New synthetic uses for chiral 1,3-dioxolan-4-ones
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The behaviour of chiral 1,3-dioxolan-4-ones, derived from reaction of mandelic and lactic acid with pivalaldehyde, as chiral acyl anion equivalents has been examined. Addition of the corresponding 5-anions to a substituted nitrostyrene and to butenolide was achieved and the structure and stereochemistry of the adducts established by X-ray crystallography. Fragmentation under flash vacuum pyrolysis (FVP) conditions occurred in the expected way (loss of Bu([superscript]t)CHO and CO) in the latter case, but in the former reductive cyclisation was used to generate functionalised lactams. An unexpected reaction of a dioxolanone anion with the dioxolanone to afford an aldol-like dimer was observed in one case. Attempts to extend the range of dioxolanones by using amino acid-derived α-hydroxy acids met with limited success. Only the 5-benzyl compound derived from phenylalanine was obtained in reasonable yield and an attempt to alkylate it led again to aldol-like dimerisation. Cycloaddition to the double bond of 2-t-butyl-5-methylene-1,3-dioxolan-4-one was used to gain access to a range of novel spiro bicyclic and polycyclic systems and fragmentation of these was expected to provide products resulting from a chiral ketene equivalent. While the epoxide derived from the cyclopentadiene Diels Alder adduct did behave in this way to give the chiral ketone in high e.e., the corresponding aziridine underwent unexpected isomerisation pointing to a stepwise fragmentation mechanism of possible general applicability in these systems. Adducts were also formed with tetracyclone and 1,3-diphenylisobenzofuran and an interesting pattern of exo/endo selectivity was observed in these cases. With tetrachlorothiophene dioxide the adduct again fragmented in an unexpected way to give tetrachlorobenzoic acid, providing further support for the intermediacy of an oxonium carboxylate species. 1,3-Dipolar cycloaddition to 2-t-butyl-5-methylene-1,3-dioxolan-4-one was achieved for the first time and a range of adducts containing novel spiro heterocyclic ring systems derived from nitrile oxides, nitrones and diazo compounds were obtained and characterised. The regiochemistry of addition as well as the relative and absolute stereochemistry was demonstrated by X-ray structures of three adducts. Upon pyrolysis some of these compounds unexpectedly lost Bu([superscript]t)CHO and CO2 to give carbene-derived products, including a β-lactam in one case. Cycloaddition reactions of the achiral 2,2-dimethyl-5-methylene-1,3-dioxolan-4-one were also briefly studied.
Thesis, PhD Doctor of Philosophy
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