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I. Stereoselective synthesis of homoallyl peroxides and synthesis of peroxyalkanols. II. 1,2,4-trioxepanes as redox protecting groups for carbonyls. III. Cutin monomer: Synthesis, stereochemistry and fungal activation
Although the number of reported peroxide natural products with important biological properties continues to increase, methodology for synthesis of peroxides has remained limited. Most peroxide natural products (or their synthetic precursors) contain a 3-peroxycarbonyl or a homoallyl unit. However there are relatively few methods suitable for even racemic synthesis of these motifs and no general approaches to their asymmetric synthesis are available. Chapter 1 describes approaches to stereoselective synthesis of homoallyl peroxides through Lewis acid mediated allylation of chiral monoperoxyacetals with allylsilanes. The stereoinduction from an existing stereocenter in the peroxyacetal is utilized to dictate stereochemistry of the newly formed peroxide via C-C bond formation. The results indicate that stereoinduction is significant from neighboring 2-iodo, 2-silyl-, 3-alkoxy and moderate from 3-acyloxy and 4-alkoxy groups, and minimal from a 4-iodosubstituent. Overall the stereochemical trends were found to loosely parallel nonperoxidic acetals, but are influenced by the unique reactivity patterns of the peroxyacetals. In the course of this study we found that acetals were more reactive than peroxyacetals and the first synthesis of a 3-sila-1,2,4-trioxepane is reported. ^ Chapter 2 describes the synthesis of peroxyalkanols utilizing the Co(II)-catalyzed triethylsilyl peroxidation. The methodology is investigated as an approach to the 1,2-dioxolane core of the plakinic acids. The Co(II)-peroxidations also offer a mild method for the synthesis of peroxyacetates which are synthetic precursors for peroxide synthesis. The chemistry developed in Chapter 3 suggests that 1,2,4-trioxepanes may complement the use of 1,3-dithianes for acid stable protection of carbonyls. Trioxepanes readily prepared and easily handled derivatives of aldehydes and ketones are stable to a variety of synthetic conditions and are easily cleaved by Zn/AcOH or Mg/MeOH to regenerate the parent carbonyls. ^ Finally Chapter 4 describes the first asymmetric synthesis of cutin monomers (R) and (S)-10,16-dihydroxyhexadecanoic acids and confirmation of the (S)(+)-absolute configuration for the 10,16-DHPA derived from tomato cutin. The individual (R) and (S)-stereoisomers are not identical in activation of gene expression in the fungal pathogen Colletotrichum trifolii. The efforts toward the preparation of synthetic cutin polymer are also reported. ^
Ahmed, Aqeel, "I. Stereoselective synthesis of homoallyl peroxides and synthesis of peroxyalkanols. II. 1,2,4-trioxepanes as redox protecting groups for carbonyls. III. Cutin monomer: Synthesis, stereochemistry and fungal activation" (2004). ETD collection for University of Nebraska - Lincoln. AAI3152598.