S_N2′ Addition of Cuprates to Acyclic Vinyloxiranes. Synthesis of Tylactone and Tylonolide Subunits

The chiral acyclic vinyloxiranes 8 and 18 undergo highly anti selective S_N2′ additions upon treatment with Et₂CuLi and (S)-PMBOMOCH₂CH(CH₃)CH₂Cu(CN)Li, respectively. The product of the former addition, diol 9, affords the α-epoxide 12 upon epoxidation with m-CPBA. Conversion to acetonide 15, a possible C-1-C-7 segment of tylactone, was effected by hydrogenation of the methylene acetonide 14 obtained from epoxide 12 through LiNEt₂ elimination and ketalization with 2,2-dimethoxypropane (2,2-DMP). Allylic alcohol 24b, a close analogue of diol 9, gave only the β-epoxide 25b upon treatment with m-CPBA. Epoxidation with magnesium monoperoxyphthalic acid (MMPP), however, yielded a separable 53:47 mixture of β-and α-epoxides 25b and 26b. The former was carried on to acetonide 29 by a sequence involving basic elimination (LiNEt₂), treatment with 2,2-DMP, and hydrogenation. Acetonide 30, a diastereomer of 29, was prepared from epoxide 26b by a parallel sequence. Acetonide 30 was converted to the lactol methyl ether 48, an intermediate in Nicolaou's synthesis of O-micinosyl tylonolide, through displacement of tosylate 43 with KCN and then reduction (DIBAH), methanolysis (HCl, MeOH), silylation (TBSOTf, 2,6-lutidine), and finally PMBOM cleavage (DDQ). An identical sequence was applied to acetonide 29 resulting in the isomeric lactol methyl ether 37.