Bicyclic sulfone synthesis

A procedure for a synthesis of dienyl sulfoxides (232 and 233) involves ring opening of dihydrothiophene 1,1-dioxide (231) by two molar equivalents of Grignard reagent (equation 141)136. The yields are usually in the 20-66% range. Similarly, treatment of the bicyclic sulfones 234, with 2 equivalents of phenylmagnesium bromide, produced a mixture of 1,4-dienylic sulfoxides (equation 142)136. 

In the synthesis of an ionophore antibiotic, a bicyclic sulfone made by oxidation of the corresponding sulfide with PhSeSePh/H202 to avoid double bond oxidation was coupled with an allylic bromide in 97% yield and high stereoselectivity. The sulfonyl group was finally eliminated in basic medium to create a trans-dxene (Scheme 66). 

The results can be rationalized as follows the first cyclization of substrate 68 gave a mixture of a-sulfonyl radical intermediates 71 and 72 (Scheme 19). These then underwent a second fully stereoselective cyclization to give the cis- and franr-fused bicyclic products 69 and 70, respectively, as single isomers. Such sequential transformations, in which two carbon-carbon bonds are formed in a single step, are attractive methods for enhancing the efficiency of organic synthesis. As y-hydroxy-a,P-unsaturated phenyl sulfones can be prepared in enantiomerically pure form,48 this procedure should be readily applied to the synthesis of enantiomerically pure bicyclic products. 

II, 2.4]-triazin-3(2/y)-one (151, R = Me, R = H) (70JHCI23I). Similarly, 150 (R = Me) reacts with a-cyanobenzyl p-toluene sulfonate lPhCH(CN)OTS] or a-cyanobenzyl bromide PhCH(CN)Br] to afford 151 (R = Me, R = Ph) (7IJHC62I). Structure 151 was proved by its reduction with diborane to 152, which was also obtained by an independent synthesis involving the reaction of 153 with thionyl chloride followed by base-catalyzed cyclization (70JHCI231 71JHC621). To ascertain whether the methyl group at the I-position in 150 (R = Me) influences the direction of cyclization, the thione 150 (R = H) was allowed to condense with a-cyanobenzyl-p-toluene sulfonate and ethyl a-bromophenylacetate. In both cases 151 (R = H, R = Ph) was obtained. Compound 150, however, on reaction with 1,2-dibromoethane, furnished another bicyclic system (154). The difference in the behavior of 150 toward dibromoethane and other reagents such as ethyl bromoacetate and a-cyanobenzyl p-toluene... 

Sammes and cowoikers devised a promising solution to the synthesis of substituted P-silyl sulfones, which has been exploited in a new approach to the antibiotic bicyclomycin (Scheme 44). In their approach the P-silyl sulfone (125) was prepared by conjugate addition of the lithium enolate of the mono-imino ether derivative (123) of a dioxopiperazine to the unsaturated P-silyl sulfone (124). After oxidative cyclization to the bicyclic system (126), the latent alkene functionality was unleashed in 89% yield on treatment with TBAF in THF at room temperature to give the basic skeleton of bicyclomycin. 

Carretero and co-workers found that dqjrotection and cyclization of the vinyl-sulfone 294 produced a 4 1 mixture of 2,3-cu-disubstituted pyrrolidine 295 and its tram isomer (Scheme 40). A/-Alkylation of the nuxture. with 3-chloro-2-chloro-methylprop-l-ene followed by chromatogttqjhy led to isolation of the pure 2,3-c/s product 296, silylation and base-initiated cyclization of which gave indolizidine 297. Ozonolysis and elimination of the sulfone group yieldied another pivotal intermediate, the bicyclic enone 298. Reduction with L-Selectride afforded an inseparable nuxture of two diastereomeric alcohols 299 (9 1). Separation was accomplished only after dihydroxylation with osmium tetroxide and peracetylation of the resdting tetrols. The synthesis of ( )-241 was completed by hyc lysis of the m or tetraacetate 300. 

A dramatic application was the asymmetric synthesis of epibatidine 70 by Simpkins.18 Diels-Alder reaction of the deactivated pyrrole 63 with the alkynyl sulfone 64 gave the bicyclic core 65 of epibatidine. Selective reduction gave the compound 66 needed for epibatidine, but in racemic form. A directed lithiation (chapter 7) and sulfonation led to achiral bis sulfone 67. 

A very interesting synthesis of medium-sized cyclic amines has been performed by selective ring cleavage of sulfonylated bicyclic amines.246 A Julia-type desulfonylation of an activated p-amino sulfone is the key step in this method, which takes place even in the presence of a hydroxyl leaving group in the P position (Eq. 140). 

Haseltine has described an enantioselective formal synthesis of pancratistatin in which the stereocontrol is driven by the aeetonide of conduritol A. The enantioselective hydrolysis (desymmetrization) of this compound was achieved with a lipase, and the aryl-cyclohexane ring bond was formed through an intramolecular cyclization of the activated benzene ring with an allylic triflate (278). Plumet reported a total synthesis of (-l-)-7-deoxypaneratistatin based on the conjugate addition of an aryl-lithium species to a bicyclic conjugated sulfone derived from furan, which enabled the efficient installation of the six stereogenic centers of the cyclohexane ring (279) (Scheme 13). 

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