Iminium ions allylic silanes

Most reactions of alkenyl and allylic silanes require strong carbon electrophiles and Lewis acid catalysts are often involved. The most useful electrophiles from a synthetic standpoint are carbonyl compounds, iminium ions, and electrophilic alkenes. 

Iminium ions are reactive electrophiles toward both alkenyl and allylic silanes. Useful techniques for closing nitrogen-containing rings are based on in situ generation of iminium ions from amines and formaldehyde.121... 

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. 

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. 

Overman et al. exercised the CBS reduction strategy during synthesis of the natural opium alkaloid (—)-morphine (50)21 (Scheme 4.3q). Enantioselective reduction of 2-allylcyclohex-2-en-l-one (51) with catecholborane in the presence of the (R)-oxazaborolidinc catalyst (l )-28a provided the corresponding (S)-cyclohexenol 52 in greater than 96% ee. Condensation of this intermediate with phenyl isocyanate, regioselective catalytic dihydroxylation of the terminal double bond, and protection of the resulting diol afforded 53 in 68% overall yield from 51. The ally lie silane 54 for the upcoming iminium ion-ally lsilane cycliza-tion step was obtained in 81% yield by a stereoselective Sn2 displacement of allylic carbamate. 

Somfai and Ahman have applied an intramolecular allyl silane addition to an AT-sulfonyl iminium ion as a key step in an alternative synthesis of (+)-anatoxin A [43]. Thus, L-pyroglutamic acid-derived compound 55 was homologated to aldehyde 126 and then to allyl silane 127 (Scheme 24). Using titanium tetrachloride, 127 could be cyclized in good yield to bicyclic olefin 128, which was converted to the alkaloid. 

An intramolecular allyl silane/N-sulfonyl iminium ion cyclization has also been used as a pivotal step in an approach to the tricyclic core of the unique marine alkaloid sarain A [46]. The starting material was aziridine ester 129 (Scheme 25) which was elaborated to amide 130. An important step in the synthetic strategy was thermolysis of 130 to an azomethine ylide, which underwent stereospecific intramolecular 1,3-dipolar cycloaddition with the Z-alkene to produce bicyclic lactam 131 [47]. This compound was then elaborated into allyl silane 132. It was then possible to replace the lactam N-benzyl functionality with a tosyl moiety, leading to 133, and subsequent reduction of the carbonyl group afforded the desired cyclization precursor a-hydroxy sulfonamide 134. Exposure of 134 to ferric chloride promoted cyclization to a single stereoisomeric tricyclic amino alkene 136 having the requisite sarain A nucleus. It is believed that the intermediate N-sulfonyl iminium ion cyclizes via the conformation shown in 135. 

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