Silane, allylic synthesis

Salinosporamide synthesis 196 Sharpless asymmetric dihydroxylation (see also Osmylation) 84,89, 141, 189 Sharpless asymmetric epoxidation 32,141 Silane, allylic synthesis 43 Sonogashira coupling (see Pd)... 

CM with Allylic Compounds Allylic compounds such as allyl silanes, allyl phosphonates, allyl halides, and primary allylic alcohols are categorized as type I olefins and are used in CM reactions with various olefins for the synthesis of natural products. The CM reaction of allyl silanes has been reported to be combined with intramolecular Sakurai-type reaction for the construction of cyclic compounds.In the concise synthesis of the indolizidine alkaloid by Marsden and McElhinney, " allyltrimehylsilane (4 equiv) was employed as a CM partner of the type I olefin 80 giving rise to the desired allylsilane 81 in 73% yield as a 3 1 mixture of E- and Z-isomers (Scheme 24.21). After the... 

Similarly, efficient tetracyclization (MeAlCl2-promoted) of the bis-allylic silane/ bis-epoxide 97 constitutes the key step in the synthesis of (+)-a-onocerin. In this case, because of the presence of the bis-allylic silane group, a double bis-annula-tion occurs, with the formation of the ethylene-bridge linked bis-decalin system present in the target compound (Scheme 8.26) [46],... 

Entries 11 to 13 are examples of iminium ion and acyliminium ion reactions. Note that in Entries 11 and 12, vinyl, rather than allylic, silane moieties are involved. Entries 14 and 15 illustrate the synthesis of (3,-y-unsaturated ketones by acylation of allylic silanes. 

Some examples of conjugate addition reactions of allylic silanes are given in Scheme 9.5. Entries 1 to 3 illustrate the synthesis of several (3-allyl ketones. Note that Entry 2 involves the creation of a quaternary carbon. Entry 4 was used in the synthesis of a terpenoid ketone, (+)-nootkatone. Entry 5 illustrates fluoride-mediated addition using tetrabutylammonium fluoride. These conditions were found to be especially effective for unsaturated esters. In Entry 6, the addition is from the convex face of the ring system. Entry 7 illustrates a ring closure by intramolecular conjugate addition. 

Each of the unsaturated cyclic amines shown below has been synthesized by reaction of an amino-substituted allylic silane under iminium ion cyclization conditions (CH2=0, TFA). By retrosynthetic analysis, identify the appropriate precursor for each cyclization. Suggest a method of synthesis of each of the required amines. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

The allyl silane (185) has been used to prepare key intermediates for the synthesis of prostaglandins 66 5 ... 

A more recent synthesis of 197 [365] is shown in Fig. 9. Enders introduced the stereogenic centre of (S)-lactic acid into the crucial position 10 in 197. The vinylsulfone B, readily available from lactic acid, was transformed into the planar chiral phenylsulfonyl-substituted (q3-allyl)tetracarbonyliron(+l) tetra-fluoroborate C showing (IR,2S,3 )-configuration. Addition of allyltrimethyl silane yielded the vinyl sulfone D which was hydrogenated to E. Alkylation with the dioxolane-derivative of l-bromoheptan-6-one (readily available from 6-bro-mohexanoic acid) afforded F. Finally, reductive removal of the sulfonyl group and deprotection of the carbonyl group furnished 197. A similar approach was used for the synthesis of 198 [366]. 

Scheme 3.7. Diastereoselective formation of /S-silyl ( )- or (Z)-ester enolates by silylcuprate conjugate addition followed by alkylation with aldehydes [49]. Stereoselective synthesis of ( )-and (Z)-allyl silanes [50].

The complexes of the composition [ Ir( t-X)(diene) 2], where X = halogen, OH, OMe (e.g. [IrCl(CO)(cod)]) appeared to be very effective catalysts for the hydrosilylation of allyl chloride by trialkoxy- and alkylalkoxy-silanes [22]. Other iridium complexes have been subsequently reported as catalysts for the synthesis of silane... 

Scheme 16 Synthesis of the allylic silane 89 by an 5 2 displacement of an allylic phosphate...

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