Excised heterocyclization

Biodesulfurization (BDS) is the excision (liberation or removal) of sulfur from organosul-fur compounds, including sulfur-bearing heterocycles, as a result of the selective cleavage of carbon-sulfur bonds in those compounds by the action of a biocatalyst. Biocatalysts capable of selective sulfur removal, without significant conversion of other components in the fuel are desirable. BDS can either be an oxidative or a reductive process, resulting in conversion of sulfur to sulfate in an oxidative process and conversion to hydrogen sulfide in a reductive process. However, the reductive processes have been rare and mostly remained elusive to development due to lack of reproducibility of the results. Moderate reaction conditions are employed, in both processes, such as ambient temperature (about 30°C) and pressure. 

Previous preparations by Scolastico were based on the Strecker synthesis of aminonitrile and lacked steroselectivity [74,75]. More recently, two formal syntheses were reported from the same ketone 71. In Rama Rao s synthesis (Scheme 11.19) [76], 71 was condensed with vinyl magnesium bromide to give the tertiary alcohol 72 as a single isomer. This compound was then transformed into the vinyl epoxide 73 that, under palladium catalysis, reacted with 4-methoxyphenyl isocyanate to produce the oxazohdinone 74 with retention of its configuration. The remainder of the synthesis consisted of heterocycle opening and adjustment of the oxidation level to provide the lactone 75. Excision of two carbons was necessary to form the known aldehyde 76, previously transformed into myriocin [74]. 

Aza-heterocycle Ring Contraction. A variety of polyaza six-ring heterocycles undergo contraction with formal excision of N, e.g. eq 14. Further heteroatoms in the ring are tolerated 1,2,3-triazines yield pyrazoles and 1,2,4-triazines yield imidazoles, with the latter usually requiring reflux temperature. The utility of indole syntheses from cinnolines " by this method should be noted. Adj acency of heteroatoms is not required conversions of pyrimidine to pyrrole and 1,3,5-triazine to imidazole have been recorded. The mechanism of these conversions is unclear, but ring dihydro derivatives are thought to be involved. 

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