The azaallyl cation-mediated [4 + 3] cycloaddition with spiro[2.4]hepta-4,6-diene by the procedure of Schmid provides the tricyclic cycloadducts of general type 3. The keto bridge of the cycloadducts 17c, 21, and 22 has been cleaved by PhI(OAc)2-l2 (Suárez cleavage), which involves ^-fragmentation of an alkoxy radical, to furnish iodo lactones 19,32, and 30a,b, respectively. Subsequent oxidation of these alkyl iodides has been investigated to develop a new synthetic route for bridgehead olefins (i.e., 33) of medium-sized carbocycles.
Total Synthesis of Colchicine.α-Methoxy-Substituted Oxyallyl [4 + 3] Cycloaddition Approach.-A new total synthesis of colchicine (VIb) is developed, based on a [4+3] cycloaddition of furan derivative (III) with the α-methoxy(trimethylsiloxy)allyl cation. The stereochemistry of intermediate (V) is assigned tentatively based on the expected approach of the allyl cation from the less hindered β-face of the furan. Complete inversion of the regioselectivity of this reaction is unexpectedly observed using the acetyl-protected furan derivative (II). This cycloaddition methodology is anticipated to also offer an efficient approach to related tropoloisoquinoline alkaloids. -(LEE, J. C.; JIN, S.-J.; CHA, J. K.; J.
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