siliranium ion, which represents a bridged non-classical pentavalent silicon cation [5,8,9]. The two diastereoisomeric siliranium ions, having synand anti-stereochemistry, are interconverted via the p-silyl cation.…”

Catalytic Asymmetric [3+2] Annulation of Allylsilanes with Isatins: Synthesis of Spirooxindoles

“…Although we expected that various allylsilanes would favor the annulation product, the allylation pathway is remarkably persistent even for bulky silyl groups. [18] Even replacing one isopropyl group on the silicon with a para-anisole group led to a drop in the selectivity for the annulation product (entry 2). Overall, the benzhydryl allylsilane provided a balance for the desired reactivity and product ratio (entry 3), and was optimal given the ease of preparation.…”

Catalytic Asymmetric [3+2] Annulation of Allylsilanes with Isatins: Synthesis of Spirooxindoles

“…[16] We screened several allylsilanes having different steric and electronic properties with the aim of identifying an allylsilane that favors the annulation pathway and also contains an oxidizable silane group (Table 3). [18] Even replacing one isopropyl group on the silicon with a para-anisole group led to a drop in the selectivity for the annulation product (entry 2). Although we expected that various allylsilanes would favor the annulation product, the allylation pathway is remarkably persistent even for bulky silyl groups.…”