Allylic oxidation 14-membered ring structures

Monosubstituted diene systems have been employed for the synthesis of bicyclic systems as well as monocyclics [17]. The simplest way to accomplish this is to construct a second ring onto an existing structure (Eq. 54). This cyclization process is initiated at the alkene lacking allylic substitution. The formation of the six-membered ring necessitates the use of methylphenylsilane as the silylat-ing reagent. Because the silane itself comprises a new stereocenter, it must be removed by oxidation to assess accurately the diastereoselectivity of cyclization. Fluxionality of the five-membered ring results in a mixture of diastereomers at the silylmethyl-substituted stereo center. 

Aziridines are key structural motifs present in natural products such as mitomycins and azinomycins and versatile building blocks which can undergo various useful transformations. Hypervalent iodine-mediated intramolecular aziridinations of allylic carbamates and reaction of A-tosyliminophenyliodinane (Phi = NTs) with double bonds have been reported to be efficient and practical routes to access these three-membered rings. Allylic carbamates 71 undergo enantioselective aziridine formation on oxidation with chiral binaphthyl hypervalent iodine compound 72 (Scheme 28) [86]. 

Oxidative extrusion of p-lactones from the ir-allyl complexes is most effectively achieved using ceric ammonium nitrate (CAN). It is essential that the structure of the complex does not preclude formation of the four-membered ring. Whilst the P-lactone 26 is readily formed as the major product from the cis complex 27, the larger 8-lactone 28 is formed from the tram complex 29 (Scheme 4.12). 

An enantioselective oxidative cyclization of 4-alkenoic acids to give y-lactone derivatives via palladium-catalyzed allylic C-H activation was also disclosed by Sasai in 2011 (Scheme 3.42) [76]. In the presence of 10mol% Pd(OAc)2 and 15mol% spiro bis(isoxazoline) ligand, a variety of five-membered ring lactones could be constructed in moderate to excellent yields. However, the enantioselectivity is highly depended on the structures of substrates. 

The structure of ( —) 2P-acetoxy-1 la-hydroxyverrucosane (544) was established as follows (Scheme 65). Jones oxidation of (544) gave a five-membered ketone (551). Acid treatment of the latter resulted in homoallylic ring expansion to homo-allylic ketoalcohol (552) which was converted via the tosylhydrazone into (548) identical with the homoallylic alcohol prepared from (547) via (545) (Scheme 64). That the 11-hydroxyl group was a was inferred from the remarkable downfield shifts exhibited by the C-10 and the C-12 methyl groups of the five-membered ketone (551) and the small solvent shift exhibited by the C-10 methyl group (553) and its homoallylic ring expanded product. 

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