Ylides, formation tetrahydro

Concerning the mechanism of O/H insertion, direct carbenoid insertion, oxonium ylide and proton transfer processes have been discussed 7). A recent contribution to this issue is furnished by the Cu(acac)2- or Rh2(OAc)4-catalyzed reaction of benz-hydryl 6-diazopenicillanate 237) with various alcohols, from which 6a-alkoxypenicil-lanates 339 and tetrahydro-l,4-thiazepines 340 resulted324. Formation of 340 is rationalized best by assuming an oxonium ylide intermediate 338 which then rearranges as shown in the formula scheme. Such an assumption is justified by the observation of thiazepine derivatives in reactions which involved deprotonation at C-6 of 6p-aminopenicillanates 325,326). It is possible that the oxonium ylide is the common intermediate for both 339 and 340. 

The Cu(I)-catalyzed cyclization for the formation of ethyl ( )-tetrahydro-4-methylene-2-phenyl-3-(phenylsulfonyl)furan-3-carboxylate 82 has been accomplished starting from propargyl alcohol and ethyl 2-phenylsulfonyl cinnamate. Upon treatment with Pd(0) and phenylvinyl zinc chloride as shown in the following scheme, the methylenetetrahydrofuran 82 can be converted to a 2,3,4-trisubstituted 2,5-dihydrofuran. In this manner, a number of substituents (aryl, vinyl and alkyl) can be introduced to C4 <00EJO1711>. Moderate yields of 2-(a-substituted N-tosyIaminomethyl)-2,5-dihydrofurans can be realized when N-tosylimines are treated with a 4-hydroxy-cis-butenyl arsonium salt or a sulfonium salt in the presence of KOH in acetonitrile. The mechanism is believed to involve a new ylide cyclization process <00T2967>. 

The reduction of A -iminopyridinium salts and their ylides (112) has been studied and found to parallel the behavior of A -alkylpyridinium salts. In protic solvents reduction leads to the formation of 1,2,3,6-tetrahydro deriva-... 

According to a recent report, however, the carbonyl ylides formed under these conditions may isomerize and lead to adducts different from those normally expected87. Thus, (5)-l-acetyl-2-(diazoacetyl)pyrrolidine (8). derived from /V-acetyl-i.-proline, upon treatment with dimethyl butynedioate in the presence of a catalytic amount of rhodium(II) acetate dimer at 25 °C affords only 10% of the expected adduct dimethyl 5,8-epoxy-2,3,5,8,9,9a-hexahydro-5-methyl-9-oxo-lH-pyrrolo[l, 2-o]azepine-6,7-dicarboxylate (9). Instead, dimethyl 1,2,8,9-tetrahydro-5-methyl-l-oxo-3a//,7//-furo[3,2-g]pyrrolizine-3a,4-dicarboxylate (10) is obtained in 87% yield. The formation of this product is explained via an isomeric ylide87 and thus occurs with complete loss of chiral information. 

The solvent dependence of this cycloaddition is remarkable. The formation of 5 requires an aprotic medium, whereas in a protic medium (e.g. CH3OH), the thieno[2,3-Z ]-5,6,7,7a-tetrahydro-l -pyrrolizine 10 is formed. As the rate of product formation is the same in both media, it is plausible that the same primary product 8 of a dipolar [2+2] cycloaddition is formed first. The reaction then proceeds to give 5 in the nonpolar medium and 10 via the ylide 9 in the polar medium [19]. 

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