Synthetic Study of Macrocyclic Terpenoids by the Pummerer-type alpha-Allylation of Chiral Selenoxides

ach_2cl_2 solution of l-menthyl 2-phenylselenenylacetates (1a) was treated with mCPBA to give thecorresponding selenoxide 2a as stable colorless needles. The ^ 1h NMR and ^ 3cnmr spectra of 2aindicated that 2a was obtained as an equilibrated 1：1 mixture at the Se-O bond. On the other hand，mCPBA oxidation of the ethyl 2-phenylselenenylacetate (1b) only gave the recoverted selenide due tothe deoxygenation from the selenoxide 2b during the usual workup. Thus，这是obvious that the steric protection was effectvie for the stabilization of seleninylfunctionality.subsequently,ach_2cl_2 solution of 2a was treated with (cf_3co) _2o或bf_3、oe_2 <t2> in the presence ofallyltrimethylsilane to affod an inseparable diastereomeric mixture (1：1) of the alpha-allylation products (l-menthyl 2- paynylselenenyl -4-pentenoate (3a)) besides selenide1a和diphenyl diselenide. One-pot alpha-allylation of 2b was also carried out by the stepwisetreatment of 1 b with mcpba, allyltrimethylsilane,and (cf_3co)_2 . the use of the allyltributylstannanes was also effective for the similaralpha-allylation of 2a，but the unavoidable contamination of organostannanes to the crude reaction mixture made thechromatogaphic purification of the products so difficult. the alpha-allylation of selenoxides 2 wasthought to proceed through the nucleophilic attack of allylsilane to the selenonium ion generated bythe Pummerer type reaction from 2.FurthermoremCPBA oxidation of the diastereomeric mixture of 3a gave -menthyl (2e)-2,4-pentadienoate （4a）through thermal syn elimination of selenoxides. Thus，the Pummerer-type alpha-allylation of selenoxides 2 was regarded as being a novel homologationreaction of carbon chains。

A CH_2Cl_2 solution of l-menthyl 2-phenylselenenylacetates (1a) was treated with mCPBA to give the corresponding selenoxide 2a as stable colorless needles. The ^1H NMR and ^3CNMR spectra of 2a indicated that 2a was obtained as an equilibrated 1 : 1 mixture at the Se-O bond. On the other hand, mCPBA oxidation of the ethyl 2-phenylselenenylacetate (1b) only gave the recoverted selenide due to the deoxygenation from the selenoxide 2b during the usual workup. Thus, it was obvious that the steric protection was effectvie for the stabilization of seleninyl functionality.Subsequently, a CH_2Cl_2 solution of 2a was treated with (CF_3CO)_2O or BF_3・OE_<t2> in the presence of allyltrimethylsilane to affod an inseparable diastereomeric mixture (1 : 1) of the alpha-allylation products (l-menthyl 2-phenylselenenyl-4-pentenoate (3a)) besides selenide 1a and diphenyl diselenide. One-pot alpha-allylation of 2b was also carried out by the stepwise treatment of 1b with mCPBA,allyltrimethylsilane, and (CF_3CO)_2O.The use of the allyltributylstannanes was also effective for the similar alpha-allylation of 2a, but the unavoidable contamination of organostannanes to the crude reaction mixture made the chromatogaphic purification of the products so difficult. The alpha-allylation of selenoxides 2 was thought to proceed through the nucleophilic attack of allylsilane to the selenonium ion generated by the Pummerer type reaction from 2.Furthermore, mCPBA oxidation of the diastereomeric mixture of 3a gave l-menthyl (2E)-2,4-pentadienoate (4a) through thermal syn elimination of selenoxides. Thus, the Pummerer-type alpha-allylation of selenoxides 2 was regarded as being a novel homologation reaction of carbon chains.