Natural product synthesis oxygen nucleophile cyclization

M-Acyliminium cyclizations of optically active mono- and di-oxygenated hydroxylactam derivatives have been used in the synthesis of a number of natural products. In case of a five-membered lactam the oxygen function adjacent to the iminium carbon directs attack of the internal nucleophile from the least hindered side, opposite to the substituent. In the examples given the size of the newly formed ring is determined by the electronic bias of the alkene substituent. 

As described in Section II, Lewis acid catalyzed desilylative carbon-carbon bond formation with an electrophile has been shown to be very versatile in organic synthesis. Occasionally, depending on the nature of the substrates (e.g. the presence of appropriate functional groups), the carbon-silicon bond may remain intact. For example, treatment of 132 with a Lewis acid affords a mixture of cyclization products 133-135 (equation 113). The isolation of 133 indicates that the carbocation intermediate thus formed is trapped by the oxygen nucleophile before elimination of the silyl moiety occurs204. 

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