Lewis acids promoting cyclization

A Lewis-acid-promoted cyclization of ethene tricarboxylate derivatived aromatic compounds 1162 provides a route to isochroman-3-ones 1163 via a Friedel-Crafts intramolecular Michael addition protocol. The substrate must possess two ///f to-positioncd electron donating groups in order for the reaction to proceed (Equation 452) <20040BC3134>. 

Lewis-acid promoted cyclizations of allylic stannanes have been successfully employed to prepare macrocyclic compounds. An enantioenriched a-oxygenated allylic stannane led to a 14-membered cembrane precursor in high yield with excellent diastereoselectivity (Eq. 64) [80]. 

Scheme 3.35. Tetrahydrofurar synthesis by means of MOM a-alkoxy alkyl cuprate conjugate additions followed by Lewis acid-promoted cyclization (MOM = methoxymethyl) [129].

The first stereocontrolled method for the preparation of 3-alkylideneoxepanes was demonstrated using this cyclization strategy. Thus, Lewis acid promoted cyclizations of vinylsilane acetals (37a) and (37b) yield the seven-membered ring alkylideneoxacycles (38a) and (38b), as illustrated in Scheme 16. 

The Lewis acid-promoted cyclization of the deuterium-labeled model 13 is found to give products corresponding to an anti Sp reaction. All of the cycliza-tions with Lewis acids are greater than 95% selective for the anti Sp reaction. The high selectivity observed demonstrates that in a sterically unbiased S/- reaction an anti orientation of the electrophile with respect to silicon is preferred. The products from both the synclinal and antiperiplanar transition structures are found to be anti selective. The arrangement of double bonds in the transition structure does not affect the relative disposition of the silicon electrofuge, which must be disposed away from the approaching electrophile. 

A thorough mechanistic study of the intramolecular allylstannane-aldehyde cy-clization has recently been reported [75b]. The intramolecular cyclization substrates described allow the double bond geometry of the allylstannane to be varied. The Lewis acid-promoted cyclization of either the E- or Z-stannane can lead to the four limiting stereoisomeric products (Scheme 10-41). 

Yamamoto [106] and Martin [107] have independently investigated the exo cyclizations of (y-alkoxyallyl)stannanes, potential precursors to polyether natural products. Yamamoto systematically studied the thermal and Lewis acid-promoted cyclizations of allylstannanes (Z)-170 and ( )-171 (Table 11-11). 

Kuroda et al. have found that the diastereoselectivity of the Lewis acid-promoted cyclization of 132 is highly dependent on the geometry of the allylsilane moiety (Scheme 10.154) [429]. The stereochemical outcomes can be rationalized by chair transition structures involving a secondary orbital interaction wifhout severe steric repulsion. 

Aziridinylbenzaldoximes 286 undergo Lewis acid promoted cyclization with scandium triflate in the presence of TMSCl to produce 5,6-dihydro-4f/-l-oxa-2,4-diazines 287 (Equation 66) <2006TL9029>. The method was further developed to produce a one-pot synthesis of 287 beginning with the aldoxime precursors, i.e., the appropriate aziridines and chlorooximes. 

Lewis acid-promoted cyclization of 5-hexenals J. A. Marshall, Chemtracts-Org. Chem. 5, 1-7 (1992). Review of alkenes as enophiles B. B. Snider, Comp. Org. Syn. 5, 1-27 (1991). Review of carbonyl compounds as enophiles idem, ibid. 2, 527-561 in conjunction with asymmetric synthesis K. Mikami, M. Shimizu, Chem. Rev. 92,1021-1050 (1992) K. Mikami et al, Synlett 1992, 255-265. 

An intramolecular cyclization of an ene with a Lewis acid-activated carbonyl also took advantage of low-energy conformers to obtain high selectivity in the reaction. The optically-enriched ketones 75 and 77 underwent Lewis acid promoted cyclization via a chair-chair transition state, which minimized steric interactions and provided quantitative yield of the optically-enriched alcohols 76 and 78, respectively. In this instance, the OTIPS group served as a conformational anchor to result in the stereoselectivity in the cyclization. ... 

Lewis Acid-Promoted Cyclization Reactions by Way of Methoxycarbonyloxonium Ion Intermediates... 

H. Hiemstra, L.L.M. Lolkema, H.H. Mooiweer and W.N. Speckamp Lewis Acid-Promoted Cyclization Reactions by Way of Methoxycarbonyloxonium Ion Intermediates ... 

Alternatively, 2-aryl-4-quinolones (124, R =Ar) are obtained from (2-amino)aceto-phenones 125 by base-induced aldol condensation with aromatic aldehydes (—>-126), Lewis acid-promoted cyclization to the 2,3-dihydro-4-quinolones 127, and their dehydrogenation with PhI(OAc)2 (127 -> 124, R=Ar) [186],... 

Alkylphenol Oxidation. 1,4-Quinone methides can be obtained by the reaction of p-alkylphenols with Ag20. These reactive compounds may undergo subsequent transformations, for example, a Lewis acid promoted cyclization (eq 5). 1,2-Quinone methides can also be formed by Ag20 oxidation of appropriately substituted phenols eqs 6 and 7). ... 

Scheme 5.28 Lewis acid-promoted cyclization of alkenylsilane bearing an acetal moiety.

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